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Text Truncated. Only the first 1MB is shown below. Download the file for the complete contents. START-INFO-DIR-ENTRY * mysql: (mysql). MySQL documentation. END-INFO-DIR-ENTRY Table of Contents ***************** General Information About MySQL MySQL, MySQL AB, and Open Source What Is MySQL What Is MySQL AB About This Manual Conventions Used in This Manual History of MySQL The Main Features of MySQL How Stable Is MySQL? How Big Can MySQL Tables Be? Year 2000 Compliance MySQL Information Sources Books About MySQL General SQL Information and Tutorials Useful MySQL-related Links MySQL Mailing Lists The MySQL Mailing Lists Asking Questions or Reporting Bugs How to Report Bugs or Problems Guidelines for Answering Question on the Mailing List MySQL Licensing and Support MySQL Licensing Policy Copyrights Used by MySQL Copyright Changes Example Licensing Situations Selling Products that use MySQL ISP MySQL Services Running a Web Server Using MySQL MySQL Licensing and Support Costs Payment information Contact Information Types of Commercial Support Basic E-mail Support Extended E-mail Support Login Support Extended Login Support Telephone Support Support for other table handlers How Standards-compatible Is MySQL? MySQL Extensions to ANSI SQL92 MySQL Differences Compared to ANSI SQL92 Running MySQL in ANSI Mode Functionality Missing from MySQL Sub-selects `SELECT INTO TABLE' Transactions Stored Procedures and Triggers Foreign Keys Reasons NOT to Use Foreign Keys constraints Views `--' as the Start of a Comment What Standards Does MySQL Follow? How to Cope Without `COMMIT'/`ROLLBACK' Known errors and design deficiencies in MySQL How MySQL Compares to Other Databases How MySQL Compares to `mSQL' How to Convert `mSQL' Tools for MySQL How `mSQL' and MySQL Client/Server Communications Protocols Differ How `mSQL' 2.0 SQL Syntax Differs from MySQL How MySQL Compares to PostgreSQL MySQL and PostgreSQL development strategies Featurewise Comparison of MySQL and PostgreSQL Benchmarking MySQL and PostgreSQL MySQL and the future (The TODO) Things that should be in 4.0 Things that must be done in the real near future Things that have to be done sometime Some things we don't have any plans to do MySQL Installation Quick Standard Installation of MySQL Installing MySQL on Linux Installing MySQL on Windows General Installation Issues How to Get MySQL Operating Systems Supported by MySQL Which MySQL Version to Use Installation Layouts How and When Updates Are Released MySQL Binaries Compiled by MySQL AB Installing a MySQL Source Distribution Quick Installation Overview Applying Patches Typical `configure' Options Installing from the Development Source Tree Problems Compiling? MIT-pthreads Notes Post-installation Setup and Testing Problems Running `mysql_install_db' Problems Starting the MySQL Server Starting and Stopping MySQL Automatically Upgrading/Downgrading MySQL Upgrading From Version 3.22 to Version 3.23 Upgrading from Version 3.21 to Version 3.22 Upgrading from Version 3.20 to Version 3.21 Upgrading to Another Architecture Operating System Specific Notes Linux Notes (All Linux Versions) Linux Notes for Binary Distributions Linux x86 Notes Linux SPARC Notes Linux Alpha Notes Linux PowerPC Notes Linux MIPS Notes Linux IA64 Notes Windows Notes Starting MySQL on Windows 95 or Windows 98 Starting MySQL on Windows NT or Windows 2000 Running MySQL on Windows Connecting to a Remote MySQL from Windows with SSH Splitting Data Across Different Disks on Windows Compiling MySQL Clients on Windows MySQL-Windows Compared to Unix MySQL Solaris Notes Solaris 2.7/2.8 Notes Solaris x86 Notes BSD Notes FreeBSD Notes NetBSD notes OpenBSD Notes OpenBSD 2.5 Notes OpenBSD 2.8 Notes BSD/OS Notes BSD/OS Version 2.x Notes BSD/OS Version 3.x Notes BSD/OS Version 4.x Notes Mac OS X Notes Mac OS X Public Beta Mac OS X Server Other Unix Notes HP-UX Notes for Binary Distributions HP-UX Version 10.20 Notes HP-UX Version 11.x Notes IBM-AIX notes SunOS 4 Notes Alpha-DEC-UNIX Notes (Tru64) Alpha-DEC-OSF1 Notes SGI Irix Notes SCO Notes SCO Unixware Version 7.0 Notes OS/2 Notes BeOS Notes Novell Netware Notes Introduction to MySQL: A MySQL Tutorial Connecting to and Disconnecting from the Server Entering Queries Creating and Using a Database Creating and Selecting a Database Creating a Table Loading Data into a Table Retrieving Information from a Table Selecting All Data Selecting Particular Rows Selecting Particular Columns Sorting Rows Date Calculations Working with `NULL' Values Pattern Matching Counting Rows Using More Than one Table Getting Information About Databases and Tables Examples of Common Queries The Maximum Value for a Column The Row Holding the Maximum of a Certain Column Maximum of Column per Group The Rows Holding the Group-wise Maximum of a Certain Field Using user variables Using Foreign Keys Searching on Two Keys Calculating visits per day Using `mysql' in Batch Mode Queries from Twin Project Find all Non-distributed Twins Show a Table on Twin Pair Status Using MySQL with Apache MySQL Database Administration Configuring MySQL mysqld Command-line Options my.cnf Option Files Installing Many Servers on the Same Machine Running Multiple MySQL Servers on the Same Machine General Security Issues and the MySQL Access Privilege System General Security Guidelines How to Make MySQL Secure Against Crackers Startup Options for `mysqld' Concerning Security What the Privilege System Does How the Privilege System Works Privileges Provided by MySQL Connecting to the MySQL Server Access Control, Stage 1: Connection Verification Access Control, Stage 2: Request Verification Causes of `Access denied' Errors MySQL User Account Management `GRANT' and `REVOKE' Syntax MySQL User Names and Passwords When Privilege Changes Take Effect Setting Up the Initial MySQL Privileges Adding New Users to MySQL Setting Up Passwords Keeping Your Password Secure Disaster Prevention and Recovery Database Backups `BACKUP TABLE' Syntax `RESTORE TABLE' Syntax `CHECK TABLE' Syntax `REPAIR TABLE' Syntax Using `myisamchk' for Table Maintenance and Crash Recovery `myisamchk' Invocation Syntax General Options for `myisamchk' Check Options for `myisamchk' Repair Options for myisamchk Other Options for `myisamchk' `myisamchk' Memory Usage Using `myisamchk' for Crash Recovery How to Check Tables for Errors How to Repair Tables Table Optimization Setting Up a Table Maintenance Regimen Getting Information About a Table Database Administration Language Reference `OPTIMIZE TABLE' Syntax `ANALYZE TABLE' Syntax `FLUSH' Syntax `KILL' Syntax `SHOW' Syntax Retrieving information about Database, Tables, Columns, and Indexes `SHOW TABLE STATUS' `SHOW STATUS' `SHOW VARIABLES' `SHOW LOGS' `SHOW PROCESSLIST' `SHOW GRANTS' `SHOW CREATE TABLE' MySQL Localization and International Usage The Character Set Used for Data and Sorting Non-English Error Messages Adding a New Character Set The character definition arrays String Collating Support Multi-byte Character Support MySQL Server-Side Scripts and Utilities Overview of the Server-Side Scripts and Utilities safe_mysqld, the wrapper around mysqld mysqld_multi, program for managing multiple MySQL servers myisampack, The MySQL Compressed Read-only Table Generator mysqld-max, An extended mysqld server MySQL Client-Side Scripts and Utilities Overview of the Client-Side Scripts and Utilities The Command-line Tool mysqladmin, Administrating a MySQL Server Using `mysqlcheck' for Table Maintenance and Crash Recovery mysqldump, Dumping Table Structure and Data mysqlhotcopy, Copying MySQL Databases and Tables mysqlimport, Importing Data from Text Files Showing Databases, Tables, and Columns perror, Explaining Error Codes How to Run SQL Commands from a Text File The MySQL Log Files The Error Log The General Query Log The Update Log The Binary Update Log The Slow Query Log Log File Maintenance Replication in MySQL Introduction Replication Implementation Overview How To Set Up Replication Replication Features and Known Problems Replication Options in my.cnf SQL Commands Related to Replication Replication FAQ Troubleshooting Replication MySQL Optimization Optimization Overview MySQL Design Limitations/Tradeoffs Portability What Have We Used MySQL For? The MySQL Benchmark Suite Using Your Own Benchmarks Optimizing `SELECT's and Other Queries `EXPLAIN' Syntax (Get Information About a `SELECT') Estimating Query Performance Speed of `SELECT' Queries How MySQL Optimizes `WHERE' Clauses How MySQL Optimizes `DISTINCT' How MySQL Optimizes `LEFT JOIN' and `RIGHT JOIN' How MySQL Optimizes `LIMIT' Speed of `INSERT' Queries Speed of `UPDATE' Queries Speed of `DELETE' Queries Other Optimization Tips Locking Issues How MySQL Locks Tables Table Locking Issues Optimizing Database Structure Design Choices Get Your Data as Small as Possible How MySQL Uses Indexes Column Indexes Multiple-Column Indexes How MySQL Opens and Closes Tables Drawbacks to Creating Large Numbers of Tables in the Same Database Why So Many Open tables? Optimizing the MySQL Server System/Compile Time and Startup Parameter Tuning Tuning Server Parameters How Compiling and Linking Affects the Speed of MySQL How MySQL Uses Memory How MySQL uses DNS `SET' Syntax Disk Issues Using Symbolic Links Using Symbolic Links for Databases Using Symbolic Links for Tables MySQL Language Reference Language Structure Literals: How to Write Strings and Numbers Strings Numbers Hexadecimal Values `NULL' Values Database, Table, Index, Column, and Alias Names Case Sensitivity in Names User Variables Comment Syntax Is MySQL Picky About Reserved Words? Column Types Numeric Types Date and Time Types Y2K Issues and Date Types The `DATETIME', `DATE', and `TIMESTAMP' Types The `TIME' Type The `YEAR' Type String Types The `CHAR' and `VARCHAR' Types The `BLOB' and `TEXT' Types The `ENUM' Type The `SET' Type Choosing the Right Type for a Column Using Column Types from Other Database Engines Column Type Storage Requirements Functions for Use in `SELECT' and `WHERE' Clauses Non-Type-Specific Operators and Functions Parenthesis Comparison Operators Logical Operators Control Flow Functions String Functions String Comparison Functions Case Sensitivity Numeric Functions Arithmetic Operations Mathematical Functions Date and Time Functions Other Functions Bit Functions Miscellaneous Functions Data Manipulation: `SELECT', `INSERT', `UPDATE', `DELETE' `SELECT' Syntax `JOIN' Syntax `INSERT' Syntax `INSERT ... SELECT' Syntax `INSERT DELAYED' syntax `UPDATE' Syntax `DELETE' Syntax `TRUNCATE' Syntax `REPLACE' Syntax `LOAD DATA INFILE' Syntax Data Definition: `CREATE', `DROP', `ALTER' `CREATE DATABASE' Syntax `DROP DATABASE' Syntax `CREATE TABLE' Syntax Silent Column Specification Changes `ALTER TABLE' Syntax `RENAME TABLE' Syntax `DROP TABLE' Syntax `CREATE INDEX' Syntax `DROP INDEX' Syntax Basic MySQL User Utility Commands `USE' Syntax `DESCRIBE' Syntax (Get Information About Columns) MySQL Transactional and Locking Commands `BEGIN/COMMIT/ROLLBACK' Syntax `LOCK TABLES/UNLOCK TABLES' Syntax `SET TRANSACTION' Syntax MySQL Full-text Search Fulltext restrictions Fine-tuning MySQL Full-text Search New Features of Full-text Search to Appear in MySQL 4.0 Full-text Search TODO MySQL Table Types MyISAM Tables Space Needed for Keys MyISAM Table Formats Static (Fixed-length) Table Characteristics Dynamic Table Characteristics Compressed Table Characteristics MyISAM table problems. Corrupted MyISAM tables. Clients is using or hasn't closed the table properly MERGE Tables ISAM Tables HEAP Tables BDB or Berkeley_DB Tables Overview of BDB Tables Installing BDB BDB startup options Some characteristic of `BDB' tables: Some things we need to fix for BDB in the near future: Operating systems supported by *BDB* Errors You May Get When Using BDB Tables InnoDB Tables InnoDB tables overview InnoDB startup options Creating InnoDB table space If something goes wrong in database creation Creating InnoDB tables Converting MyISAM tables to InnoDB Adding and removing InnoDB data and log files Backing up and recovering an InnoDB database Checkpoints Moving an InnoDB database to another machine InnoDB transaction model Consistent read Locking reads Next-key locking: avoiding the phantom problem Locks set by different SQL statements in InnoDB Deadlock detection and rollback An example of how the consistent read works in InnoDB Performance tuning tips The InnoDB Monitor Implementation of multiversioning Table and index structures Physical structure of an index Insert buffering Adaptive hash indexes Physical record structure How an auto-increment column works in InnoDB File space management and disk i/o Disk i/o File space management Defragmenting a table Error handling Some restrictions on InnoDB tables InnoDB contact information MySQL APIs MySQL PHP API Common Problems with MySQL and PHP MySQL Perl API `DBI' with `DBD::mysql' The `DBI' Interface More `DBI'/`DBD' Information MySQL ODBC Support How To Install MyODBC How to Fill in the Various Fields in the ODBC Administrator Program Connect parameters for MyODBC How to Report Problems with MyODBC Programs Known to Work with MyODBC How to Get the Value of an `AUTO_INCREMENT' Column in ODBC Reporting Problems with MyODBC MySQL C API C API Datatypes C API Function Overview C API Function Descriptions `mysql_affected_rows()' `mysql_close()' `mysql_connect()' `mysql_change_user()' `mysql_character_set_name()' `mysql_create_db()' `mysql_data_seek()' `mysql_debug()' `mysql_drop_db()' `mysql_dump_debug_info()' `mysql_eof()' `mysql_errno()' `mysql_error()' `mysql_escape_string()' `mysql_fetch_field()' `mysql_fetch_fields()' `mysql_fetch_field_direct()' `mysql_fetch_lengths()' `mysql_fetch_row()' `mysql_field_count()' `mysql_field_seek()' `mysql_field_tell()' `mysql_free_result()' `mysql_get_client_info()' `mysql_get_host_info()' `mysql_get_proto_info()' `mysql_get_server_info()' `mysql_info()' `mysql_init()' `mysql_insert_id()' `mysql_kill()' `mysql_list_dbs()' `mysql_list_fields()' `mysql_list_processes()' `mysql_list_tables()' `mysql_num_fields()' `mysql_num_rows()' `mysql_options()' `mysql_ping()' `mysql_query()' `mysql_real_connect()' `mysql_real_escape_string()' `mysql_real_query()' `mysql_reload()' `mysql_row_seek()' `mysql_row_tell()' `mysql_select_db()' `mysql_shutdown()' `mysql_stat()' `mysql_store_result()' `mysql_thread_id()' `mysql_use_result()' C Threaded Function Descriptions `my_init()' `my_thread_init()' `my_thread_end()' Common questions and problems when using the C API Why Is It that After `mysql_query()' Returns Success, `mysql_store_result()' Sometimes Returns `NULL?' What Results Can I Get From a Query? How Can I Get the Unique ID for the Last Inserted Row? Problems Linking with the C API Building Client Programs How to Make a Threaded Client MySQL C++ APIs Borland C++ MySQL Java Connectivity (JDBC) MySQL Python APIs MySQL Tcl APIs MySQL Eiffel wrapper Extending MySQL Adding New Functions to MySQL `CREATE FUNCTION/DROP FUNCTION' Syntax Adding a New User-definable Function UDF Calling Sequences Argument Processing Return Values and Error Handling Compiling and Installing User-definable Functions Adding a New Native Function Adding New Procedures to MySQL Procedure Analyse Writing a Procedure MySQL Internals MySQL Threads MySQL Test Suite Running the MySQL Test Suite Extending the MySQL Test Suite Reporting Bugs in the MySQL Test Suite Problems and Common Errors How to Determine What Is Causing Problems Some Common Errors When Using MySQL `Access denied' Error `MySQL server has gone away' Error `Can't connect to [local] MySQL server' error `Host '...' is blocked' Error `Too many connections' Error `Some non-transactional changed tables couldn't be rolled back' Error `Out of memory' Error `Packet too large' Error Communication Errors / Aborted Connection `The table is full' Error `Can't create/write to file' Error `Commands out of sync' Error in Client `Ignoring user' Error `Table 'xxx' doesn't exist' Error `Cant' initialize character set xxx' error. File Not Found Installation Related Issues Problems When Linking with the MySQL Client Library How to Run MySQL As a Normal User Problems with File Permissions Administration Related Issues What To Do If MySQL Keeps Crashing How to Reset a Forgotten Password How MySQL Handles a Full Disk Where MySQL Stores Temporary Files How to Protect or change the MySQL socket file `/tmp/mysql.sock' Time Zone Problems Query Related Issues Case Sensitivity in Searches Problems Using `DATE' Columns Problems with `NULL' Values Problems with `alias' Deleting Rows from Related Tables Solving Problems with No Matching Rows Table Definition Related Issues Problems with `ALTER TABLE'. How To Change the Order of Columns in a Table TEMPORARY TABLE problems Some MySQL Users General News Sites Some Web Search Engines Some Information Search Engines Concentrated on Some Area Online Magazines Web Sites that Use MySQL as a Backend Some Domain/Internet/Web and Related Services Web Sites that Use `PHP' and MySQL Some MySQL Consultants Programming Uncategorized Pages MySQL customer usage Contributed Programs APIs Clients Web Tools Performance Benchmarking Tools Authentication Tools Converters Using MySQL with Other Products Useful Tools RPMs for Common Tools (Most Are for RedHat 6.1) Useful Functions Windows programs Uncategorized Credits Developers at MySQL AB Contributors to MySQL Supporters to MySQL MySQL change history Changes in release 4.0.x (Development; Alpha) Changes in release 4.0.0 Changes in release 3.23.x (Stable) Changes in release 3.23.44 Changes in release 3.23.43 Changes in release 3.23.42 Changes in release 3.23.41 Changes in release 3.23.40 Changes in release 3.23.39 Changes in release 3.23.38 Changes in release 3.23.37 Changes in release 3.23.36 Changes in release 3.23.35 Changes in release 3.23.34a Changes in release 3.23.34 Changes in release 3.23.33 Changes in release 3.23.32 Changes in release 3.23.31 Changes in release 3.23.30 Changes in release 3.23.29 Changes in release 3.23.28 Changes in release 3.23.27 Changes in release 3.23.26 Changes in release 3.23.25 Changes in release 3.23.24 Changes in release 3.23.23 Changes in release 3.23.22 Changes in release 3.23.21 Changes in release 3.23.20 Changes in release 3.23.19 Changes in release 3.23.18 Changes in release 3.23.17 Changes in release 3.23.16 Changes in release 3.23.15 Changes in release 3.23.14 Changes in release 3.23.13 Changes in release 3.23.12 Changes in release 3.23.11 Changes in release 3.23.10 Changes in release 3.23.9 Changes in release 3.23.8 Changes in release 3.23.7 Changes in release 3.23.6 Changes in release 3.23.5 Changes in release 3.23.4 Changes in release 3.23.3 Changes in release 3.23.2 Changes in release 3.23.1 Changes in release 3.23.0 Changes in release 3.22.x (Older; Still supported) Changes in release 3.22.35 Changes in release 3.22.34 Changes in release 3.22.33 Changes in release 3.22.32 Changes in release 3.22.31 Changes in release 3.22.30 Changes in release 3.22.29 Changes in release 3.22.28 Changes in release 3.22.27 Changes in release 3.22.26 Changes in release 3.22.25 Changes in release 3.22.24 Changes in release 3.22.23 Changes in release 3.22.22 Changes in release 3.22.21 Changes in release 3.22.20 Changes in release 3.22.19 Changes in release 3.22.18 Changes in release 3.22.17 Changes in release 3.22.16 Changes in release 3.22.15 Changes in release 3.22.14 Changes in release 3.22.13 Changes in release 3.22.12 Changes in release 3.22.11 Changes in release 3.22.10 Changes in release 3.22.9 Changes in release 3.22.8 Changes in release 3.22.7 Changes in release 3.22.6 Changes in release 3.22.5 Changes in release 3.22.4 Changes in release 3.22.3 Changes in release 3.22.2 Changes in release 3.22.1 Changes in release 3.22.0 Changes in release 3.21.x Changes in release 3.21.33 Changes in release 3.21.32 Changes in release 3.21.31 Changes in release 3.21.30 Changes in release 3.21.29 Changes in release 3.21.28 Changes in release 3.21.27 Changes in release 3.21.26 Changes in release 3.21.25 Changes in release 3.21.24 Changes in release 3.21.23 Changes in release 3.21.22 Changes in release 3.21.21a Changes in release 3.21.21 Changes in release 3.21.20 Changes in release 3.21.19 Changes in release 3.21.18 Changes in release 3.21.17 Changes in release 3.21.16 Changes in release 3.21.15 Changes in release 3.21.14b Changes in release 3.21.14a Changes in release 3.21.13 Changes in release 3.21.12 Changes in release 3.21.11 Changes in release 3.21.10 Changes in release 3.21.9 Changes in release 3.21.8 Changes in release 3.21.7 Changes in release 3.21.6 Changes in release 3.21.5 Changes in release 3.21.4 Changes in release 3.21.3 Changes in release 3.21.2 Changes in release 3.21.0 Changes in release 3.20.x Changes in release 3.20.18 Changes in release 3.20.17 Changes in release 3.20.16 Changes in release 3.20.15 Changes in release 3.20.14 Changes in release 3.20.13 Changes in release 3.20.11 Changes in release 3.20.10 Changes in release 3.20.9 Changes in release 3.20.8 Changes in release 3.20.7 Changes in release 3.20.6 Changes in release 3.20.3 Changes in release 3.20.0 Changes in release 3.19.x Changes in release 3.19.5 Changes in release 3.19.4 Changes in release 3.19.3 Comments on porting to other systems Debugging a MySQL server Compiling MYSQL for debugging. Creating trace files Debugging mysqld under gdb Using a stack trace Using log files to find cause of errors in mysqld Making a test case when you experience table corruption Debugging a MySQL client The DBUG package. Locking methods Comments about RTS threads Differences between different thread packages Environment Variables Description of MySQL regular expression syntax What is Unireg? GNU GENERAL PUBLIC LICENSE Preamble How to Apply These Terms to Your New Programs GNU LESSER GENERAL PUBLIC LICENSE Preamble How to Apply These Terms to Your New Libraries Pieces of the manual in transit Installing a MySQL Binary Distribution Perl Installation Comments Installing Perl on Unix Installing ActiveState Perl on Windows Installing the MySQL Perl Distribution on Windows Problems Using the Perl `DBI'/`DBD' Interface Functions for Use with `GROUP BY' Clauses SQL command, type and function index Concept Index This is a manual for MySQL. This version is about the 3.23.44 version of MySQL. You can find a manual about any older version of MySQL in the binary or source distribution for that version. General Information About MySQL ******************************* This is the MySQL reference manual; it documents MySQL Version 3.23.44. As MySQL is work in progress, the manual gets updated frequently. There is a very good chance that this version is out of date, unless you are looking at it online. The most recent version of this manual is available at `http://www.mysql.com/documentation/' in many different formats. If you have a hard time finding information in the manual, you can try the searchable PHP version at `http://www.mysql.com/documentation/manual.php'. MySQL is a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server. MySQL is free software. It is licensed with the *GNU GENERAL PUBLIC LICENSE* `http://www.gnu.org/'. *Note Licensing and Support::. The MySQL home page (http://www.mysql.com/) provides the latest information about MySQL. The following list describes some useful sections of the manual: * For information about the company behind MySQL, see *Note What is MySQL AB::. * For a discussion of MySQL's capabilities, see *Note Features::. * For installation instructions, see *Note Installing::. * For tips on porting MySQL to new architectures or operating systems, see *Note Porting::. * For information about upgrading from a Version 3.22 release, see *Note Upgrading-from-3.22::. * For a tutorial introduction to MySQL, see *Note Tutorial::. * For examples of SQL and benchmarking information, see the benchmarking directory (`sql-bench' in the distribution). * For a history of new features and bug fixes, see *Note News::. * For a list of currently known bugs and misfeatures, see *Note Bugs::. * For future plans, see *Note TODO::. * For a list of all the contributors to this project, see *Note Credits::. *IMPORTANT:* Reports of errors (often called bugs), as well as questions and comments, should be sent to the mailing list at <mysql@lists.mysql.com>. *Note Bug reports::. The `mysqlbug' script should be used to generate bug reports. For source distributions, the `mysqlbug' script can be found in the `scripts' directory. For binary distributions, `mysqlbug' can be found in the `bin' directory. If you have found a sensitive security bug in MySQL, you should send an email to <security@mysql.com>. If you have any suggestions concerning additions or corrections to this manual, please send them to the manual team at <docs@mysql.com>. This is a reference manual; it does not provide general instruction on SQL or relational database concepts. If you want general information about SQL, see *Note General-SQL::. For books that focus more specifically on MySQL, see *Note MySQL-Books::. MySQL, MySQL AB, and Open Source ================================ What Is MySQL ------------- MySQL, the most popular Open Source SQL database, is provided by MySQL AB. MySQL AB is a commercial company that builds its business providing services around the MySQL database. *Note What is MySQL AB::. MySQL is a database management system. A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL. Since computers are very good at handling large amounts of data, database management plays a central role in computing, as stand-alone utilities, or as parts of other applications. MySQL is a relational database management system. A relational database stores data in separate tables rather than putting all the data in one big storeroom. This adds speed and flexibility. The tables are linked by defined relations making it possible to combine data from several tables on request. The SQL part of MySQL stands for "Structured Query Language" - the most common standardized language used to access databases. MySQL is Open Source Software. Open Source means that it is possible for anyone to use and modify. Anybody can download MySQL from the Internet and use it without paying anything. Anybody so inclined can study the source code and change it to fit their needs. MySQL uses the GPL (GNU General Public License) `http://www.gnu.org', to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL into a commercial application you can buy a commercially licensed version from us. Why use MySQL? MySQL is very fast, reliable, and easy to use. If that is what you are looking for, you should give it a try. MySQL also has a very practical set of features developed in very close cooperation with our users. You can find a performance comparison of MySQL to some other database managers on our benchmark page. *Note MySQL Benchmarks::. MySQL was originally developed to handle very large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Though under constant development, MySQL today offers a rich and very useful set of functions. The connectivity, speed, and security make MySQL highly suited for accessing databases on the Internet. The technical features of MySQL For advanced technical information, see *Note Reference::. MySQL is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and several programming interfaces. We also provide MySQL as a multi-threaded library which you can link into your application to get a smaller, faster, easier to manage product. MySQL has a lot of contributed software available. It is very likely that you will find that your favorite application or language already supports MySQL. The official way to pronounce MySQL is "My Ess Que Ell" (not MY-SEQUEL). But we try to avoid correcting people who say MY-SEQUEL. What Is MySQL AB ---------------- MySQL AB is the Swedish company owned and run by the MySQL founders and main developers. We are dedicated to developing MySQL and spreading our database to new users. MySQL AB owns the copyright to the MySQL server source code and the MySQL trademark. A significant amount of revenues from our services goes to developing MySQL. *Note What-is::. MySQL AB has been profitable providing MySQL from the start. We don't get any outside funding, but have earned all our money ourselves. We are searching after partners that would like to support our development of MySQL so that we could accelerate the development pace. If you are interested in doing this, you can email <partner@mysql.com> about this! MySQL AB has currently 20+ people (`http://www.mysql.com/development/team.html') on its payroll and is growing rapidly. Our main sources of income are: * Commercial high quality support for MySQL provided by the MySQL developers themselves. If you are interested in purchasing a support contract, please visit `https://order.mysql.com/' to view our support options or to order support. * Consulting services. We have developers and consultants in 12 countries and partners in many other countries that can help you with almost any MySQL related issues. If you need consulting services, please email a good description of your needs to <info@mysql.com>! If we can't handle this ourselves we can usually find a partner or a developer that can help you with your problems. * We sell licenses for using MySQL as an embedded database. *Note Cost::. If you have a commercial product for which you need a fast, high quality database, but you can't afford to make your product Open Source, you can buy the right to use the MySQL server under a normal commercial copyright. If you are interested in this you can buy MySQL licenses at `https://order.mysql.com/' or contact us at <licensing@mysql.com>. * Advertising. `http://www.mysql.com/' is a very popular web site with more than 10,000,000 page views per months (January 2001). By putting a banner on this you are guaranteed to reach a lot of potential customers in the Open source, Linux and database community. If you are interested in this email <advertising@mysql.com>. * We are building a partner program to be able to provide MySQL services in every country. If you are interested in becoming a partner of MySQL AB please visit `http://www.mysql.com/information/partners.html' or email <partner@mysql.com>. * We provide MySQL training through our partner programs. For more information, please email <info@mysql.com>. * The MySQL brand has, since 1995, been associated with speed and reliability, and is known to be something you can depend upon. If you are interested in using the MySQL trademark in your marketing, you can email <info@mysql.com> about this. The MySQL core values show our dedication to MySQL and Open Source. We want MySQL to be: * The best and the most used database in the world. * Available and affordable for all. * Easy to use. * Continuously improved while remaining fast and safe. * Fun to use and improve. * Free from bugs. MySQL AB and the people of MySQL AB: * Promote Open Source Philosophy and support the Open Source Community. * Aim to be good citizens. * Prefer partners that share our values and mind-set. * Answer mail and give support. * Are a virtual company, networking with others. * Work against software patents. About This Manual ----------------- This manual is currently available in Texinfo, plain text, Info, HTML, PostScript, and PDF versions. The primary document is the Texinfo file. The HTML version is produced automatically using a modified version of `texi2html'. The plain text and Info versions are produced with `makeinfo'. The Postscript version is produced using `texi2dvi' and `dvips'. The PDF version is produced with `pdftex'. This manual is written and maintained by David Axmark, Michael (Monty) Widenius, Jeremy Cole, and Paul DuBois. For other contributors, see *Note Credits::. Conventions Used in This Manual ------------------------------- This manual uses certain typographical conventions: `constant' Constant-width font is used for command names and options; SQL statements; database, table and column names; C and Perl code; and environment variables. Example: "To see how `mysqladmin' works, invoke it with the `--help' option." `filename' Constant-width font with surrounding quotes is used for filenames and pathnames. Example: "The distribution is installed under the `/usr/local/' directory." `c' Constant-width font with surrounding quotes is also used to indicate character sequences. Example: "To specify a wild card, use the `%' character." _italic_ Italic font is used for emphasis, _like this_. *boldface* Boldface font is used for access privilege names (for example, "do not grant the *process* privilege lightly") and occasionally to convey *especially strong emphasis*. When commands are shown that are meant to be executed by a particular program, the program is indicated by a prompt shown before the command. For example, `shell>' indicates a command that you execute from your login shell, and `mysql>' indicates a command that you execute from the `mysql' client program: shell> type a shell command here mysql> type a mysql command here Shell commands are shown using Bourne shell syntax. If you are using a `csh'-style shell, you may need to issue commands slightly differently. For example, the sequence to set an environment variable and run a command looks like this in Bourne shell syntax: shell> VARNAME=value some_command For `csh', you would execute the sequence like this: shell> setenv VARNAME value shell> some_command Often, database, table, and column names must be substituted into commands. To indicate that such substitution is necessary, this manual uses `db_name', `tbl_name' and `col_name'. For example, you might see a statement like this: mysql> SELECT col_name FROM db_name.tbl_name; This means that if you were to enter a similar statement, you would supply your own database, table, and column names, perhaps like this: mysql> SELECT author_name FROM biblio_db.author_list; SQL statements may be written in uppercase or lowercase. When this manual shows a SQL statement, uppercase is used for particular keywords if those keywords are under discussion (to emphasize them) and lowercase is used for the rest of the statement. For example, you might see the following in a discussion of the `SELECT' statement: mysql> SELECT count(*) FROM tbl_name; On the other hand, in a discussion of the `COUNT()' function, the same statement would be written like this: mysql> select COUNT(*) from tbl_name; If no particular emphasis is intended, all keywords are written uniformly in uppercase. In syntax descriptions, square brackets (`[' and `]') are used to indicate optional words or clauses: DROP TABLE [IF EXISTS] tbl_name When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (`|'). When one member from a set of choices *may* be chosen, the alternatives are listed within square brackets (`[' and `]'): TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str) When one member from a set of choices *must* be chosen, the alternatives are listed within braces (`{' and `}'): {DESCRIBE | DESC} tbl_name {col_name | wild} History of MySQL ---------------- We once started out with the intention of using `mSQL' to connect to our tables using our own fast low-level (ISAM) routines. However, after some testing we came to the conclusion that `mSQL' was not fast enough nor flexible enough for our needs. This resulted in a new SQL interface to our database but with almost the same API interface as `mSQL'. This API was chosen to ease porting of third-party code. The derivation of the name MySQL is not perfectly clear. Our base directory and a large number of our libraries and tools have had the prefix "my" for well over 10 years. However, Monty's daughter (some years younger) is also named My. Which of the two gave its name to MySQL is still a mystery, even for us. The Main Features of MySQL -------------------------- The following list describes some of the important characteristics of MySQL: * Fully multi-threaded using kernel threads. This means it can easily use multiple CPUs if available. * C, C++, Eiffel, Java, Perl, PHP, Python and Tcl APIs. *Note Clients::. * Works on many different platforms. *Note Which OS::. * Many column types: signed/unsigned integers 1, 2, 3, 4, and 8 bytes long, `FLOAT', `DOUBLE', `CHAR', `VARCHAR', `TEXT', `BLOB', `DATE', `TIME', `DATETIME', `TIMESTAMP', `YEAR', `SET', and `ENUM' types. *Note Column types::. * Very fast joins using an optimized one-sweep multi-join. * Full operator and function support in the `SELECT' and `WHERE' parts of queries. For example: mysql> SELECT CONCAT(first_name, " ", last_name) FROM tbl_name WHERE income/dependents > 10000 AND age > 30; * SQL functions are implemented through a highly optimized class library and should be as fast as possible! Usually there isn't any memory allocation at all after query initialization. * Full support for SQL `GROUP BY' and `ORDER BY' clauses. Support for group functions (`COUNT()', `COUNT(DISTINCT ...)', `AVG()', `STD()', `SUM()', `MAX()' and `MIN()'). * Support for `LEFT OUTER JOIN' and `RIGHT OUTER JOIN' with ANSI SQL and ODBC syntax. * You can mix tables from different databases in the same query (as of Version 3.22). * A privilege and password system that is very flexible and secure, and allows host-based verification. Passwords are secure because all password traffic is encrypted when you connect to a server. * ODBC (Open-DataBase-Connectivity) support for Win32 (with source). All ODBC 2.5 functions and many others. For example, you can use MS Access to connect to your MySQL server. *Note ODBC::. * Very fast B-tree disk tables with index compression. * Up to 32 indexes per table are allowed. Each index may consist of 1 to 16 columns or parts of columns. The maximum index length is 500 bytes (this may be changed when compiling MySQL). An index may use a prefix of a `CHAR' or `VARCHAR' field. * Fixed-length and variable-length records. * In-memory hash tables which are used as temporary tables. * Handles large databases. We are using MySQL with some databases that contain 50,000,000 records and we know of users that uses MySQL with 60,000 tables and about 5,000,000,000 rows * All columns have default values. You can use `INSERT' to insert a subset of a table's columns; those columns that are not explicitly given values are set to their default values. * Uses GNU Automake, Autoconf, and Libtool for portability. * Written in C and C++. Tested with a broad range of different compilers. * A very fast thread-based memory allocation system. * No memory leaks. MySQL has been tested with Purify, a commercial memory leakage detector. * Includes `myisamchk', a very fast utility for table checking, optimization, and repair. All of the functionality of `myisamchk' is also available through the SQL interface as well. *Note MySQL Database Administration::. * Full support for several different character sets, including ISO-8859-1 (Latin1), big5, ujis, and more. For example, the Scandinavian characters `a*', `a"' and `o"' are allowed in table and column names. * All data are saved in the chosen character set. All comparisons for normal string columns are case insensitive. * Sorting is done according to the chosen character set (the Swedish way by default). It is possible to change this when the MySQL server is started up. To see an example of very advanced sorting, look at the Czech sorting code. MySQL supports many different character sets that can be specified at compile and run time. * Aliases on tables and columns are allowed as in the SQL92 standard. * `DELETE', `INSERT', `REPLACE', and `UPDATE' return the number of rows that were changed (affected). It is possible to return the number of rows matched instead by setting a flag when connecting to the server. * Function names do not clash with table or column names. For example, `ABS' is a valid column name. The only restriction is that for a function call, no spaces are allowed between the function name and the `(' that follows it. *Note Reserved words::. * All MySQL programs can be invoked with the `--help' or `-?' options to obtain online assistance. * The server can provide error messages to clients in many languages. *Note Languages::. * Clients may connect to the MySQL server using TCP/IP Sockets, Unix Sockets (Unix), or Named Pipes (NT). * The MySQL-specific `SHOW' command can be used to retrieve information about databases, tables, and indexes. The `EXPLAIN' command can be used to determine how the optimizer resolves a query. How Stable Is MySQL? -------------------- This section addresses the questions "How stable is MySQL?" and "Can I depend on MySQL in this project?" We will try to clarify some issues and to answer some of the more important questions that seem to concern many people. This section has been put together from information gathered from the mailing list (which is very active in reporting bugs). At TcX, MySQL has worked without any problems in our projects since mid-1996. When MySQL was released to a wider public, we noticed that there were some pieces of "untested code" that were quickly found by the new users who made queries in a manner different than our own. Each new release has had fewer portability problems than the previous one (even though each has had many new features). Each release of MySQL has been usable, and there have been problems only when users start to use code from the "gray zones." Naturally, outside users don't know what the gray zones are; this section attempts to indicate those that are currently known. The descriptions deal with Version 3.23 of MySQL. All known and reported bugs are fixed in the latest version, with the exception of the bugs listed in the bugs section, which are things that are design-related. *Note Bugs::. MySQL is written in multiple layers and different independent modules. These modules are listed below with an indication of how well-tested each of them is: *The ISAM table handler -- Stable* This manages storage and retrieval of all data in MySQL Version 3.22 and earlier. In all MySQL releases there hasn't been a single (reported) bug in this code. The only known way to get a corrupted table is to kill the server in the middle of an update. Even that is unlikely to destroy any data beyond rescue, because all data are flushed to disk between each query. There hasn't been a single bug report about lost data because of bugs in MySQL. *The MyISAM table handler -- Stable* This is new in MySQL Version 3.23. It's largely based on the ISAM table code but has a lot of new and very useful features. *The parser and lexical analyser -- Stable* There hasn't been a single reported bug in this system for a long time. *The C client code -- Stable* No known problems. In early Version 3.20 releases, there were some limitations in the send/receive buffer size. As of Version 3.21, the buffer size is now dynamic up to a default of 16M. *Standard client programs -- Stable* These include `mysql', `mysqladmin', `mysqlshow', `mysqldump', and `mysqlimport'. *Basic SQL -- Stable* The basic SQL function system and string classes and dynamic memory handling. Not a single reported bug in this system. *Query optimizer -- Stable* *Range optimizer -- Stable* *Join optimizer -- Stable* *Locking -- Gamma* This is very system-dependent. On some systems there are big problems using standard OS locking (`fcntl()'). In these cases, you should run the MySQL daemon with the `--skip-locking' flag. Problems are known to occur on some Linux systems, and on SunOS when using NFS-mounted file systems. *Linux threads -- Stable* The major problem found has been with the `fcntl()' call, which is fixed by using the `--skip-locking' option to `mysqld'. Some people have reported lockup problems with Version 0.5. LinuxThreads will need to be recompiled if you plan to use 1000+ concurrent connections. Although it is possible to run that many connections with the default LinuxThreads (however, you will never go above 1021), the default stack spacing of 2 MB makes the application unstable, and we have been able to reproduce a coredump after creating 1021 idle connections. *Note Linux::. *Solaris 2.5+ pthreads -- Stable* We use this for all our production work. *MIT-pthreads (Other systems) -- Stable* There have been no reported bugs since Version 3.20.15 and no known bugs since Version 3.20.16. On some systems, there is a "misfeature" where some operations are quite slow (a 1/20 second sleep is done between each query). Of course, MIT-pthreads may slow down everything a bit, but index-based `SELECT' statements are usually done in one time frame so there shouldn't be a mutex locking/thread juggling. *Other thread implementions -- Beta - Gamma* The ports to other systems are still very new and may have bugs, possibly in MySQL, but most often in the thread implementation itself. *`LOAD DATA ...', `INSERT ... SELECT' -- Stable* Some people thought they had found bugs here, but these usually have turned out to be misunderstandings. Please check the manual before reporting problems! *`ALTER TABLE' -- Stable* Small changes in Version 3.22.12. *DBD -- Stable* Now maintained by Jochen Wiedmann (<wiedmann@neckar-alb.de>). Thanks! *`mysqlaccess' -- Stable* Written and maintained by Yves Carlier (<Yves.Carlier@rug.ac.be>). Thanks! *`GRANT' -- Stable* Big changes made in MySQL Version 3.22.12. **MyODBC* (uses ODBC SDK 2.5) -- Gamma* It seems to work well with some programs. *Replication - Beta / Gamma* We are still working on replication, so don't expect this to be rock solid yet. On the other hand, some MySQL users are already using this with good results. *BDB Tables - Beta* The Berkeley DB code is very stable, but we are still improving the interface between MySQL and BDB tables, so it will take some time before this is as tested as the other table types. *InnoDB Tables - Beta* This is a recent addition to `MySQL'. They appear to work good and can be used after some initial testing. *Automatic recovery of MyISAM tables - Beta* This only affects the new code that checks if the table was closed properly on open and executes an automatic check/repair of the table if it wasn't. *MERGE tables - Beta / Gamma* The usage of keys on `MERGE' tables is still not that tested. The other part of the `MERGE' code is quite well tested. *FULLTEXT - Beta* Text search seems to work, but is still not widely used. MySQL AB provides e-mail support for paying customers, but the MySQL mailing list usually provides answers to common questions. Bugs are usually fixed right away with a patch; for serious bugs, there is almost always a new release. How Big Can MySQL Tables Be? ---------------------------- MySQL Version 3.22 has a 4G limit on table size. With the new `MyISAM' in MySQL Version 3.23 the maximum table size is pushed up to 8 million terabytes (2 ^ 63 bytes). Note, however, that operating systems have their own file size limits. Here are some examples: *Operating System* *File Size Limit* Linux-Intel 32 bit 2G, 4G or more, depends on Linux version Linux-Alpha 8T (?) Solaris 2.5.1 2G (possible 4G with patch) Solaris 2.6 4G Solaris 2.7 Intel 4G Solaris 2.7 ULTRA-SPARC 8T (?) On Linux 2.2 you can get bigger tables than 2G by using the LFS patch for the ext2 file system. On Linux 2.4 there exists also patches for ReiserFS to get support for big files. This means that the table size for MySQL is normally limited by the operating system. By default, MySQL tables have a maximum size of about 4G. You can check the maximum table size for a table with the `SHOW TABLE STATUS' command or with the `myisamchk -dv table_name'. *Note SHOW::. If you need bigger tables than 4G (and your operating system supports this), you should set the `AVG_ROW_LENGTH' and `MAX_ROWS' parameter when you create your table. *Note CREATE TABLE::. You can also set these later with `ALTER TABLE'. *Note ALTER TABLE::. If your big table is going to be read-only, you could use `myisampack' to merge and compress many tables to one. `myisampack' usually compresses a table by at least 50%, so you can have, in effect, much bigger tables. *Note `myisampack': myisampack. You can go around the operating system file limit for `MyISAM' data files by using the `RAID' option. *Note CREATE TABLE::. Another solution can be the included MERGE library, which allows you to handle a collection of identical tables as one. *Note MERGE tables: MERGE. Year 2000 Compliance -------------------- MySQL itself has no problems with Year 2000 (Y2K) compliance: * MySQL uses Unix time functions and has no problems with dates until `2069'; all 2-digit years are regarded to be in the range `1970' to `2069', which means that if you store `01' in a `year' column, MySQL treats it as `2001'. * All MySQL date functions are stored in one file `sql/time.cc' and coded very carefully to be year 2000-safe. * In MySQL Version 3.22 and later, the new `YEAR' column type can store years `0' and `1901' to `2155' in 1 byte and display them using 2 or 4 digits. You may run into problems with applications that use MySQL in a way that is not Y2K-safe. For example, many old applications store or manipulate years using 2-digit values (which are ambiguous) rather than 4-digit values. This problem may be compounded by applications that use values such as `00' or `99' as "missing" value indicators. Unfortunately, these problems may be difficult to fix, because different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions. Here is a simple demonstration illustrating that MySQL doesn't have any problems with dates until the year 2030: mysql> DROP TABLE IF EXISTS y2k; Query OK, 0 rows affected (0.01 sec) mysql> CREATE TABLE y2k (date date, date_time datetime, time_stamp timestamp); Query OK, 0 rows affected (0.00 sec) mysql> INSERT INTO y2k VALUES -> ("1998-12-31","1998-12-31 23:59:59",19981231235959), -> ("1999-01-01","1999-01-01 00:00:00",19990101000000), -> ("1999-09-09","1999-09-09 23:59:59",19990909235959), -> ("2000-01-01","2000-01-01 00:00:00",20000101000000), -> ("2000-02-28","2000-02-28 00:00:00",20000228000000), -> ("2000-02-29","2000-02-29 00:00:00",20000229000000), -> ("2000-03-01","2000-03-01 00:00:00",20000301000000), -> ("2000-12-31","2000-12-31 23:59:59",20001231235959), -> ("2001-01-01","2001-01-01 00:00:00",20010101000000), -> ("2004-12-31","2004-12-31 23:59:59",20041231235959), -> ("2005-01-01","2005-01-01 00:00:00",20050101000000), -> ("2030-01-01","2030-01-01 00:00:00",20300101000000), -> ("2050-01-01","2050-01-01 00:00:00",20500101000000); Query OK, 13 rows affected (0.01 sec) Records: 13 Duplicates: 0 Warnings: 0 mysql> SELECT * FROM y2k; +------------+---------------------+----------------+ | date | date_time | time_stamp | +------------+---------------------+----------------+ | 1998-12-31 | 1998-12-31 23:59:59 | 19981231235959 | | 1999-01-01 | 1999-01-01 00:00:00 | 19990101000000 | | 1999-09-09 | 1999-09-09 23:59:59 | 19990909235959 | | 2000-01-01 | 2000-01-01 00:00:00 | 20000101000000 | | 2000-02-28 | 2000-02-28 00:00:00 | 20000228000000 | | 2000-02-29 | 2000-02-29 00:00:00 | 20000229000000 | | 2000-03-01 | 2000-03-01 00:00:00 | 20000301000000 | | 2000-12-31 | 2000-12-31 23:59:59 | 20001231235959 | | 2001-01-01 | 2001-01-01 00:00:00 | 20010101000000 | | 2004-12-31 | 2004-12-31 23:59:59 | 20041231235959 | | 2005-01-01 | 2005-01-01 00:00:00 | 20050101000000 | | 2030-01-01 | 2030-01-01 00:00:00 | 20300101000000 | | 2050-01-01 | 2050-01-01 00:00:00 | 00000000000000 | +------------+---------------------+----------------+ 13 rows in set (0.00 sec) This shows that the `DATE' and `DATETIME' types will not give any problems with future dates (they handle dates until the year 9999). The `TIMESTAMP' type, which is used to store the current time, has a range up to only `2030-01-01'. `TIMESTAMP' has a range of `1970' to `2030' on 32-bit machines (signed value). On 64-bit machines it handles times up to `2106' (unsigned value). Even though MySQL is Y2K-compliant, it is your responsibility to provide unambiguous input. See *Note Y2K issues:: for MySQL's rules for dealing with ambiguous date input data (data containing 2-digit year values). MySQL Information Sources ========================= Books About MySQL ----------------- For the latest book information, with user comments, please visit `http://www.mysql.com/portal/books/html/index.html'. While this manual is still the right place for up to date technical information, its primary goal is to contain everything there is to know about MySQL. It is sometimes nice to have a bound book to read in bed or while you travel. Here is a list of books about MySQL and related subjects (in English). By purchasing a book through these hyperlinks provided herein, you are contributing to the development of MySQL. _MySQL_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=0735709211&bfmtype=book) Publisher New Riders Author Paul DuBois Pub Date 1st Edition December 1999 ISBN 0735709211 Pages 800 Price $49.99 US Downloadable examples `samp_db' distribution (http://www.kitebird.com/mysql-book/) Errata are available here (http://www.kitebird.com/mysql-book/errata.html) Foreword by Michael "Monty" Widenius, MySQL Moderator. In _MySQL_, Paul DuBois provides you with a comprehensive guide to one of the most popular relational database systems. Paul has contributed to the online documentation for MySQL and is an active member of the MySQL community. The principal MySQL developer, Monty Widenius, and a network of his fellow developers reviewed the manuscript, and provided Paul with the kind of insight no one else could supply. Instead of merely giving you a general overview of MySQL, Paul teaches you how to make the most of its capabilities. Through two sample database applications that run throughout the book, he gives you solutions to problems you're sure to face. He helps you integrate MySQL efficiently with third-party tools, such as PHP and Perl, enabling you to generate dynamic Web pages through database queries. He teaches you to write programs that access MySQL databases, and also provides a comprehensive set of references to column types, operators, functions, SQL syntax, MySQL programming, C API, Perl `DBI', and PHP API. _MySQL_ simply gives you the kind of information you won't find anywhere else. If you use MySQL, this book provides you with: * An introduction to MySQL and SQL. * Coverage of MySQL's data types and how to use them. * Thorough treatment of how to write client programs in C. * A guide to using the Perl `DBI' and PHP APIs for developing command-line and Web-based applications. * Tips on administrative issues such as user accounts, backup, crash recovery, and security. * Help in choosing an ISP for MySQL access. * A comprehensive reference for MySQL's data types, operators, functions, and SQL statements and utilities. * Complete reference guides for MySQL's C API, the Perl `DBI' API, and PHP's MySQL-related functions. _MySQL & mSQL_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=1565924347&bfmtype=book) Publisher O'Reilly Authors Randy Jay Yarger, George Reese & Tim King Pub Date 1st Edition July 1999 ISBN 1-56592-434-7, Order Number: 4347 Pages 506 Price $34.95 This book teaches you how to use MySQL and `mSQL', two popular and robust database products that support key subsets of SQL on both Linux and Unix systems. Anyone who knows basic C, Java, Perl, or Python can write a program to interact with a database, either as a stand-alone application or through a Web page. This book takes you through the whole process, from installation and configuration to programming interfaces and basic administration. Includes plenty of tutorial material. _Sams' Teach Yourself MySQL in 21 Days_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=0672319144&bfmtype=book) Publisher Sams Authors Mark Maslakowski and Tony Butcher Pub Date June 2000 ISBN 0672319144 Pages 650 Price $39.99 Sams' _Teach Yourself MySQL in 21 Days_ is for intermediate Linux users who want to move into databases. A large share of the audience is Web developers who need a database to store large amounts of information that can be retrieved via the Web. Sams' _Teach Yourself MySQL in 21 Days_ is a practical, step-by-step tutorial. The reader will learn to design and employ this open source database technology into his or her Web site using practical, hands-on examples to follow. _E-Commerce Solutions with MySQL_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=0761524452&bfmtype=book) Publisher Prima Communications, Inc. Authors N/A Pub Date January 2000 ISBN 0761524452 Pages 500 Price $39.99 No description available. _MySQL and PHP from Scratch_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=0789724405&bfmtype=book) Publisher Que Authors N/A Pub Date September 2000 ISBN 0789724405 Pages 550 Price $34.99 This book puts together information on installing, setting up, and troubleshooting Apache, MySQL, PHP3, and IMP into one complete volume. You also learn how each piece is part of a whole by learning, step-by-step, how to create a web-based e-mail system. Learn to run the equivalent of Active Server Pages (ASP) using PHP3, set up an e-commerce site using a database and the Apache web server, and create a data entry system (such as sales, product quality tracking, customer preferences, etc) that no installation in the PC. _Professional MySQL Programming_ Available Barnes and Noble (http://shop.barnesandnoble.com/bookSearch/isbnInquiry.asp?isbn=1861005164) Publisher Wrox Press, Inc. Authors N/A Pub Date Late 2001 ISBN 1861005164 Pages 1000 Price $49.99 No description available. _Professional Linux Programming_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=1861003013&bfmtype=book) Publisher Wrox Press, Inc. Authors N/A Pub Date September 2000 ISBN 1861003013 Pages 1155 Price $47.99 In this follow-up to the best-selling _Beginning Linux Programming_, you will learn from the authors' real-world knowledge and experience of developing software for Linux; you'll be taken through the development of a sample 'DVD Store' application, with 'theme' chapters addressing different aspects of its implementation. Meanwhile, individual "take-a-break" chapters cover important topics that go beyond the bounds of the central theme. All focus on the practical aspects of programming, showing how crucial it is to choose the right tools for the job, use them as they should be used, and get things right first time. _PHP and MySQL Web Development_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=0672317842&bfmtype=book) Publisher Sams Authors Luke Welling, Laura Thomson Pub Date March 2001 ISBN 0672317842 Pages 700 Price $49.99 _PHP and MySQL Web Development_ introduces you to the advantages of implementing both MySQL and PHP. These advantages are detailed through the provision of both statistics and several case studies. A practical web application is developed throughout the book, providing you with the tools necessary to implement a functional online database. Each function is developed separately, allowing you the choice to incorporate only those parts that you would like to implement. Programming concepts of the PHP language are highlighted, including functions which tie MySQL support into a PHP script and advanced topics regarding table manipulation. *Books recommended by the MySQL Developers* _SQL-99 Complete, Really_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=0879305681&bfmtype=book) Publisher CMP Books Authors Peter Gulutzan, Trudy Pelzer Pub Date April 1999 ISBN 0879305681 Pages 1104 Price $55.96 This book contains complete descriptions of the new standards for syntax, data structures, and retrieval processes of SQL databases. As an example-based reference manual, it includes all of the CLI functions, information, schema tables, and status codes, as well as a working SQL database provided on the companion disk. _C, A reference manual_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=0133262243&bfmtype=book) Publisher Prentice Hall Authors Samuel P. Harbison, Guy L. Steele Pub Date September 1994 ISBN 0133262243 Pages 480 Price $35.99 A new and improved revision of the bestselling C language reference. This manual introduces the notion of "Clean C", writing C code that can be compiled as a C++ program, C programming style that emphasizes correctness, portability, maintainability, and incorporates the ISO C Amendment 1 (1994) which specifies new facilities for writing portable, international programs in C. _C++ for Real Programmers_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=0120499428&bfmtype=book) Publisher Academic Press, Incorporated Authors Jeff Alger, Jim Keogh Pub Date February 1998 ISBN 0120499428 Pages 388 Price $39.95 _C++ For Real Programmers_ bridges the gap between C++ as described in beginner and intermediate-level books and C++ as it is practiced by experts. Numerous valuable techniques are described, organized into three simple themes: indirection, class hierarchies, and memory management. It also provides in-depth coverage of template creation, exception handling, pointers and optimization techniques. The focus of the book is on ANSI C++ and, as such, is compiler independent. _C++ For Real Programmers_ is a revision of _Secrets of the C++ Masters_ and includes a new appendix comparing C++ with Java. The book comes with a 3.5" disk for Windows with source code. _Algorithms in C_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=0201514257&bfmtype=book) Publisher Addison Wesley Longman, Inc. Authors Robert Sedgewick Pub Date April 1990 ISBN 0201514257 Pages 648 Price $45.75 _Algorithms in C_ describes a variety of algorithms in a number of areas of interest, including: sorting, searching, string-processing, and geometric, graph and mathematical algorithms. The book emphasizes fundamental techniques, providing readers with the tools to confidently implement, run, and debug useful algorithms. _Multithreaded Programming with Pthreads_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=0136807291&bfmtype=book) Publisher Prentice Hall Authors Bil Lewis, Daniel J. Berg Pub Date October 1997 ISBN 0136807291 Pages 432 Price $34.95 Based on the best-selling _Threads Primer_, _Multithreaded Programming with Pthreads_ gives you a solid understanding of Posix threads: what they are, how they work, when to use them, and how to optimize them. It retains the clarity and humor of _Threads Primer_, but includes expanded comparisons to Win32 and OS/2 implementations. Code examples tested on all of the major UNIX platforms are featured along with detailed explanations of how and why they use threads. _Programming the PERL DBI: Database Programming with PERL_ Available Barnes and Noble (http://service.bfast.com/bfast/click?bfmid=2181&sourceid=34233559&bfpid=1565926994&bfmtype=book) Publisher O'Reilly & Associates, Incorporated Authors Alligator Descartes, Tim Bunce Pub Date February 2000 ISBN 1565926994 Pages 400 Price $27.96 _Programming the Perl DBI_ is coauthored by Alligator Descartes, one of the most active members of the DBI community, and by Tim Bunce, the inventor of DBI. For the uninitiated, the book explains the architecture of DBI and shows you how to write DBI-based programs. For the experienced DBI dabbler, this book explains DBI's nuances and the peculiarities of each individual DBD. The book includes: * An introduction to DBI and its design. * How to construct queries and bind parameters. * Working with database, driver, and statement handles. * Debugging techniques. * Coverage of each existing DBD. * A complete reference to DBI. General SQL Information and Tutorials ------------------------------------- The following book has been recommended by several people on the MySQL mailing list: Judith S. Bowman, Sandra L. Emerson and Marcy Darnovsky _The Practical SQL Handbook: Using Structured Query Language_ Second Edition Addison-Wesley ISBN 0-201-62623-3 http://www.awl.com The following book has also received some recommendations by MySQL users: Martin Gruber _Understanding SQL_ ISBN 0-89588-644-8 Publisher Sybex 510 523 8233 Alameda, CA USA A SQL tutorial is available on the net at http://w3.one.net/~jhoffman/sqltut.htm Useful MySQL-related Links -------------------------- Apart from the following links, you can find and download a lot of MySQL programs, tools and APIs from the Contrib directory (http://www.mysql.com/Downloads/Contrib/). MySQL Tutorials and Manuals --------------------- MySQL Myths Debunked (http://michael.bacarella.com/research/mysqlmyths.html) MySQL used in the real world. `http://www.4t2.com/mysql' Information about the German MySQL mailing list. `http://www2.rent-a-database.de/mysql/' MySQL handbook in German. `http://www.bitmover.com:8888//home/bk/mysql' Web access to the MySQL BitKeeper repository. `http://www.analysisandsolutions.com/code/mybasic.htm' Beginners MySQL Tutorial on how to install and set up MySQL on a Windows machine. `http://www.devshed.com/Server_Side/MySQL/' A lot of MySQL tutorials. `http://mysql.hitstar.com/' MySQL manual in Chinese. `http://www.linuxplanet.com/linuxplanet/tutorials/1046/1/' Setting Up a MySQL-based Web site. `http://www.hotwired.com/webmonkey/backend/tutorials/tutorial1.html' MySQL-Perl tutorial. `http://www.iserver.com/support/contrib/perl5/modules.html' Installing new Perl modules that require locally installed modules. `http://www.hotwired.com/webmonkey/databases/tutorials/tutorial4.html' PHP/MySQL Tutorial. `http://www.useractive.com/' Hands on tutorial for MySQL. Porting MySQL/Using MySQL on Different Systems ---------------------------------------------- `http://www.entropy.ch/software/macosx/mysql/' Binary of MySQL for Mac OS X Client. Includes information of how to build and use MySQL on Mac OS X. `http://xclave.macnn.com/MySQL/' The Mac OS Xclave. Running MySQL on Mac OS X. `http://www.prnet.de/RegEx/mysql.html' MySQL for Mac OS X Server. `http://www.latencyzero.com/macosx/mysql.html' Building MySQL for Mac OS X. `http://www.essencesw.com/Software/mysqllib.html' New Client libraries for the Mac OS Classic (Macintosh). `http://www.lilback.com/macsql/' Client libraries for Mac OS Classic (Macintosh). `http://sixk.maniasys.com/index_en.html' MySQL for Amiga Perl-related Links ------------------ `http://dbimysql.photoflux.com/' Perl DBI with MySQL FAQ. MySQL Discussion Forums ----------------------- `http://www.weberdev.com/' Examples using MySQL; (check Top 20) `http://futurerealm.com/forum/futureforum.htm' FutureForum Web Discussion Software. Commercial Applications that Support MySQL ------------------------------------------ `http://www.supportwizard.com/' SupportWizard; Interactive helpdesk on the Web (This product includes a licensed copy of MySQL.) `http://www.sonork.com/' Sonork, Instant Messenger that is not only Internet oriented. It's focused on private networks and on small to medium companies. Client is free, server is free for up to 5 seats. `http://www.stweb.org/' StWeb - Stratos Web and Application server - An easy-to-use, cross platform, Internet/Intranet development and deployment system for development of web-enabled applications. The standard version of StWeb has a native interface to MySQL database. `http://www.rightnowtech.com/' Right Now Web; Web automation for customer service. `http://www.icaap.org/Bazaar/' Bazaar; Interactive Discussion Forums with Web interface. `http://www.phonesweep.com/' PhoneSweepT is the world's first commercial Telephone Scanner. Many break-ins in recent years have come not through the Internet, but through unauthorized dial-up modems. PhoneSweep lets you find these modems by repeatedly placing phone calls to every phone number that your organization controls. PhoneSweep has a built-in expert system that can recognize more than 250 different kinds of remote-access programs, including Carbon Copy(TM), pcANYWHERE(TM), and Windows NT RAS. All information is stored in the SQL database. It then generates a comprehensive report detailing which services were discovered on which dial-up numbers in your organization. SQL Clients and Report Writers ------------------------------ urSQL (http://www.urbanresearch.com/software/utils/urbsql/index.html) SQL Editor and Query Utility. Custom syntax highlighting, editable results grid, exportable result-sets, basic MySQL admin functions, Etc.. For Windows. MySQL Data Manager (http://www.edatanew.com/) MySQL Data Manager * is platform independent web client (written in perl) for MySQL server over TCP/IP. `http://ksql.sourceforge.net/' KDE MySQL client. `http://www.ecker-software.de' A Windows GUI client by David Ecker. `http://www.icaap.org/software/kiosk/' Kiosk; a MySQL client for database management. Written in Perl. Will be a part of Bazaar. `http://www.casestudio.com/' Db design tool that supports MySQL 3.23. `http://home.skif.net/~voland/zeos/eng/index.html' Zeos - A client that supports MySQL, Interbase and PostgreSQL. `http://www.geocities.com/SiliconValley/Ridge/4280/GenericReportWriter/grwhome.html' A free report writer in Java `http://www.javaframework.de' MySQLExport - Export of MySQL create statements and data in a lot of different formats (SQL, HTML, CVS, text, ZIP, GZIP...) `http://dlabs.4t2.com' M2D, a MySQL Administration client for Windows. M2D supports administration of MySQL databases, creation of new databases and tables, editing, and more. `http://dlabs.4t2.com' Dexter, a small server written in Perl which can be used as a proxy server for MySQL or as a database extender. `http://www.scibit.com/Products/Software/Utils/Mascon.asp' Mascon is a powerful Win32 GUI for administering MySQL databases. `http://www.rtlabs.com/' MacSQL Monitor. GUI for MySQL, ODBC, and JDBC databases for the Mac OS. Distributions that Include MySQL -------------------------------- `http://www.suse.com/' SuSE Linux (6.1 and above) `http://www.redhat.com/' RedHat Linux (7.0 and above) `http://distro.conectiva.com.br' Conectiva Linux (4.0 and above) Web Development Tools that Support MySQL ---------------------------------------- `http://www.php.net/' PHP: A server-side HTML-embedded scripting language. `http://www.midgard-project.org' The Midgard Application Server; a powerful Web development environment based on MySQL and PHP. `http://www.smartworker.org' SmartWorker is a platform for Web application development. `http://xsp.lentus.se/' XSP: e(X)tendible (S)erver (P)ages and is a HTML embedded tag language written in Java (previously known as XTAGS.) `http://www.dbServ.de/' dbServ is an extension to a web server to integrate database output into your HTML code. You may use any HTML function in your output. Only the client will stop you. It works as standalone server or as Java servlet. `http://www.chilisoft.com/' Platform independent ASP from Chili!Soft `http://www.voicenet.com/~zellert/tjFM' A JDBC driver for MySQL. `http://www.wernhart.priv.at/php/' MySQL + PHP demos. `http://www.dbwww.com/' ForwardSQL: HTML interface to manipulate MySQL databases. `http://www.daa.com.au/~james/www-sql/' WWW-SQL: Display database information. `http://www.minivend.com/minivend/' Minivend: A Web shopping cart. `http://www.heitml.com/' HeiTML: A server-side extension of HTML and a 4GL language at the same time. `http://www.metahtml.com/' Metahtml: A Dynamic Programming Language for WWW Applications. `http://www.binevolve.com/' VelocityGen for Perl and Tcl. `http://hawkeye.net/' Hawkeye Internet Server Suite. `http://www.fastflow.com/' Network Database Connection For Linux `http://www.wdbi.net/' WDBI: Web browser as a universal front end to databases which supports MySQL well. `http://www.webgroove.com/' WebGroove Script: HTML compiler and server-side scripting language. `http://www.ihtml.com/' A server-side Web site scripting language. `ftp://ftp.igc.apc.org/pub/myodbc/README' How to use MySQL with ColdFusion on Solaris. `http://calistra.com/MySQL/' Calistra's ODBC MySQL Administrator. `http://www.webmerger.com' Webmerger - This CGI tool interprets files and generates dynamic output based on a set of simple tags. Ready-to-run drivers for MySQL and PostgreSQL through ODBC. `http://phpclub.net/' PHPclub - Tips and tricks for PHP. `http://www.penguinservices.com/scripts' MySQL and Perl Scripts. `http://www.widgetchuck.com' The Widgetchuck; Web Site Tools and Gadgets `http://www.adcycle.com/' AdCycle - advertising management software. `http://sourceforge.net/projects/pwpage/' pwPage - provides an extremely fast and simple approach to the creation of database forms. That is, if a database table exists and an HTML page has been constructed using a few simple guidelines, pwPage can be immediately used for table data selections, insertions, updates, deletions and selectable table content reviewing. `http://www.omnis-software.com/products/studio/studio.html' OMNIS Studio is a rapid application development (RAD) tool. `http://www.webplus.com' talentsoft Web+ 4.6 - a powerful and comprehensive development language for use in creating web-based client/server applications without writing complicated, low-level, and time-consuming CGI programs. Database Design Tools with MySQL Support ---------------------------------------- `http://www.mysql.com/documentation/dezign/' "DeZign for databases" is a database development tool that uses an entity relationship diagram (ERD). Web Servers with MySQL Tools ---------------------------- `ftp://ftp.kcilink.com/pub/' mod_auth_mysql, An Apache authentication module. `http://www.roxen.com/' The Roxen Challenger Web server. Extensions for Other Programs ----------------------------- `http://www.seawood.org/msql_bind/' MySQL support for BIND (The Internet Domain Name Server). `http://www.inet-interactive.com/sendmail/' MySQL support for Sendmail and Procmail. Using MySQL with Other Programs ------------------------------- `http://www.iserver.com/support/addonhelp/database/mysql/msaccess.html' Using MySQL with Access. `http://www.iserver.com/support/contrib/perl5/modules.html' Installing new Perl modules that require locally installed modules. ODBC-related Links ------------------ `http://www.iodbc.org/' Popular iODBC Driver Manager (libiodbc) now available as Open Source. `http://users.ids.net/~bjepson/freeODBC/' The FreeODBC Pages. `http://genix.net/unixODBC/' The unixODBC Project goals are to develop and promote unixODBC to be the definitive standard for ODBC on the Linux platform. This is to include GUI support for KDE. `http://www.sw-soft.com/products/BtrieveODBC/' A MySQL-based ODBC driver for Btrieve. *API*-related Links ------------------- `http://www.jppp.com/' Partially implemented TDataset-compatible components for MySQL. `http://www.riverstyx.net/qpopmysql/' qpopmysql - A patch to allow POP3 authentication from a MySQL database. There's also a link to Paul Khavkine's patch for Procmail to allow any MTA to deliver to users in a MySQL database. `http://www.pbc.ottawa.on.ca' Visual Basic class generator for Active X. `http://www.essencesw.com/Software/mysqllib.html' New Client libraries for the Mac OS Classic (Macintosh). `http://www.lilback.com/macsql/' Client libraries for the Macintosh. `http://www.essencesw.com/Plugins/mysqlplug.html' Plugin for REALbasic (for Macintosh) `http://www.iis.ee.ethz.ch/~neeri/macintosh/gusi-qa.html' A library that emulates BSD sockets and pthreads on Macintosh. This can be used if you want to compile the MySQL client library on Mac. It could probably even be sued to port MySQL to Macintosh, but we don't know of anyone that has tried that. `http://www.dedecker.net/jessie/scmdb/' SCMDB - an add-on for SCM that ports the MySQL C library to scheme (SCM). With this library scheme developers can make connections to a MySQL database and use embedded SQL in their programs. Other MySQL-related Links ------------------------- SAT (http://www.satisoft.com/) The Small Application Toolkit (SAT) is a collection of utilities intended to simplify the development of small, multi-user, GUI based applications in a (Microsoft -or- X) Windows Client / Unix Server environment. `http://www.wix.com/mysql-hosting/' Registry of Web providers who support MySQL. `http://www.softagency.co.jp/mysql/index.en.html' Links about using MySQL in Japan/Asia. `http://abattoir.cc.ndsu.nodak.edu/~nem/mysql/udf/' MySQL UDF Registry. `http://www.open.com.au/products.html' Commercial Web defect tracking system. `http://www.stonekeep.com/pts/' PTS: Project Tracking System. `http://tomato.nvgc.vt.edu/~hroberts/mot' Job and software tracking system. `http://www.cynergi.net/exportsql/' ExportSQL: A script to export data from Access95+. `http://SAL.KachinaTech.COM/H/1/MYSQL.html' SAL (Scientific Applications on Linux) MySQL entry. `http://www.infotech-nj.com/itech/index.shtml' A consulting company which mentions MySQL in the right company. `http://www.pmpcs.com/' PMP Computer Solutions. Database developers using MySQL and `mSQL'. `http://www.aewa.org/' Airborne Early Warning Association. `http://www.dedserius.com/y2kmatrix/' Y2K tester. SQL and Database Interfaces --------------------------- `http://java.sun.com/products/jdbc/' The JDBC database access API. `http://www.gagme.com/mysql' Patch for `mSQL' Tcl. `http://www.amsoft.ru/easysql/' EasySQL: An ODBC-like driver manager. `http://www.lightlink.com/hessling/rexxsql.html' A REXX interface to SQL databases. `http://www.mytcl.cx/' Tcl interface based on tcl-sql with many bugfixes. `http://www.binevolve.com/~tdarugar/tcl-sql/' Tcl interface. `http://www.contrib.andrew.cmu.edu/~shadow/sql.html' SQL Reference Page with a lot of interesting links. Examples of MySQL Use --------------------- `http://www.little6.com/about/linux/' Little6 Inc., An online contract and job finding site that is powered by MySQL, PHP3, and Linux. `http://www.delec.com/is/products/prep/examples/BookShelf/index.html' DELECis - A tool that makes it very easy to create an automatically generated table documentation. They have used MySQL as an example. `http://www.worldrecords.com' World Records - A search engine for information about music that uses MySQL and PHP. `http://www.webtechniques.com/archives/1998/01/note/' A Contact Database using MySQL and PHP. `http://modems.rosenet.net/mysql/' Web based interface and Community Calendar with PHP. `http://www.odbsoft.com/cook/sources.htm' Perl package to generate html from a SQL table structure and for generating SQL statements from an html form. `http://www.gusnet.cx/proj/telsql/' Basic telephone database using `DBI'/`DBD'. `http://tecfa.unige.ch/guides/java/staf2x/ex/jdbc/coffee-break' JDBC examples by Daniel K. Schneider. `http://www.spade.com/linux/howto/PostgreSQL-HOWTO-41.html' SQL BNF `http://www.ooc.com/' Object Oriented Concepts Inc; CORBA applications with examples in source. `http://www.pbc.ottawa.on.ca/' DBWiz; Includes an example of how to manage cursors in VB. `http://keilor.cs.umass.edu/pluribus/' Pluribus is a free search engine that learns to improve the quality of its results over time. Pluribus works by recording which pages a user prefers among those returned for a query. A user votes for a page by selecting it; Pluribus then uses that knowledge to improve the quality of the results when someone else submits the same (or similar) query. Uses PHP and MySQL. `http://www.stopbit.com/' Stopbit - A technology news site using MySQL and PHP. `http://www.linuxsupportline.com/~kalendar/' KDE based calendar manager - The calendar manager has both single user (file based) and multi-user (MySQL database) support. `http://tim.desert.net/~tim/imger/' Example of storing/retrieving images with MySQL and CGI. `http://www.penguinservices.com/scripts' Online shopping cart system. `http://www.city-gallery.com/album/' Old Photo Album - The album is a collaborative popular history of photography project that generates all pages from data stored in a MySQL database. Pages are dynamically generated through a php3 interface to the database content. Users contribute images and descriptions. Contributed images are stored on the web server to avoid storing them in the database as BLOBs. All other information is stored on the shared MySQL server. General Database Links ---------------------- `http://www.pcslink.com/~ej/dbweb.html' Database Jump Site `http://black.hole-in-the.net/guy/webdb/' Homepage of the webdb-l (Web Databases) mailing list. `http://www.symbolstone.org/technology/perl/DBI/index.html' Perl `DBI'/`DBD' modules homepage. `http://www.student.uni-koeln.de/cygwin/' Cygwin tools. Unix on top of Windows. `http://dbasecentral.com/' dbasecentral.com; Development and distribution of powerful and easy-to-use database applications and systems. `http://www.tek-tips.com/' Tek-Tips Forums are 800+ independent peer-to-peer non-commercial support forums for Computer Professionals. Features include automatic e-mail notification of responses, a links library, and member confidentiality guaranteed. `http://www.public.asu.edu/~peterjn/btree/' B-Trees: Balanced Tree Data Structures. `http://www.fit.qut.edu.au/~maire/baobab/lecture/sld001.htm' A lecture about B-Trees. There are also many Web pages that use MySQL. *Note Users::. Send any additions to this list to <webmaster@mysql.com>. We now require that you show a MySQL logo somewhere if you wish your site to be added. It is okay to have it on a "used tools" page or something similar. MySQL Mailing Lists ------------------- This section introduces you to the MySQL mailing lists, and gives some guidelines as to how to use them. The MySQL Mailing Lists ....................... To subscribe to the main MySQL mailing list, send a message to the electronic mail address <mysql-subscribe@lists.mysql.com>. To unsubscribe from the main MySQL mailing list, send a message to the electronic mail address <mysql-unsubscribe@lists.mysql.com>. Only the address to which you send your messages is significant. The subject line and the body of the message are ignored. If your reply address is not valid, you can specify your address explicitly. Adding a hyphen to the subscribe or unsubscribe command word, followed by your address with the `@' character in your address replaced by a `='. For example, to subscribe `your_name@host.domain', send a message to `mysql-subscribe-your_name=host.domain@lists.mysql.com'. Mail to <mysql-subscribe@lists.mysql.com> or <mysql-unsubscribe@lists.mysql.com> is handled automatically by the ezmlm mailing list processor. Information about ezmlm is available at The ezmlm Website (http://www.ezmlm.org). To post a message to the list itself, send your message to `mysql@lists.mysql.com'. However, please _do not_ send mail about subscribing or unsubscribing to <mysql@lists.mysql.com>, because any mail sent to that address is distributed automatically to thousands of other users. Your local site may have many subscribers to <mysql@lists.mysql.com>. If so, it may have a local mailing list, so that messages sent from `lists.mysql.com' to your site are propagated to the local list. In such cases, please contact your system administrator to be added to or dropped from the local MySQL list. If you wish to have traffic for a mailing list go to a separate mailbox in your mail program, set up a filter based on the message headers. You can use either the `List-ID:' or `Delivered-To:' headers to identify list messages. The following MySQL mailing lists exist: `<announce-subscribe@lists.mysql.com> announce' This is for announcement of new versions of MySQL and related programs. This is a low volume list all MySQL users should subscribe to. `<mysql-subscribe@lists.mysql.com> mysql' The main list for general MySQL discussion. Please note that some topics are better discussed on the more-specialized lists. If you post to the wrong list, you may not get an answer! `<mysql-digest-subscribe@lists.mysql.com> mysql-digest' The `mysql' list in digest form. That means you get all individual messages, sent as one large mail message once a day. `<bugs-subscribe@lists.mysql.com> bugs' On this list you should only post a full, repeatable bug report using the `mysqlbug' script (if you are running on Windows, you should include a description of the operating system and the MySQL version). Preferably, you should test the problem using the latest stable or development version of MySQL before posting! Anyone should be able to repeat the bug by just using `mysql test < script' on the included test case. All bugs posted on this list will be corrected or documented in the next MySQL release! If there are only small code changes involved, we will also post a patch that fixes the problem. `<bugs-digest-subscribe@lists.mysql.com> bugs-digest' The `bugs' list in digest form. `<internals-subscribe@lists.mysql.com> internals' A list for people who work on the MySQL code. On this list one can also discuss MySQL development and post patches. `<internals-digest-subscribe@lists.mysql.com> internals-digest' A digest version of the `internals' list. `<java-subscribe@lists.mysql.com> java' Discussion about MySQL and Java. Mostly about the JDBC drivers. `<java-digest-subscribe@lists.mysql.com> java-digest' A digest version of the `java' list. `<win32-subscribe@lists.mysql.com> win32' All things concerning MySQL on Microsoft operating systems such as Win95, Win98, NT, and Win2000. `<win32-digest-subscribe@lists.mysql.com> win32-digest' A digest version of the `win32' list. `<myodbc-subscribe@lists.mysql.com> myodbc' All things about connecting to MySQL with ODBC. `<myodbc-digest-subscribe@lists.mysql.com> myodbc-digest' A digest version of the `myodbc' list. `<plusplus-subscribe@lists.mysql.com> plusplus' All things concerning programming with the C++ API to MySQL. `<plusplus-digest-subscribe@lists.mysql.com> plusplus-digest' A digest version of the `plusplus' list. `<msql-mysql-modules-subscribe@lists.mysql.com> msql-mysql-modules' A list about the Perl support in MySQL. msql-mysql-modules `<msql-mysql-modules-digest-subscribe@lists.mysql.com> msql-mysql-modules-digest' A digest version of the `msql-mysql-modules' list. You subscribe or unsubscribe to all lists in the same way as described above. In your subscribe or unsubscribe message, just put the appropriate mailing list name rather than `mysql'. For example, to subscribe to or unsubscribe from the `myodbc' list, send a message to <myodbc-subscribe@lists.mysql.com> or <myodbc-unsubscribe@lists.mysql.com>. If you can't get an answer for your questions from the mailing list, one option is to pay for support from MySQL AB, which will put you in direct contact with MySQL developers. *Note Support::. The following table shows some MySQL mailing in other languages than English. Note that these are not operated by MySQL AB, so we can't guarantee the quality on these. `<mysql-france-subscribe@yahoogroups.com> A French mailing list' `<list@tinc.net> A Korean mailing list' Email `subscribe mysql your@email.address' to this list. `<mysql-de-request@lists.4t2.com> A German mailing list' Email `subscribe mysql-de your@email.address' to this list. You can find information about this mailing list at `http://www.4t2.com/mysql'. `<mysql-br-request@listas.linkway.com.br> A Portugese mailing list' Email `subscribe mysql-br your@email.address' to this list. `<mysql-alta@elistas.net> A Spanish mailing list' Email `subscribe mysql your@email.address' to this list. Asking Questions or Reporting Bugs .................................. Before posting a bug report or question, please do the following: * Start by searching the MySQL online manual at: `http://www.mysql.com/documentation/manual.php' We try to keep the manual up to date by updating it frequently with solutions to newly found problems! * Search the MySQL mailing list archives: `http://www.mysql.com/documentation/' * You can also use `http://www.mysql.com/search.html' to search all the Web pages (including the manual) that are located at `http://www.mysql.com/'. If you can't find an answer in the manual or the archives, check with your local MySQL expert. If you still can't find an answer to your question, go ahead and read the next section about how to send mail to <mysql@lists.mysql.com>. How to Report Bugs or Problems .............................. Writing a good bug report takes patience, but doing it right the first time saves time for us and for you. A good bug report containing a full test case for the bug will make it very likely that we will fix it in the next release. This section will help you write your report correctly so that you don't waste your time doing things that may not help us much or at all. We encourage everyone to use the `mysqlbug' script to generate a bug report (or a report about any problem), if possible. `mysqlbug' can be found in the `scripts' directory in the source distribution, or, for a binary distribution, in the `bin' directory under your MySQL installation directory. If you are unable to use `mysqlbug', you should still include all the necessary information listed in this section. The `mysqlbug' script helps you generate a report by determining much of the following information automatically, but if something important is missing, please include it with your message! Please read this section carefully and make sure that all the information described here is included in your report. The normal place to report bugs and problems is <mysql@lists.mysql.com>. If you can make a test case that clearly demonstrates the bug, you should post it to the <bugs@lists.mysql.com> list. Note that on this list you should only post a full, repeatable bug report using the `mysqlbug' script. If you are running on Windows, you should include a description of the operating system and the MySQL version. Preferably, you should test the problem using the latest stable or development version of MySQL before posting! Anyone should be able to repeat the bug by just using "`mysql test < script'" on the included test case or run the shell or perl script that is included in the bug report. All bugs posted on the `bugs' list will be corrected or documented in the next MySQL release! If there are only small code changes involved to correct this problem, we will also post a patch that fixes the problem. Remember that it is possible to respond to a message containing too much information, but not to one containing too little. Often people omit facts because they think they know the cause of a problem and assume that some details don't matter. A good principle is: if you are in doubt about stating something, state it! It is a thousand times faster and less troublesome to write a couple of lines more in your report than to be forced to ask again and wait for the answer because you didn't include enough information the first time. The most common errors are that people don't indicate the version number of the MySQL distribution they are using, or don't indicate what platform they have MySQL installed on (including the platform version number). This is highly relevant information, and in 99 cases out of 100 the bug report is useless without it! Very often we get questions like, "Why doesn't this work for me?" then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has been fixed already in newer MySQL versions. Sometimes the error is platform dependent; in such cases, it is next to impossible to fix anything without knowing the operating system and the version number of the platform. Remember also to provide information about your compiler, if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL-related. Most compilers are under development all the time and become better version by version. To determine whether or not your problem depends on your compiler, we need to know what compiler is used. Note that every compiling problem should be regarded as a bug report and reported accordingly. It is most helpful when a good description of the problem is included in the bug report. That is, a good example of all the things you did that led to the problem and the problem itself exactly described. The best reports are those that include a full example showing how to reproduce the bug or problem. *Note Reproduceable test case::. If a program produces an error message, it is very important to include the message in your report! If we try to search for something from the archives using programs, it is better that the error message reported exactly matches the one that the program produces. (Even the case should be observed!) You should never try to remember what the error message was; instead, copy and paste the entire message into your report! If you have a problem with MyODBC, you should try to generate a MyODBC trace file. *Note MyODBC bug report::. Please remember that many of the people who will read your report will do so using an 80-column display. When generating reports or examples using the `mysql' command line tool, you should therefore use the `--vertical' option (or the `\G' statement terminator) for output that would exceed the available width for such a display (for example, with the `EXPLAIN SELECT' statement; see the example below). Please include the following information in your report: * The version number of the MySQL distribution you are using (for example, MySQL Version 3.22.22). You can find out which version you are running by executing `mysqladmin version'. `mysqladmin' can be found in the `bin' directory under your MySQL installation directory. * The manufacturer and model of the machine you are working on. * The operating system name and version. For most operating systems, you can get this information by executing the Unix command `uname -a'. * Sometimes the amount of memory (real and virtual) is relevant. If in doubt, include these values. * If you are using a source distribution of MySQL, the name and version number of the compiler used is needed. If you have a binary distribution, the distribution name is needed. * If the problem occurs during compilation, include the exact error message(s) and also a few lines of context around the offending code in the file where the error occurred. * If `mysqld' died, you should also report the query that crashed `mysqld'. You can usually find this out by running `mysqld' with logging enabled. *Note Using log files::. * If any database table is related to the problem, include the output from `mysqldump --no-data db_name tbl_name1 tbl_name2 ...'. This is very easy to do and is a powerful way to get information about any table in a database that will help us create a situation matching the one you have. * For speed-related bugs or problems with `SELECT' statements, you should always include the output of `EXPLAIN SELECT ...', and at least the number of rows that the `SELECT' statement produces. The more information you give about your situation, the more likely it is that someone can help you! For example, the following is an example of a very good bug report (it should of course be posted with the `mysqlbug' script): Example run using the `mysql' command line tool (note the use of the `\G' statement terminator for statements whose output width would otherwise exceed that of an 80-column display device): mysql> SHOW VARIABLES; mysql> SHOW COLUMNS FROM ...\G <output from SHOW COLUMNS> mysql> EXPLAIN SELECT ...\G <output from EXPLAIN> mysql> FLUSH STATUS; mysql> SELECT ...; <A short version of the output from SELECT, including the time taken to run the query> mysql> SHOW STATUS; <output from SHOW STATUS> * If a bug or problem occurs while running *mysqld*, try to provide an input script that will reproduce the anomaly. This script should include any necessary source files. The more closely the script can reproduce your situation, the better. If you can make a repeatable test case, you should post this to <bugs@lists.mysql.com> for a high priority treatment! If you can't provide a script, you should at least include the output from `mysqladmin variables extended-status processlist' in your mail to provide some information of how your system is performing! * If you can't produce a test case in a few rows, or if the test table is too big to be mailed to the mailing list (more than 10 rows), you should dump your tables using `mysqldump' and create a `README' file that describes your problem. Create a compressed archive of your files using `tar' and `gzip' or `zip', and use `ftp' to transfer the archive to `ftp://support.mysql.com/pub/mysql/secret/'. Then send a short description of the problem to <bugs@lists.mysql.com>. * If you think that MySQL produces a strange result from a query, include not only the result, but also your opinion of what the result should be, and an account describing the basis for your opinion. * When giving an example of the problem, it's better to use the variable names, table names, etc., that exist in your actual situation than to come up with new names. The problem could be related to the name of a variable or table! These cases are rare, perhaps, but it is better to be safe than sorry. After all, it should be easier for you to provide an example that uses your actual situation, and it is by all means better for us. In case you have data you don't want to show to others, you can use `ftp' to transfer it to `ftp://support.mysql.com/pub/mysql/secret/'. If the data are really top secret and you don't want to show them even to us, then go ahead and provide an example using other names, but please regard this as the last choice. * Include all the options given to the relevant programs, if possible. For example, indicate the options that you use when you start the `mysqld' daemon and that you use to run any MySQL client programs. The options to programs like `mysqld' and `mysql', and to the `configure' script, are often keys to answers and are very relevant! It is never a bad idea to include them anyway! If you use any modules, such as Perl or PHP, please include the version number(s) of those as well. * If your question is related to the privilege system, please include the output of `mysqlaccess', the output of `mysqladmin reload', and all the error messages you get when trying to connect! When you test your privileges, you should first run `mysqlaccess'. After this, execute `mysqladmin reload version' and try to connect with the program that gives you trouble. `mysqlaccess' can be found in the `bin' directory under your MySQL installation directory. * If you have a patch for a bug, that is good, but don't assume the patch is all we need, or that we will use it, if you don't provide some necessary information, such as test cases showing the bug that your patch fixes. We might find problems with your patch or we might not understand it at all; if so, we can't use it. If we can't verify exactly what the patch is meant for, we won't use it. Test cases will help us here. Show that the patch will handle all the situations that may occur. If we find a borderline case (even a rare one) where the patch won't work, it may be useless. * Guesses about what the bug is, why it occurs, or what it depends on, are usually wrong. Even the MySQL team can't guess such things without first using a debugger to determine the real cause of a bug. * Indicate in your mail message that you have checked the reference manual and mail archive so others know that you have tried to solve the problem yourself. * If you get a `parse error', please check your syntax closely! If you can't find something wrong with it, it's extremely likely that your current version of MySQL doesn't support the query you are using. If you are using the current version and the manual at `http://www.mysql.com/documentation/manual.php' doesn't cover the syntax you are using, MySQL doesn't support your query. In this case, your only options are to implement the syntax yourself or e-mail <mysql-licensing@mysql.com> and ask for an offer to implement it! If the manual covers the syntax you are using, but you have an older version of MySQL, you should check the MySQL change history to see when the syntax was implemented. In this case, you have the option of upgrading to a newer version of MySQL. *Note News::. * If you have a problem such that your data appears corrupt or you get errors when you access some particular table, you should first check and then try repairing your tables with `myisamchk' or `CHECK TABLE' and `REPAIR TABLE'. *Note MySQL Database Administration::. * If you often get corrupted tables you should try to find out when and why this happens! In this case, the `mysql-data-directory/'hostname'.err' file may contain some information about what happened. *Note Error log::. Please include any relevant information from this file in your bug report! Normally `mysqld' should *NEVER* crash a table if nothing killed it in the middle of an update! If you can find the cause of `mysqld' dying, it's much easier for us to provide you with a fix for the problem! *Note What is crashing::. * If possible, download and install the most recent version of MySQL and check whether or not it solves your problem. All versions of MySQL are thoroughly tested and should work without problems! We believe in making everything as backward compatible as possible, and you should be able to switch MySQL versions in minutes! *Note Which version::. If you are a support customer, please cross-post the bug report to <mysql-support@mysql.com> for higher priority treatment, as well as to the appropriate mailing list to see if someone else has experienced (and perhaps solved) the problem. For information on reporting bugs in *MyODBC*, see *Note ODBC Problems::. For solutions to some common problems, see *Note Problems::. When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarize the answers and send the summary to the mailing list so that others may have the benefit of responses you received that helped you solve your problem! Guidelines for Answering Question on the Mailing List ..................................................... If you consider your answer to have broad interest, you may want to post it to the mailing list instead of replying directly to the individual who asked. Try to make your answer general enough that people other than the original poster may benefit from it. When you post to the list, please make sure that your answer is not a duplication of a previous answer. Try to summarize the essential part of the question in your reply; don't feel obliged to quote the entire original message. Please don't post mail messages from your browser with HTML mode turned on! Many users don't read mail with a browser! MySQL Licensing and Support =========================== This section describes MySQL support and licensing arrangements: * The copyrights under which MySQL is distributed (*note Copyright::) * Sample situations illustrating when a license is required (*note Licensing examples::) * Support costs (*note Cost::) and support benefits (*note Support::) * Commercial licensing costs MySQL Licensing Policy ---------------------- The formal terms of the GPL license can be found at *Note GPL license::. Basically, our licensing policy and interpretation of the GPL is as follows: Note that older versions of MySQL are still using a more strict license (http://www.mysql.com/support/arrangements/mypl.html). See the documentation for that version for more information. If you need a commercial MySQL license, because the GPL license doesn't suit your application, you can buy one at `https://order.mysql.com/'. For normal internal use, MySQL costs nothing. You do not have to pay us if you do not want to. A license is required if: - You link a program, that is not free software, with code from the MySQL server or clients that has the GPL copyright. This happens for example when you use MySQL as an embedded server in your applications or when you add not free extensions to the MySQL server. In this case your application/code would also become GPL through the GPL license that acts as a virus. By licensing MySQL server from MySQL AB under a commercial license you will avoid this problem. See `http://www.gnu.org/copyleft/gpl-faq.html'. - You have a commercial application that ONLY works with MySQL and ships the application with the MySQL server. This is because we view this as linking even if it is done over the network. - You have a distribution of MySQL and you don't provide the source code for your copy of the MySQL server, as defined in the GPL license. A license is *NOT* required if: - You do not need a license to include the client code in commercial programs. The client part of MySQL licensed with the LGPL `GNU Library General Public License'. The `mysql' command-line client includes code from the `readline' library that is under the `GPL'. - If your use of MySQL does not require a license, but you like MySQL and want to encourage further development, you are certainly welcome to purchase a license or MySQL support anyway. - If you use MySQL in a commercial context such that you profit by its use, we ask that you further the development of MySQL by purchasing some level of support. We feel that if MySQL helps your business, it is reasonable to ask that you help MySQL. (Otherwise, if you ask us support questions, you are not only using for free something into which we've put a lot a work, you're asking us to provide free support, too.) For circumstances under which a MySQL license is required, you need a license per machine that runs the `mysqld' server. However, a multiple-CPU machine counts as a single machine, and there is no restriction on the number of MySQL servers that run on one machine, or on the number of clients concurrently connected to a server running on that machine! If you have any questions as to whether or not a license is required for your particular use of MySQL, please read this again and then contact us. *Note Contact information::. If you require a MySQL license, the easiest way to pay for it is to use the license form on MySQL's secure server at `https://order.mysql.com/'. Other forms of payment are discussed in *Note Payment information::. Copyrights Used by MySQL ------------------------ There are several different copyrights on the MySQL distribution: 1. The MySQL-specific source needed to build the `mysqlclient' library is licensed under the `LGPL' and programs in the `client' directory is GPL. Each file has a header that shows which copyright is used for that file. 2. The client library and the (GNU `getopt') library are covered by the "GNU LIBRARY GENERAL PUBLIC LICENSE." *Note LGPL license::. 3. Some parts of the source (the `regexp' library) are covered by a Berkeley-style copyright. 4. All the source in the server and the (GNU `readline') library is covered by the "GNU GENERAL PUBLIC LICENSE." *Note GPL license::. This is also available as the file `COPYING' in the distributions. One goal is that the SQL client library should be free enough that it is possible to add MySQL support into commercial products without a license. For this reason, we chose the LGPL license for the client code. This means that you can use MySQL for free with any program that uses any of the free software licenses. MySQL is also free for any end user for his own or company usage. However, if you use MySQL for something important to you, you may want to help secure its development by purchasing licenses or a support contract. *Note Support::. Copyright Changes ................. Version 3.22 of MySQL is still using a more strict license. See the documentation for that version for more information. Example Licensing Situations ---------------------------- This section describes some situations illustrating whether or not you must license the MySQL server. Generally these examples involve providing MySQL as an integral part of a product. Note that a single MySQL license covers any number of CPUs and `mysqld' servers on a machine! There is no artificial limit on the number of clients that connect to the server in any way. Selling Products that use MySQL ............................... To determine whether or not you need a MySQL license when selling your application, you should ask whether the proper functioning of your application is dependent on the use of MySQL and whether you include the MySQL server with your product. There are several cases to consider: * Does your application require MySQL to function properly? * If your product requires MySQL, you need a license for any machine that runs the `mysqld' server. For example, if you've designed your application around MySQL, then you've really made a commercial product that requires the engine, so you need a license. * If your application does not require MySQL, you do not need to obtain a license. For example, if using MySQL just adds some new optional features to your product (such as adding logging to a database if MySQL is used rather than logging to a text file), it should fall within normal use, and a license would not be required. * In other words, you need a license if you sell a product designed specifically for use with MySQL or that requires the MySQL server to function at all. This is true whether or not you provide MySQL for your client as part of your product distribution. * It also depends on what you're doing for the client. Do you plan to provide your client with detailed instructions on installing MySQL with your software? Then your product may be contingent on the use of MySQL; if so, you need to buy a license. If you are simply tying into a database that you expect already to have been installed by the time your software is purchased, then you probably don't need a license. ISP MySQL Services .................. Internet Service Providers (ISPs) often host MySQL servers for their customers. With the GPL license this does not require a license. On the other hand, we do encourage people to use ISPs that have MySQL support, as this will give them the confidence that if they have some problem with their MySQL installation, their ISP will be able to solve the problem for them (in some cases with the help from the MySQL development team). All ISPs that want to keep themselves up-to-date should subscribe to our `announce' mailing list so that they can be aware of fatal issues that may be relevant for their MySQL installations. Note that if the ISP doesn't have a license for MySQL, it should give its customers at least read access to the source of the MySQL installation so that its customer can verify that it is patched correctly. Running a Web Server Using MySQL ................................ If you use MySQL in conjunction with a Web server on Unix, you don't have to pay for a license. This is true even if you run a commercial Web server that uses MySQL, because you are not selling an embedded MySQL version yourself. However, in this case we would like you to purchase MySQL support, because MySQL is helping your enterprise. MySQL Licensing and Support Costs --------------------------------- Our current license prices are shown below. To make a purchase, please visit `https://order.mysql.com/'. If you pay by credit card, the currency is EURO (European Union Euro) so the prices will differ slightly. *Number of licenses* *Per copy* 1-9 230 EURO 10-24 138 EURO 25-49 117 EURO 50-99 102 EURO 100-249 91 EURO 250-499 76 EURO 500-999 66 EURO For high volume (OEM) purchases, please contact <sales@mysql.com>. For OEM purchases, you must act as the middle-man for eventual problems or extension requests from your users. We also require that OEM customers have at least an extended e-mail support contract. Note that OEM licenses only apply for products where the user doesn't have direct access to the MySQL server (embedded system). In other words, the MySQL server should only be used with the application that was supplied you. If you have a low-margin, high-volume product, you can always talk to us about other terms (for example, a percent of the sale price). If you do, please be informative about your product, pricing, market, and any other information that may be relevant. A full-price license is not a support agreement and includes very minimal support. This means that we try to answer any relevant questions. If the answer is in the documentation, we will direct you to the appropriate section. If you have not purchased a license or support, we probably will not answer at all. If you discover what we consider a real bug, we are likely to fix it in any case. But if you pay for support we will notify you about the fix status instead of just fixing it in a later release. More comprehensive support is sold separately. Descriptions of what each level of support includes are given in *Note Support::. Costs for the various types of commercial support are shown below. Support level prices are in EURO (European Union Euro). One EURO is about 1.06 USD. *Type of support* *Cost per year* Basic e-mail support. *Note Basic EURO 200 email support::. Extended e-mail support *Note EURO 1000 Extended email support::. Login support *Note Login EURO 2000 support::. Extended login support *Note EURO 5000 Extended login support::. Telephone support *Note Telephone EURO 12000 support::. You may upgrade from any lower level of support to a higher level of support for the difference in price between the two support levels. We do also provide telephone support (mostly emergency support but also 24/7 support). This support option doesn't however have a fixed price but is negotiated for case to case. If you are interested in this option you can email <sales@mysql.com> and tell us about your needs. Note that as our sales staff is very busy, it may take some time until your request is handled. Our support staff does however always answer promptly to support questions! Payment information ................... Currently we can take SWIFT payments, checks, or credit cards. Payment should be made to: Postgirot Bank AB 105 06 STOCKHOLM, SWEDEN MySQL AB BOX 6434 11382 STOCKHOLM, SWEDEN SWIFT address: PGSI SESS Account number: 96 77 06 - 3 Specify: license and/or support and your name and e-mail address. In Europe and Japan you can use EuroGiro (that should be less expensive) to the same account. If you want to pay by check, make it payable to "MySQL Finland AB" and mail it to the address below: MySQL AB BOX 6434, Torsgatan 21 11382 STOCKHOLM, SWEDEN If you want to pay by credit card over the Internet, you can use MySQL AB's secure license form (https://order.mysql.com/). You can also print a copy of the license form, fill it in, and send it by fax to: +46-8-729 69 05 If you want us to bill you, you can use the license form and write "bill us" in the comment field. You can also mail a message to <sales@mysql.com> (*not* `mysql@lists.mysql.com'!) with your company information and ask us to bill you. Contact Information ................... For commercial licensing, please contact the MySQL licensing team. The much preferred method is by e-mail to <licensing@mysql.com>. Fax is also possible but handling of these may take much longer (Fax +46-8-729 69 05). If you represent a business that is interested in partnering with MySQL, please send e-mail to <partner@mysql.com>. For timely, precise answers to technical questions about MySQL you should order (https://order.mysql.com/) one of our support contracts (http://www.mysql.com/support/arrangements/types.html). MySQL support is provided by the MySQL developers so the standard is extremely high. If you are interested in placing a banner advertisement on our Web site, please send e-mail to <advertising@mysql.com>. If you are interested in any of the jobs listed in our jobs (http://www.mysql.com/development/jobs/) section, please send e-mail to <jobs@mysql.com>. For general discussion amongst our many users, please direct your attention to the appropriate mailing list (http://www.mysql.com/documentation/lists.html). For general information inquires, please send e-mail to <info@mysql.com>. For questions or comments about the workings or content of the Web site, please send e-mail to <webmaster@mysql.com>. Types of Commercial Support --------------------------- The following is true of all support options: * The support is per year. * We will fix, or provide a reasonable workaround for any repeatable bug. * We will give a reasonable effort to find and fix any other MySQL related bug. * The higher level of support contract the more effort we will put into finding a solution to your problems. * The following is true for all support contracts except Basic email support: For non-bug related things, like helping you optimize your queries or your system, extending MySQL with new functionality, etc., we charge 200 EURO/hour, which is deducted from your support contract. In other words, if you have login support (2000 EURO), you can expect us to work up to 10 hours to help you with things like this. Basic E-mail Support .................... Basic e-mail support is a very inexpensive support option and should be thought of more as a way to support our development of MySQL than as a real support option. We at MySQL do give a lot of free support in all the different MySQL lists, and the money we get from basic e-mail support is largely used to make this possible. At this support level, the MySQL mailing lists are the preferred means of communication. Questions normally should be mailed to the primary mailing list (<mysql@lists.mysql.com>) or one of the other regular lists (for example, <win32@lists.mysql.com> for Windows-related MySQL questions), as someone else already may have experienced and solved the problem you have. *Note Asking questions::. However, by purchasing basic e-mail support, you also have access to the support address <mysql-support@mysql.com>, which is not available as part of the minimal support that you get by purchasing a MySQL license. This means that for especially critical questions, you can cross-post your message to <mysql-support@mysql.com>. (If the message contains sensitive data, you should post only to <mysql-support@mysql.com>.) *REMEMBER!* to ALWAYS include your registration number and expiration date when you send a message to <mysql-support@mysql.com>. Note that if you have encountered a critical, repeatable bug, and follow the rules outlined in the manual section of how to report bugs and send it to <bugs@lists.mysql.com>, we promise to try to fix this as soon as possible, regardless of your support level! *Note Bug reports::. Basic e-mail support includes the following types of service: * If your question is already answered in the manual, we will inform you of the correct section in which you can find the answer. If the answer is not in the manual, we will point you in the right direction to solve your problem. * We guarantee a timely answer for your e-mail messages. We can't guarantee that we can solve any problem, but at least you will receive an answer if we can contact you by e-mail. * We will help with unexpected problems when you install MySQL from a binary distribution on supported platforms. This level of support does not cover installing MySQL from a source distribution. Supported platforms are those for which MySQL is known to work. *Note Which OS::. * We will help you with bugs and missing features. Any bugs that are found are fixed for the next MySQL release. If the bug is critical for you, we will mail you a patch for it as soon the bug is fixed. Critical bugs always have the highest priority for us, and we ensure that they are fixed as soon as possible. * Your suggestions for the further development of MySQL will be taken into consideration. By taking email support you have already helped the further development of MySQL. If you want to have more input, upgrade to a higher level of support. * If you want us to help optimize your system, you must upgrade to a higher level of support. Extended E-mail Support ....................... Extended e-mail support includes everything in basic e-mail support with these additions: * Your e-mail will be dealt with before mail from basic e-mail support users and non-registered users. * Your suggestions for the further development of MySQL will receive strong consideration. Simple extensions that suit the basic goals of MySQL are implemented in a matter of days. By taking extended e-mail support you have already helped the further development of MySQL. * Typical situations that are covered by extended e-mail support are: - We will answer and (within reason) solve questions that relate to possible bugs in MySQL. As soon as the bug is found and corrected, we will mail a patch for it. - We will help with unexpected problems when you install MySQL from a source or binary distribution on supported platforms. - We will answer questions about missing features and offer hints how to work around them. - We will provide hints on optimizing `mysqld' for your situation. * You are allowed to influence the priority of items on the MySQL TODO List. *Note TODO::. This will ensure that the features you really need will be implemented sooner than they might be otherwise. Login Support ............. Login support includes everything in extended e-mail support with these additions: * Your e-mail will be dealt with even before e-mail from extended e-mail support users. * Your suggestions for the further development of MySQL will be taken into very high consideration. Realistic extensions that can be implemented in a couple of hours and that suit the basic goals of MySQL will be implemented as soon as possible. * If you have a very specific problem, we can try to log in on your system to solve the problem "in place." * Like any database vendor, we can't guarantee that we can rescue any data from crashed tables, but if the worst happens, we will help you rescue as much as possible. MySQL has proven itself very reliable, but anything is possible due to circumstances beyond our control (for example, if your system crashes or someone kills the server by executing a `kill -9' command). * We will provide hints on optimizing your system and your queries. * You are allowed to call a MySQL developer (in moderation) and discuss your MySQL-related problems. This option is however only to be used as a last result during an emergency after we have failed to grasp the total problem with email. To make efficient use of our time we need to first get all facts about the problem, before talking on phone, to be able to work as efficiently as possible on solving the problem. Extended Login Support ...................... Extended login support includes everything in login support with these additions: * Your e-mail has the highest possible priority. * We will actively examine your system and help you optimize it and your queries. We may also optimize and/or extend MySQL to better suit your needs. * You may also request special extensions just for you. For example: mysql> select MY_FUNC(col1,col2) from table; * We will provide a binary distribution of all important MySQL releases for your system, as long as we can get an account on a similar system. In the worst case, we may require access to your system to be able to create a binary distribution. * If you can provide accommodations and pay for traveler fares, you can even get a MySQL developer to visit you and offer you help with your troubles. Extended login support entitles you to one personal encounter per year, but we are always very flexible towards our customers! If the visit takes 16 hours or more, the first 8 hours is without charge. For the hours above 8 hours, you will be charged with a rate that is at least 20 % less than our standard rates. Telephone Support ................. Telephone support includes everything in extended login support with these additions: * We will provide you with a dynamic web page showing the current list of `MySQL' developers that you can phone when you have a critical problem. * For non critical problem, you can request a MySQL developer to phone back within 48 hours to discuss `MySQL' related issues. Support for other table handlers ................................ To get support for `BDB' tables, `InnoDB' tables you have to pay an additional 30% on the standard support price for each of the table handlers you would like to have support for. We at `MySQL AB' will help you create a proper bug report for the table handler and submit it to the developers for the specific table handler. We will also do our best to ensure that you will get a timely answer or solution from the developers of the table handler. Even if we are quite confident that we can solve most problems within a timely manner, we can't guarantee a quick solution for any problems you can get with the different table handlers. We will however do our best to help you get the problem solved. How Standards-compatible Is MySQL? ================================== This section describes how MySQL relates to the ANSI SQL standards. MySQL has many extensions to the ANSI SQL standards, and here you will find out what they are, and how to use them. You will also find information about functionality missing from MySQL, and how to work around some differences. MySQL Extensions to ANSI SQL92 ------------------------------ MySQL includes some extensions that you probably will not find in other SQL databases. Be warned that if you use them, your code will not be portable to other SQL servers. In some cases, you can write code that includes MySQL extensions, but is still portable, by using comments of the form `/*! ... */'. In this case, MySQL will parse and execute the code within the comment as it would any other MySQL statement, but other SQL servers will ignore the extensions. For example: SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ... If you add a version number after the `'!'', the syntax will only be executed if the MySQL version is equal to or newer than the used version number: CREATE /*!32302 TEMPORARY */ TABLE (a int); The above means that if you have Version 3.23.02 or newer, then MySQL will use the `TEMPORARY' keyword. MySQL extensions are listed below: * The field types `MEDIUMINT', `SET', `ENUM', and the different `BLOB' and `TEXT' types. * The field attributes `AUTO_INCREMENT', `BINARY', `NULL', `UNSIGNED', and `ZEROFILL'. * All string comparisons are case insensitive by default, with sort ordering determined by the current character set (ISO-8859-1 Latin1 by default). If you don't like this, you should declare your columns with the `BINARY' attribute or use the `BINARY' cast, which causes comparisons to be done according to the ASCII order used on the MySQL server host. * MySQL maps each database to a directory under the MySQL data directory, and tables within a database to filenames in the database directory. This has a few implications: - Database names and table names are case sensitive in MySQL on operating systems that have case-sensitive filenames (like most Unix systems). *Note Name case sensitivity::. - Database, table, index, column, or alias names may begin with a digit (but may not consist solely of digits). - You can use standard system commands to backup, rename, move, delete, and copy tables. For example, to rename a table, rename the `.MYD', `.MYI', and `.frm' files to which the table corresponds. * In SQL statements, you can access tables from different databases with the `db_name.tbl_name' syntax. Some SQL servers provide the same functionality but call this `User space'. MySQL doesn't support tablespaces as in: `create table ralph.my_table...IN my_tablespace'. * `LIKE' is allowed on numeric columns. * Use of `INTO OUTFILE' and `STRAIGHT_JOIN' in a `SELECT' statement. *Note `SELECT': SELECT. * The `SQL_SMALL_RESULT' option in a `SELECT' statement. * `EXPLAIN SELECT' to get a description on how tables are joined. * Use of index names, indexes on a prefix of a field, and use of `INDEX' or `KEY' in a `CREATE TABLE' statement. *Note `CREATE TABLE': CREATE TABLE. * Use of `TEMPORARY' or `IF NOT EXISTS' with `CREATE TABLE'. * Use of `COUNT(DISTINCT list)' where 'list' is more than one element. * Use of `CHANGE col_name', `DROP col_name', or `DROP INDEX', `IGNORE' or `RENAME' in an `ALTER TABLE' statement. *Note `ALTER TABLE': ALTER TABLE. * Use of `RENAME TABLE'. *Note `RENAME TABLE': RENAME TABLE. * Use of multiple `ADD', `ALTER', `DROP', or `CHANGE' clauses in an `ALTER TABLE' statement. * Use of `DROP TABLE' with the keywords `IF EXISTS'. * You can drop multiple tables with a single `DROP TABLE' statement. * The `LIMIT' clause of the `DELETE' statement. * The `DELAYED' clause of the `INSERT' and `REPLACE' statements. * The `LOW_PRIORITY' clause of the `INSERT', `REPLACE', `DELETE', and `UPDATE' statements. * Use of `LOAD DATA INFILE'. In many cases, this syntax is compatible with Oracle's `LOAD DATA INFILE'. *Note `LOAD DATA': LOAD DATA. * The `ANALYZE TABLE', `CHECK TABLE', `OPTIMIZE TABLE', and `REPAIR TABLE' statements. * The `SHOW' statement. *Note `SHOW': SHOW. * Strings may be enclosed by either `"' or `'', not just by `''. * Use of the escape `\' character. * The `SET OPTION' statement. *Note `SET OPTION': SET OPTION. * You don't need to name all selected columns in the `GROUP BY' part. This gives better performance for some very specific, but quite normal queries. *Note Group by functions::. * One can specify `ASC' and `DESC' with `GROUP BY'. * To make it easier for users who come from other SQL environments, MySQL supports aliases for many functions. For example, all string functions support both ANSI SQL syntax and ODBC syntax. * MySQL understands the `||' and `&&' operators to mean logical OR and AND, as in the C programming language. In MySQL, `||' and `OR' are synonyms, as are `&&' and `AND'. Because of this nice syntax, MySQL doesn't support the ANSI SQL `||' operator for string concatenation; use `CONCAT()' instead. Because `CONCAT()' takes any number of arguments, it's easy to convert use of the `||' operator to MySQL. * `CREATE DATABASE' or `DROP DATABASE'. *Note `CREATE DATABASE': CREATE DATABASE. * The `%' operator is a synonym for `MOD()'. That is, `N % M' is equivalent to `MOD(N,M)'. `%' is supported for C programmers and for compatibility with PostgreSQL. * The `=', `<>', `<=' ,`<', `>=',`>', `<<', `>>', `<=>', `AND', `OR', or `LIKE' operators may be used in column comparisons to the left of the `FROM' in `SELECT' statements. For example: mysql> SELECT col1=1 AND col2=2 FROM tbl_name; * The `LAST_INSERT_ID()' function. *Note `mysql_insert_id()': mysql_insert_id. * The `REGEXP' and `NOT REGEXP' extended regular expression operators. * `CONCAT()' or `CHAR()' with one argument or more than two arguments. (In MySQL, these functions can take any number of arguments.) * The `BIT_COUNT()', `CASE', `ELT()', `FROM_DAYS()', `FORMAT()', `IF()', `PASSWORD()', `ENCRYPT()', `md5()', `ENCODE()', `DECODE()', `PERIOD_ADD()', `PERIOD_DIFF()', `TO_DAYS()', or `WEEKDAY()' functions. * Use of `TRIM()' to trim substrings. ANSI SQL only supports removal of single characters. * The `GROUP BY' functions `STD()', `BIT_OR()', and `BIT_AND()'. * Use of `REPLACE' instead of `DELETE' + `INSERT'. *Note `REPLACE': REPLACE. * The `FLUSH flush_option' statement. * The possibility to set variables in a statement with `:=': SELECT @a:=SUM(total),@b=COUNT(*),@a/@b AS avg FROM test_table; SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3; MySQL Differences Compared to ANSI SQL92 ---------------------------------------- We try to make MySQL follow the ANSI SQL standard and the ODBC SQL standard, but in some cases MySQL does some things differently: * `--' is only a comment if followed by a white space. *Note Missing comments::. * For `VARCHAR' columns, trailing spaces are removed when the value is stored. *Note Bugs::. * In some cases, `CHAR' columns are silently changed to `VARCHAR' columns. *Note Silent column changes::. * Privileges for a table are not automatically revoked when you delete a table. You must explicitly issue a `REVOKE' to revoke privileges for a table. *Note `GRANT': GRANT. * `NULL AND FALSE' will evaluate to `NULL' and not to `FALSE'. This is because we don't think it's good to have to evaluate a lot of extra conditions in this case. Running MySQL in ANSI Mode -------------------------- If you start `mysqld' with the `--ansi' option, the following behavior of MySQL changes: * `||' is string concatenation instead of `OR'. * You can have any number of spaces between a function name and the `('. This forces all function names to be treated as reserved words. * `"' will be an identifier quote character (like the MySQL ``' quote character) and not a string quote character. * `REAL' will be a synonym for `FLOAT' instead of a synonym of `DOUBLE'. * The default transaction isolation level is `SERIALIZABLE'. *Note SET TRANSACTION::. This is the same as using `--sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,SERIALIZE,ONLY_FULL_GROUP_BY'. Functionality Missing from MySQL -------------------------------- The following functionality is missing in the current version of MySQL. For a prioritized list indicating when new extensions may be added to MySQL, you should consult the online MySQL TODO list (http://www.mysql.com/documentation/manual.php?section=TODO). That is the latest version of the TODO list in this manual. *Note TODO::. Sub-selects ........... MySQL currently only supports sub selects of the form `INSERT ... SELECT ...' and `REPLACE ... SELECT ...'. You can however use the function `IN()' in other contexts. In many cases you can rewrite the query without a sub-select: SELECT * FROM table1 WHERE id IN (SELECT id FROM table2); This can be re-written as: SELECT table1.* FROM table1,table2 WHERE table1.id=table2.id; The queries: SELECT * FROM table1 WHERE id NOT IN (SELECT id FROM table2); SELECT * FROM table1 WHERE NOT EXISTS (SELECT id FROM table2 where table1.id=table2.id); Can be rewritten as: SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id where table2.id IS NULL For more complicated subqueries you can often create temporary tables to hold the subquery. In some cases, however this option will not work. The most frequently encountered of these cases arises with `DELETE' statements, for which standard SQL does not support joins (except in sub-selects). For this situation there are two options available until subqueries are supported by MySQL. The first option is to use a procedural programming language (such as Perl or PHP) to submit a `SELECT' query to obtain the primary keys for the records to be deleted, and then use these values to construct the `DELETE' statement (`DELETE FROM ... WHERE ... IN (key1, key2, ...)'). The second option is to use interactive SQL to contruct a set of `DELETE' statements automatically, using the MySQL extension `CONCAT()' (in lieu of the standard `||' operator). For example: SELECT CONCAT('DELETE FROM tab1 WHERE pkid = ', tab1.pkid, ';') FROM tab1, tab2 WHERE tab1.col1 = tab2.col2; You can place this query in a script file and redirect input from it to the `mysql' command-line interpreter, piping its output back to a second instance of the interpreter: prompt> mysql --skip-column-names mydb < myscript.sql | mysql mydb MySQL 4.0 supports multi-table deletes that can be used to efficiently delete rows based on information from one table or even from many tables at the same time. `SELECT INTO TABLE' ................... MySQL doesn't yet support the Oracle SQL extension: `SELECT ... INTO TABLE ...'. MySQL supports instead the ANSI SQL syntax `INSERT INTO ... SELECT ...', which is basically the same thing. *Note INSERT SELECT::. INSERT INTO tblTemp2 (fldID) SELECT tblTemp1.fldOrder_ID FROM tblTemp1 WHERE tblTemp1.fldOrder_ID > 100; Alternatively, you can use `SELECT INTO OUTFILE...' or `CREATE TABLE ... SELECT' to solve your problem. Transactions ............ As MySQL does nowadays support transactions, the following discussion is only valid if you are only using the non-transaction-safe table types. *Note COMMIT::. The question is often asked, by the curious and the critical, "Why is MySQL not a transactional database?" or "Why does MySQL not support transactions?" MySQL has made a conscious decision to support another paradigm for data integrity, "atomic operations." It is our thinking and experience that atomic operations offer equal or even better integrity with much better performance. We, nonetheless, appreciate and understand the transactional database paradigm and plan, within the next few releases, to introduce transaction-safe tables on a per table basis. We will be giving our users the possibility to decide if they need the speed of atomic operations or if they need to use transactional features in their applications. How does one use the features of MySQL to maintain rigorous integrity and how do these features compare with the transactional paradigm? First, in the transactional paradigm, if your applications are written in a way that is dependent on the calling of "rollback" instead of "commit" in critical situations, then transactions are more convenient. Moreover, transactions ensure that unfinished updates or corrupting activities are not committed to the database; the server is given the opportunity to do an automatic rollback and your database is saved. MySQL, in almost all cases, allows you to solve for potential problems by including simple checks before updates and by running simple scripts that check the databases for inconsistencies and automatically repair or warn if such occurs. Note that just by using the MySQL log or even adding one extra log, one can normally fix tables perfectly with no data integrity loss. Moreover, fatal transactional updates can be rewritten to be atomic. In fact,we will go so far as to say that all integrity problems that transactions solve can be done with `LOCK TABLES' or atomic updates, ensuring that you never will get an automatic abort from the database, which is a common problem with transactional databases. Not even transactions can prevent all loss if the server goes down. In such cases even a transactional system can lose data. The difference between different systems lies in just how small the time-lap is where they could lose data. No system is 100% secure, only "secure enough." Even Oracle, reputed to be the safest of transactional databases, is reported to sometimes lose data in such situations. To be safe with MySQL, you only need to have backups and have the update logging turned on. With this you can recover from any situation that you could with any transactional database. It is, of course, always good to have backups, independent of which database you use. The transactional paradigm has its benefits and its drawbacks. Many users and application developers depend on the ease with which they can code around problems where an abort appears to be, or is necessary, and they may have to do a little more work with MySQL to either think differently or write more. If you are new to the atomic operations paradigm, or more familiar or more comfortable with transactions, do not jump to the conclusion that MySQL has not addressed these issues. Reliability and integrity are foremost in our minds. Recent estimates indicate that there are more than 1,000,000 `mysqld' servers currently running, many of which are in production environments. We hear very, very seldom from our users that they have lost any data, and in almost all of those cases user error is involved. This is, in our opinion, the best proof of MySQL's stability and reliability. Lastly, in situations where integrity is of highest importance, MySQL's current features allow for transaction-level or better reliability and integrity. If you lock tables with `LOCK TABLES', all updates will stall until any integrity checks are made. If you only obtain a read lock (as opposed to a write lock), then reads and inserts are still allowed to happen. The new inserted records will not be seen by any of the clients that have a `READ' lock until they release their read locks. With `INSERT DELAYED' you can queue inserts into a local queue, until the locks are released, without having the client wait for the insert to complete. *Note INSERT DELAYED::. "Atomic," in the sense that we mean it, is nothing magical. It only means that you can be sure that while each specific update is running, no other user can interfere with it, and there will never be an automatic rollback (which can happen on transaction based systems if you are not very careful). MySQL also guarantees that there will not be any dirty reads. You can find some example of how to write atomic updates in the commit-rollback section. *Note Commit-rollback::. We have thought quite a bit about integrity and performance, and we believe that our atomic operations paradigm allows for both high reliability and extremely high performance, on the order of three to five times the speed of the fastest and most optimally tuned of transactional databases. We didn't leave out transactions because they are hard to do. The main reason we went with atomic operations as opposed to transactions is that by doing this we could apply many speed optimizations that would not otherwise have been possible. Many of our users who have speed foremost in their minds are not at all concerned about transactions. For them transactions are not an issue. For those of our users who are concerned with or have wondered about transactions vis-a-vis MySQL, there is a "MySQL way" as we have outlined above. For those where safety is more important than speed, we recommend them to use the `BDB', or `InnoDB' tables for all their critical data. *Note Table types::. One final note: We are currently working on a safe replication schema that we believe to be better than any commercial replication system we know of. This system will work most reliably under the atomic operations, non-transactional, paradigm. Stay tuned. Stored Procedures and Triggers .............................. A stored procedure is a set of SQL commands that can be compiled and stored in the server. Once this has been done, clients don't need to keep reissuing the entire query but can refer to the stored procedure. This provides better performance because the query has to be parsed only once, and less information needs to be sent between the server and the client. You can also raise the conceptual level by having libraries of functions in the server. A trigger is a stored procedure that is invoked when a particular event occurs. For example, you can install a stored procedure that is triggered each time a record is deleted from a transaction table and that automatically deletes the corresponding customer from a customer table when all his transactions are deleted. The planned update language will be able to handle stored procedures, but without triggers. Triggers usually slow down everything, even queries for which they are not needed. To see when MySQL might get stored procedures, see *Note TODO::. Foreign Keys ............ Note that foreign keys in SQL are not used to join tables, but are used mostly for checking referential integrity (foreign key constraints). If you want to get results from multiple tables from a `SELECT' statement, you do this by joining tables: SELECT * from table1,table2 where table1.id = table2.id; *Note `JOIN': JOIN. *Note example-Foreign keys::. The `FOREIGN KEY' syntax in MySQL exists only for compatibility with other SQL vendors' `CREATE TABLE' commands; it doesn't do anything. The `FOREIGN KEY' syntax without `ON DELETE ...' is mostly used for documentation purposes. Some ODBC applications may use this to produce automatic `WHERE' clauses, but this is usually easy to override. `FOREIGN KEY' is sometimes used as a constraint check, but this check is unnecessary in practice if rows are inserted into the tables in the right order. MySQL only supports these clauses because some applications require them to exist (regardless of whether or not they work). In MySQL, you can work around the problem of `ON DELETE ...' not being implemented by adding the appropriate `DELETE' statement to an application when you delete records from a table that has a foreign key. In practice this is as quick (in some cases quicker) and much more portable than using foreign keys. In the near future we will extend the `FOREIGN KEY' implementation so that at least the information will be saved in the table specification file and may be retrieved by `mysqldump' and ODBC. At a later stage we will implement the foreign key constraints for application that can't easily be coded to avoid them. Reasons NOT to Use Foreign Keys constraints ........................................... There are so many problems with foreign key constraints that we don't know where to start: * Foreign key constraints make life very complicated, because the foreign key definitions must be stored in a database and implementing them would destroy the whole "nice approach" of using files that can be moved, copied, and removed. * The speed impact is terrible for `INSERT' and `UPDATE' statements, and in this case almost all `FOREIGN KEY' constraint checks are useless because you usually insert records in the right tables in the right order, anyway. * There is also a need to hold locks on many more tables when updating one table, because the side effects can cascade through the entire database. It's MUCH faster to delete records from one table first and subsequently delete them from the other tables. * You can no longer restore a table by doing a full delete from the table and then restoring all records (from a new source or from a backup). * If you use foreign key constraints you can't dump and restore tables unless you do so in a very specific order. * It's very easy to do "allowed" circular definitions that make the tables impossible to re-create each table with a single create statement, even if the definition works and is usable. * It's very easy to overlook `FOREIGN KEY ... ON DELETE' rules when one codes an application. It's not unusual that one loses a lot of important information just because a wrong or misused `ON DELETE' rule. The only nice aspect of `FOREIGN KEY' is that it gives ODBC and some other client programs the ability to see how a table is connected and to use this to show connection diagrams and to help in building applications. MySQL will soon store `FOREIGN KEY' definitions so that a client can ask for and receive an answer about how the original connection was made. The current `.frm' file format does not have any place for it. At a later stage we will implement the foreign key constraints for application that can't easily be coded to avoid them. Views ..... MySQL doesn't yet support views, but we plan to implement these to about 4.1. Views are mostly useful for letting users access a set of relations as one table (in read-only mode). Many SQL databases don't allow one to update any rows in a view, but you have to do the updates in the separate tables. As MySQL is mostly used in applications and on web system where the application writer has full control on the database usage, most of our users haven't regarded views to be very important. (At least no one has been interested enough in this to be prepared to finance the implementation of views). One doesn't need views in MySQL to restrict access to columns as MySQL has a very sophisticated privilege system. *Note Privilege system::. `--' as the Start of a Comment .............................. Some other SQL databases use `--' to start comments. MySQL has `#' as the start comment character, even if the `mysql' command-line tool removes all lines that start with `--'. You can also use the C comment style `/* this is a comment */' with MySQL. *Note Comments::. MySQL Version 3.23.3 and above supports the `--' comment style only if the comment is followed by a space. This is because this degenerate comment style has caused many problems with automatically generated SQL queries that have used something like the following code, where we automatically insert the value of the payment for `!payment!': UPDATE tbl_name SET credit=credit-!payment! What do you think will happen when the value of `payment' is negative? Because `1--1' is legal in SQL, we think it is terrible that `--' means start comment. In MySQL Version 3.23 you can, however, use: `1-- This is a comment' The following discussion only concerns you if you are running a MySQL version earlier than Version 3.23: If you have a SQL program in a text file that contains `--' comments you should use: shell> replace " --" " #" < text-file-with-funny-comments.sql \ | mysql database instead of the usual: shell> mysql database < text-file-with-funny-comments.sql You can also edit the command file "in place" to change the `--' comments to `#' comments: shell> replace " --" " #" -- text-file-with-funny-comments.sql Change them back with this command: shell> replace " #" " --" -- text-file-with-funny-comments.sql What Standards Does MySQL Follow? --------------------------------- Entry level SQL92. ODBC levels 0-2. How to Cope Without `COMMIT'/`ROLLBACK' --------------------------------------- The following mostly applies only for `ISAM', `MyISAM', and `HEAP' tables. If you only use transaction-safe tables (`BDB', or `InnoDB' tables) in an an update, you can do `COMMIT' and `ROLLBACK' also with MySQL. *Note COMMIT::. The problem with handling `COMMIT'-`ROLLBACK' efficiently with the above table types would require a completely different table layout than MySQL uses today. The table type would also need extra threads that do automatic cleanups on the tables, and the disk usage would be much higher. This would make these table types about 2-4 times slower than they are today. For the moment, we prefer implementing the SQL server language (something like stored procedures). With this you would very seldom really need `COMMIT'-`ROLLBACK.' This would also give much better performance. Loops that need transactions normally can be coded with the help of `LOCK TABLES', and you don't need cursors when you can update records on the fly. We at TcX had a greater need for a real fast database than a 100% general database. Whenever we find a way to implement these features without any speed loss, we will probably do it. For the moment, there are many more important things to do. Check the TODO for how we prioritize things at the moment. (Customers with higher levels of support can alter this, so things may be reprioritized.) The current problem is actually `ROLLBACK'. Without `ROLLBACK', you can do any kind of `COMMIT' action with `LOCK TABLES'. To support `ROLLBACK' with the above table types, MySQL would have to be changed to store all old records that were updated and revert everything back to the starting point if `ROLLBACK' was issued. For simple cases, this isn't that hard to do (the current `isamlog' could be used for this purpose), but it would be much more difficult to implement `ROLLBACK' for `ALTER/DROP/CREATE TABLE'. To avoid using `ROLLBACK', you can use the following strategy: 1. Use `LOCK TABLES ...' to lock all the tables you want to access. 2. Test conditions. 3. Update if everything is okay. 4. Use `UNLOCK TABLES' to release your locks. This is usually a much faster method than using transactions with possible `ROLLBACK's, although not always. The only situation this solution doesn't handle is when someone kills the threads in the middle of an update. In this case, all locks will be released but some of the updates may not have been executed. You can also use functions to update records in a single operation. You can get a very efficient application by using the following techniques: * Modify fields relative to their current value. * Update only those fields that actually have changed. For example, when we are doing updates to some customer information, we update only the customer data that has changed and test only that none of the changed data, or data that depend on the changed data, has changed compared to the original row. The test for changed data is done with the `WHERE' clause in the `UPDATE' statement. If the record wasn't updated, we give the client a message: "Some of the data you have changed have been changed by another user". Then we show the old row versus the new row in a window, so the user can decide which version of the customer record he should use. This gives us something that is similar to column locking but is actually even better, because we only update some of the columns, using values that are relative to their current values. This means that typical `UPDATE' statements look something like these: UPDATE tablename SET pay_back=pay_back+'relative change'; UPDATE customer SET customer_date='current_date', address='new address', phone='new phone', money_he_owes_us=money_he_owes_us+'new_money' WHERE customer_id=id AND address='old address' AND phone='old phone'; As you can see, this is very efficient and works even if another client has changed the values in the `pay_back' or `money_he_owes_us' columns. In many cases, users have wanted `ROLLBACK' and/or `LOCK TABLES' for the purpose of managing unique identifiers for some tables. This can be handled much more efficiently by using an `AUTO_INCREMENT' column and either the SQL function `LAST_INSERT_ID()' or the C API function `mysql_insert_id()'. *Note `mysql_insert_id()': mysql_insert_id. At MySQL AB, we have never had any need for row-level locking because we have always been able to code around it. Some cases really need row locking, but they are very few. If you want row-level locking, you can use a flag column in the table and do something like this: UPDATE tbl_name SET row_flag=1 WHERE id=ID; MySQL returns 1 for the number of affected rows if the row was found and `row_flag' wasn't already 1 in the original row. You can think of it as MySQL changed the above query to: UPDATE tbl_name SET row_flag=1 WHERE id=ID and row_flag <> 1; Known errors and design deficiencies in MySQL --------------------------------------------- The following problems are known and have a very high priority to get fixed: * `ANALYZE TABLE' on a BDB table may in some case make the table unusable until one has restarted `mysqld'. When this happens you will see errors like the following in the MySQL error file: 001207 22:07:56 bdb: log_flush: LSN past current end-of-log * Don't execute `ALTER TABLE' on a `BDB' table on which you are running not completed multi-statement transactions. (The transaction will probably be ignored). * `ANALYZE TABLE', `OPTIMIZE TABLE' and `REPAIR TABLE' may cause problems on tables for which you are using `INSERT DELAYED'. * Doing a `LOCK TABLE ..' and `FLUSH TABLES ..' doesn't guarantee that there isn't a half-finished transaction in progress on the table. * BDB tables are a bit slow to open. If you have many BDB tables in a database, it will take a long time to use the `mysql' client on the database if you are not using the `-A' option or if you are using `rehash'. This is especially notable when you have a big table cache. * Th current replication protocol cannot deal with `LOAD DATA INFILE' and line terminator characters of more than 1 character. The following problems are known and will be fixed in due time: * For the moment `MATCH' only works with `SELECT' statements. * When using `SET CHARACTER SET', one can't use translated characters in database, table and column names. * `DELETE FROM merge_table' used without a `WHERE' will only clear the mapping for the table, not delete everything in the mapped tables * You cannot build in another directory when using MIT-pthreads. Because this requires changes to MIT-pthreads, we are not likely to fix this. * `BLOB' values can't "reliably" be used in `GROUP BY' or `ORDER BY' or `DISTINCT'. Only the first `max_sort_length' bytes (default 1024) are used when comparing `BLOB'bs in these cases. This can be changed with the `-O max_sort_length' option to `mysqld'. A workaround for most cases is to use a substring: `SELECT DISTINCT LEFT(blob,2048) FROM tbl_name'. * Calculation is done with `BIGINT' or `DOUBLE' (both are normally 64 bits long). It depends on the function which precision one gets. The general rule is that bit functions are done with `BIGINT' precision, `IF', and `ELT()' with `BIGINT' or `DOUBLE' precision and the rest with `DOUBLE' precision. One should try to avoid using bigger unsigned long long values than 63 bits (9223372036854775807) for anything else than bit fields! * All string columns, except `BLOB' and `TEXT' columns, automatically have all trailing spaces removed when retrieved. For `CHAR' types this is okay, and may be regarded as a feature according to ANSI SQL92. The bug is that in MySQL, `VARCHAR' columns are treated the same way. * You can only have up to 255 `ENUM' and `SET' columns in one table. * `safe_mysqld' re-directs all messages from `mysqld' to the `mysqld' log. One problem with this is that if you execute `mysqladmin refresh' to close and reopen the log, `stdout' and `stderr' are still redirected to the old log. If you use `--log' extensively, you should edit `safe_mysqld' to log to `'hostname'.err' instead of `'hostname'.log' so you can easily reclaim the space for the old log by deleting the old one and executing `mysqladmin refresh'. * In the `UPDATE' statement, columns are updated from left to right. If you refer to an updated column, you will get the updated value instead of the original value. For example: mysql> UPDATE tbl_name SET KEY=KEY+1,KEY=KEY+1; This will update `KEY' with `2' instead of with `1'. * You can't use temporary tables more than once in the same query. For example, the following doesn't work. select * from temporary_table, temporary_table as t2; * `RENAME' doesn't work with `TEMPORARY' tables. * The optimizer may handle `DISTINCT' differently if you are using 'hidden' columns in a join or not. In a join, hidden columns are counted as part of the result (even if they are not shown) while in normal queries hidden columns doesn't participate in the `DISTINCT' comparison. We will probably change this in the future to never compare the hidden columns when executing `DISTINCT' An example of this is: SELECT DISTINCT mp3id FROM band_downloads WHERE userid = 9 ORDER BY id DESC; and SELECT DISTINCT band_downloads.mp3id, FROM band_downloads,band_mp3 WHERE band_downloads.userid = 9 AND band_mp3.id = band_downloads.mp3id ORDER BY band_downloads.id DESC; In the second case you may in MySQL 3.23.x get two identical rows in the result set (because the hidden 'id' column may differ). Note that the this only happens for queries where you don't have the ORDER BY columns in the result, something that is you are not allowed to do in ANSI SQL. * Because MySQL allows you to work with table types that doesn't support transactions (and thus can't `rollback' data) some things behaves a little different in MySQL than in other SQL servers: (This is just to ensure that MySQL never need to do a rollback for a SQL command). This may be a little awkward at times as column Because MySQL allows you to work with table types that don't support transactions, and thus can't `rollback' data, some things behave a little differently in MySQL than in other SQL servers. This is just to ensure that MySQL never need to do a rollback for a SQL command. This may be a little awkward at times as column values must be checked in the application, but this will actually give you a nice speed increase as it allows MySQL to do some optimizations that otherwise would be very hard to do. If you set a column to an incorrect value, MySQL will, instead of doing a rollback, store the `best possible value' in the column: - If you try to store a value outside the range in a numerical column, MySQL will instead store the smallest or biggest possible value in the column. - If you try to store a string that doesn't start with a number into a numerical column, MySQL will store 0 into it. - If you try to store `NULL' into a column that doesn't take `NULL' values, MySQL will store 0 or `''' (empty string) in it instead. (This behavior can, however, be changed with the -DDONT_USE_DEFAULT_FIELDS compile option). - MySQL allows you to store some wrong date values into `DATE' and `DATETIME' columns. (Like 2000-02-31 or 2000-02-00). If the date is totally wrong, MySQL will store the special 0000-00-00 date value in the column. - If you set an `enum' to an unsupported value, it will be set to the error value 'empty string', with numeric value 0. * If you execute a `PROCEDURE' on a query that returns an empty set, in some cases the `PROCEDURE' will not transform the columns. * Creation of a table of type `MERGE' doesn't check if the underlying tables are of compatible types. * MySQL can't yet handle `NaN', `-Inf' and `Inf' values in double. Using these will cause problems when trying to export and import data. We should as an intermediate solution change `NaN' to `NULL' (if possible) and `-Inf' and `Inf' to the Minimum respective maximum possible `double' value. * `LIMIT' on negative numbers are treated as big positive numbers. * If you use `ALTER TABLE' to first add an `UNIQUE' index to a table used in a `MERGE' table and then use `ALTER TABLE' to add a normal index on the `MERGE' table, the key order will be different for the tables if there was an old not-unique key in the table. This is because `ALTER TABLE' puts `UNIQUE' keys before normal keys to be able to detect duplicate keys as early as possible. The following are known bugs in earlier versions of MySQL: * You can get a hung thread if you do a `DROP TABLE' on a table that is one among many tables that is locked with `LOCK TABLES'. * In the following case you can get a core dump: - Delayed insert handler has pending inserts to a table. - `LOCK table' with `WRITE' - `FLUSH TABLES' * Before MySQL Version 3.23.2 an `UPDATE' that updated a key with a `WHERE' on the same key may have failed because the key was used to search for records and the same row may have been found multiple times: UPDATE tbl_name SET KEY=KEY+1 WHERE KEY > 100; A workaround is to use: mysql> UPDATE tbl_name SET KEY=KEY+1 WHERE KEY+0 > 100; This will work because MySQL will not use index on expressions in the `WHERE' clause. * Before MySQL Version 3.23, all numeric types where treated as fixed-point fields. That means you had to specify how many decimals a floating-point field shall have. All results were returned with the correct number of decimals. For platform-specific bugs, see the sections about compiling and porting. How MySQL Compares to Other Databases ===================================== This section compares MySQL to other popular databases. This section has been written by the MySQL developers, so it should be read with that in mind. There are no factual errors contained in this section that we know of. If you find something which you believe to be an error, please contact us about it at <docs@mysql.com>. For a list of all supported limits, functions, and types, see the `crash-me' Web page at `http://www.mysql.com/information/crash-me.php'. How MySQL Compares to `mSQL' ---------------------------- *Performance* For a true comparison of speed, consult the growing MySQL benchmark suite. *Note MySQL Benchmarks::. Because there is no thread creation overhead, a small parser, few features, and simple security, `mSQL' should be quicker at: * Tests that perform repeated connects and disconnects, running a very simple query during each connection. * `INSERT' operations into very simple tables with few columns and keys. * `CREATE TABLE' and `DROP TABLE'. * `SELECT' on something that isn't an index. (A table scan is very easy.) Because these operations are so simple, it is hard to be better at them when you have a higher startup overhead. After the connection is established, MySQL should perform much better. On the other hand, MySQL is much faster than `mSQL' (and most other SQL implementations) on the following: * Complex `SELECT' operations. * Retrieving large results (MySQL has a better, faster, and safer protocol). * Tables with variable-length strings, because MySQL has more efficient handling and can have indexes on `VARCHAR' columns. * Handling tables with many columns. * Handling tables with large record lengths. * `SELECT' with many expressions. * `SELECT' on large tables. * Handling many connections at the same time. MySQL is fully multi-threaded. Each connection has its own thread, which means that no thread has to wait for another (unless a thread is modifying a table another thread wants to access). In `mSQL', once one connection is established, all others must wait until the first has finished, regardless of whether the connection is running a query that is short or long. When the first connection terminates, the next can be served, while all the others wait again, etc. * Joins. `mSQL' can become pathologically slow if you change the order of tables in a `SELECT'. In the benchmark suite, a time more than 15000 times slower than MySQL was seen. This is due to `mSQL''s lack of a join optimizer to order tables in the optimal order. However, if you put the tables in exactly the right order in `mSQL'2 and the `WHERE' is simple and uses index columns, the join will be relatively fast! *Note MySQL Benchmarks::. * `ORDER BY' and `GROUP BY'. * `DISTINCT'. * Using `TEXT' or `BLOB' columns. *SQL Features* * `GROUP BY' and `HAVING'. `mSQL' does not support `GROUP BY' at all. MySQL supports a full `GROUP BY' with both `HAVING' and the following functions: `COUNT()', `AVG()', `MIN()', `MAX()', `SUM()', and `STD()'. `COUNT(*)' is optimized to return very quickly if the `SELECT' retrieves from one table, no other columns are retrieved, and there is no `WHERE' clause. `MIN()' and `MAX()' may take string arguments. * `INSERT' and `UPDATE' with calculations. MySQL can do calculations in an `INSERT' or `UPDATE'. For example: mysql> UPDATE SET x=x*10+y WHERE x<20; * Aliasing. MySQL has column aliasing. * Qualifying column names. In MySQL, if a column name is unique among the tables used in a query, you do not have to use the full qualifier. * `SELECT' with functions. MySQL has many functions (too many to list here; see *Note Functions::). *Disk Space Efficiency* That is, how small can you make your tables? MySQL has very precise types, so you can create tables that take very little space. An example of a useful MySQL datatype is the `MEDIUMINT' that is 3 bytes long. If you have 100,000,000 records, saving even one byte per record is very important. `mSQL2' has a more limited set of column types, so it is more difficult to get small tables. *Stability* This is harder to judge objectively. For a discussion of MySQL stability, see *Note Stability::. We have no experience with `mSQL' stability, so we cannot say anything about that. *Price* Another important issue is the license. MySQL has a more flexible license than `mSQL', and is also less expensive than `mSQL'. Whichever product you choose to use, remember to at least consider paying for a license or e-mail support. (You are required to get a license if you include MySQL with a product that you sell, of course.) *Perl Interfaces* MySQL has basically the same interfaces to Perl as `mSQL' with some added features. *JDBC (Java)* MySQL currently has a lot of different JDBC drivers: * The mm driver: A type 4 JDBC driver by Mark Matthews <mmatthew@ecn.purdue.edu>. This is released under the LGPL. * The Resin driver. This is a commercial JDBC driver released under open source. `http://www.caucho.com/projects/jdbc-mysql/index.xtp' * The gwe driver: A Java interface by GWE technologies (not supported anymore). * The jms driver: An improved gwe driver by Xiaokun Kelvin ZHU <X.Zhu@brad.ac.uk> (not supported anymore). * The twz driver: A type 4 JDBC driver by Terrence W. Zellers <zellert@voicenet.com>. This is commercial but is free for private and educational use (not supported anymore). The recommended driver is the mm driver. The Resin driver may also be good (at least the benchmarks looks good), but we haven't received that much information about this yet. We know that `mSQL' has a JDBC driver, but we have too little experience with it to compare. *Rate of Development* MySQL has a very small team of developers, but we are quite used to coding C and C++ very rapidly. Because threads, functions, `GROUP BY', and so on are still not implemented in `mSQL', it has a lot of catching up to do. To get some perspective on this, you can view the `mSQL' `HISTORY' file for the last year and compare it with the News section of the MySQL Reference Manual (*note News::). It should be pretty obvious which one has developed most rapidly. *Utility Programs* Both `mSQL' and MySQL have many interesting third-party tools. Because it is very easy to port upward (from `mSQL' to MySQL), almost all the interesting applications that are available for `mSQL' are also available for MySQL. MySQL comes with a simple `msql2mysql' program that fixes differences in spelling between `mSQL' and MySQL for the most-used C API functions. For example, it changes instances of `msqlConnect()' to `mysql_connect()'. Converting a client program from `mSQL' to MySQL usually takes a couple of minutes. How to Convert `mSQL' Tools for MySQL ..................................... According to our experience, it would just take a few hours to convert tools such as `msql-tcl' and `msqljava' that use the `mSQL' C API so that they work with the MySQL C API. The conversion procedure is: 1. Run the shell script `msql2mysql' on the source. This requires the `replace' program, which is distributed with MySQL. 2. Compile. 3. Fix all compiler errors. Differences between the `mSQL' C API and the MySQL C API are: * MySQL uses a `MYSQL' structure as a connection type (`mSQL' uses an `int'). * `mysql_connect()' takes a pointer to a `MYSQL' structure as a parameter. It is easy to define one globally or to use `malloc()' to get one. `mysql_connect()' also takes two parameters for specifying the user and password. You may set these to `NULL, NULL' for default use. * `mysql_error()' takes the `MYSQL' structure as a parameter. Just add the parameter to your old `msql_error()' code if you are porting old code. * MySQL returns an error number and a text error message for all errors. `mSQL' returns only a text error message. * Some incompatibilities exist as a result of MySQL supporting multiple connections to the server from the same process. How `mSQL' and MySQL Client/Server Communications Protocols Differ .................................................................. There are enough differences that it is impossible (or at least not easy) to support both. The most significant ways in which the MySQL protocol differs from the `mSQL' protocol are listed below: * A message buffer may contain many result rows. * The message buffers are dynamically enlarged if the query or the result is bigger than the current buffer, up to a configurable server and client limit. * All packets are numbered to catch duplicated or missing packets. * All column values are sent in ASCII. The lengths of columns and rows are sent in packed binary coding (1, 2, or 3 bytes). * MySQL can read in the result unbuffered (without having to store the full set in the client). * If a single read/write takes more than 30 seconds, the server closes the connection. * If a connection is idle for 8 hours, the server closes the connection. How `mSQL' 2.0 SQL Syntax Differs from MySQL ............................................ *Column types* `MySQL' Has the following additional types (among others; *note `CREATE TABLE': CREATE TABLE.): * `ENUM' type for one of a set of strings. * `SET' type for many of a set of strings. * `BIGINT' type for 64-bit integers. `' MySQL also supports the following additional type attributes: * `UNSIGNED' option for integer columns. * `ZEROFILL' option for integer columns. * `AUTO_INCREMENT' option for integer columns that are a `PRIMARY KEY'. *Note `mysql_insert_id()': mysql_insert_id. * `DEFAULT' value for all columns. `mSQL2' `mSQL' column types correspond to the MySQL types shown below: `mSQL' *Corresponding MySQL type* *type* `CHAR(len)'`CHAR(len)' `TEXT(len)'`TEXT(len)'. `len' is the maximal length. And `LIKE' works. `INT' `INT'. With many more options! `REAL' `REAL'. Or `FLOAT'. Both 4- and 8-byte versions are available. `UINT' `INT UNSIGNED' `DATE' `DATE'. Uses ANSI SQL format rather than `mSQL''s own format. `TIME' `TIME' `MONEY' `DECIMAL(12,2)'. A fixed-point value with two decimals. *Index Creation* `MySQL' Indexes may be specified at table creation time with the `CREATE TABLE' statement. `mSQL' Indexes must be created after the table has been created, with separate `CREATE INDEX' statements. *To Insert a Unique Identifier into a Table* `MySQL' Use `AUTO_INCREMENT' as a column type specifier. *Note `mysql_insert_id()': mysql_insert_id. `mSQL' Create a `SEQUENCE' on a table and select the `_seq' column. *To Obtain a Unique Identifier for a Row* `MySQL' Add a `PRIMARY KEY' or `UNIQUE' key to the table and use this. New in Version 3.23.11: If the `PRIMARY' or `UNIQUE' key consists of only one column and this is of type integer, one can also refer to it as `_rowid'. `mSQL' Use the `_rowid' column. Observe that `_rowid' may change over time depending on many factors. *To Get the Time a Column Was Last Modified* `MySQL' Add a `TIMESTAMP' column to the table. This column is automatically set to the current date and time for `INSERT' or `UPDATE' statements if you don't give the column a value or if you give it a `NULL' value. `mSQL' Use the `_timestamp' column. *`NULL' Value Comparisons* `MySQL' MySQL follows ANSI SQL, and a comparison with `NULL' is always `NULL'. `mSQL' In `mSQL', `NULL = NULL' is TRUE. You must change `=NULL' to `IS NULL' and `<>NULL' to `IS NOT NULL' when porting old code from `mSQL' to MySQL. *String Comparisons* `MySQL' Normally, string comparisons are performed in case-independent fashion with the sort order determined by the current character set (ISO-8859-1 Latin1 by default). If you don't like this, declare your columns with the `BINARY' attribute, which causes comparisons to be done according to the ASCII order used on the MySQL server host. `mSQL' All string comparisons are performed in case-sensitive fashion with sorting in ASCII order. *Case-insensitive Searching* `MySQL' `LIKE' is a case-insensitive or case-sensitive operator, depending on the columns involved. If possible, MySQL uses indexes if the `LIKE' argument doesn't start with a wild-card character. `mSQL' Use `CLIKE'. *Handling of Trailing Spaces* `MySQL' Strips all spaces at the end of `CHAR' and `VARCHAR' columns. Use a `TEXT' column if this behavior is not desired. `mSQL' Retains trailing space. *`WHERE' Clauses* `MySQL' MySQL correctly prioritizes everything (`AND' is evaluated before `OR'). To get `mSQL' behavior in MySQL, use parentheses (as shown in an example below). `mSQL' Evaluates everything from left to right. This means that some logical calculations with more than three arguments cannot be expressed in any way. It also means you must change some queries when you upgrade to MySQL. You do this easily by adding parentheses. Suppose you have the following `mSQL' query: mysql> SELECT * FROM table WHERE a=1 AND b=2 OR a=3 AND b=4; To make MySQL evaluate this the way that `mSQL' would, you must add parentheses: mysql> SELECT * FROM table WHERE (a=1 AND (b=2 OR (a=3 AND (b=4)))); *Access Control* `MySQL' Has tables to store grant (permission) options per user, host, and database. *Note Privileges::. `mSQL' Has a file `mSQL.acl' in which you can grant read/write privileges for users. `' How MySQL Compares to PostgreSQL -------------------------------- When reading the following, please note that both products are continually evolving. We at MySQL AB and the PostgreSQL developers are both working on making our respective database as good as possible, so we are both a serious choice to any commercial database. The following comparison is made by us at MySQL AB. We have tried to be as accurate and fair as possible, but because we don't have a full knowledge of all PostgreSQL features while we know MySQL througly, we may have got some things wrong. We will however correct these when they come to our attention. We would first like to note that PostgreSQL and MySQL are both widely used products, but with different design goals, even if we are both striving to be ANSI SQL compatible. This means that for some applications MySQL is more suited, while for others PostgreSQL is more suited. When choosing which database to use, you should first check if the database's feature set satisfies your application. If you need raw speed, MySQL is probably your best choice. If you need some of the extra features that only PostgreSQL can offer, you should use `PostgreSQL'. MySQL and PostgreSQL development strategies ........................................... When adding things to MySQL we take pride to do an optimal, definite solution. The code should be so good that we shouldn't have any need to change it in the foreseeable future. We also do not like to sacrifice speed for features but instead will do our utmost to find a solution that will give maximal throughput. This means that development will take a little longer, but the end result will be well worth this. This kind of development is only possible because all server code are checked by one of a few (currently two) persons before it's included in the MySQL server. We at MySQL AB believe in frequent releases to be able to push out new features quickly to our users. Because of this we do a new small release about every three weeks, and a major branch every year. All releases are throughly tested with our testing tools on a lot of different platforms. PostgreSQL is based on a kernel with lots of contributors. In this setup it makes sense to prioritize adding a lot of new features, instead of implementing them optimally, because one can always optimize things later if there arises a need for this. Another big difference between MySQL and PostgreSQL is that nearly all of the code in the MySQL server are coded by developers that are employed by MySQL AB and are still working on the server code. The exceptions are the transaction engines, and the regexp library. This is in sharp contrast to the PostgreSQL code where the majority of the code is coded by a big group of people with different backgrounds. It was only recently that the PostgreSQL developers announced that their current developer group had finally had time to take a look at all the code in the current PostgreSQL release. Both of the above development methods has it's own merits and drawbacks. We here at MySQL AB think of course that our model is better because our model gives better code consistency, more optimal and reusable code, and in our opinion, fewer bugs. Because we are the authors of the MySQL server code, we are better able to coordinate new features and releases. Featurewise Comparison of MySQL and PostgreSQL .............................................. On the crash-me (http://www.mysql.com/information/crash-me.php) page you can find a list of those database constructs and limits that one can detect automatically with a program. Note however that a lot of the numerical limits may be changed with startup options for respective database. The above web page is however extremely useful when you want to ensure that your applications works with many different databases or when you want to convert your application from one datbase to another. MySQL offers the following advantages over PostgreSQL: * `MySQL' is generally much faster than PostgreSQL. *Note MySQL-PostgreSQL benchmarks::. * MySQL has a much larger user base than PostgreSQL, therefor the code is more tested and has historically been more stable than PostgreSQL. MySQL is the much more used in production environments than PostgreSQL, mostly thanks to that MySQL AB, formerly TCX DataKonsult AB, has provided top quality commercial support for MySQL from the day it was released, whereas until recently PostgreSQL was unsupported. * MySQL works better on Windows than PostgreSQL does. MySQL runs as a native Windows application (a service on NT/Win2000/WinXP), while PostgreSQL is run under the cygwin emulation. We have heard that PostgreSQL is not yet that stable on Windows but we haven't been able to verify this ourselves. * MySQL has more APIs to other languages and is supported by more existing programs than PostgreSQL. *Note Contrib::. * MySQL works on 24/7 heavy duty systems. In most circumstances you never have to run any cleanups on MySQL. PostgreSQL doesn't yet support 24/7 systems because you have to run `VACUUM()' once in a while to reclaim space from `UPDATE' and `DELETE' commands and to perform statistics analyzes that are critical to get good performance with PostgreSQL. `VACUUM()' is also needed after adding a lot of new rows to a table. On a busy system with lots of changes, `VACUUM()' must be run very frequently, in the worst cases even many times a day. During the `VACUUM()' run, which may take hours if the database is big, the database is from a production standpoint, practically dead. The PostgreSQL team has fixing this on their TODO, but we assume that this is not an easy thing to fix permanently. * A working, tested replication feature used by sites like: - Yahoo Finance (`http://finance.yahoo.com') - Mobile.de (`http://www.mobile.de/') - Slashdot (`http://www.slashdot.org') * Included in the MySQL distribution are two different testing suites, `mysql-test-run' and crash-me (http://www.mysql.com/information/crash-me.php), as well as a benchmark suite. The test system is actively updated with code to test each new feature and almost all repeatable bugs that have come to our attention. We test MySQL with these on a lot of platforms before every release. These tests are more sophisticated than anything we have seen from PostgreSQL, and they ensures that the MySQL is kept to a high standard. * There are far more books in print about MySQL than about PostgreSQL. O'Reilly, Sams, Que, and New Riders are all major publishers with books about MySQL. All MySQL features are also documented in the MySQL on-line manual, because when a new feature is implemented, the MySQL developers are required to document it before it's included in the source. * MySQL supports more of the standard ODBC functions than `PostgreSQL'. * MySQL has a much more sophisticated `ALTER TABLE'. * MySQL has support for tables without transactions for applications that need all speed they can get. The tables may be memory based, `HEAP' tables or disk based `MyISAM'. *Note Table types::. * MySQL has support for two different table handlers that support transactions, `BerkeleyDB' and `InnoDB'. Because every transaction engine performs differently under different conditions, this gives the application writer more options to find an optimal solution for his or her setup. *Note Table types::. * `MERGE' tables gives you a unique way to instantly make a view over a set of identical tables and use these as one. This is perfect for systems where you have log files that you order for example by month. *Note MERGE::. * The option to compress read-only tables, but still have direct access to the rows in the table, gives you better performance by minimizing disk reads. This is very useful when you are archiving things. *Note myisampack::. * MySQL has internal support for fulltext search. *Note Fulltext Search::. * You can access many databases from the same connection (depending of course on your privileges). * MySQL is coded from the start to be multi-threaded while PostgreSQL uses processes. Context switching and access to common storage areas is much faster between threads than between separate processes, this gives MySQL a big speed advantage in multi-user applications and also makes it easier for MySQL to take full advantage of symmetric multiprocessor (SMP) systems. * MySQL has a much more sophisticated privilege system than PostgreSQL. While PostgreSQL only supports `INSERT', `SELECT', and `UPDATE/DELETE' grants per user on a database or a table, MySQL allows you to define a full set of different privileges on database, table and column level. MySQL also allows you to specify the privilege on host and user combinations. *Note GRANT::. * MySQL supports a compressed client/server protocol which improves performance over slow links. * MySQL employs a "table handler" concept, and is the only relational database we know of built around this concept. This allows different low-level table types to be swapped into the SQL engine, and each table type can be optimized for different performance characteristics. * All MySQL table types (except *InnoDB*) are implemented as files (one table per file), which makes it really easy to backup, move, delete and even symlink databases and tables, even when the server is down. * Tools to repair and optimize *MyISAM* tables (the most common MySQL table type). A repair tool is only needed when a physical corruption of a data file happens, usually from a hardware failure. It allows a majority of the data to be recovered. * Upgrading MySQL is painless. When you are upgrading MySQL, you don't need to dump/restore your data, as you have to do with most PostgreSQL upgrades. Drawbacks with MySQL compared to PostgreSQL: * The transaction support in MySQL is not yet as well tested as PostgreSQL's system. * Because MySQL uses threads, which are not yet flawless on many OSes, one must either use binaries from `http://www.mysql.com/downloads', or carefully follow our instructions on `http://www.mysql.com/doc/I/n/Installing_source.html' to get an optimal binary that works in all cases. * Table locking, as used by the non-transactional `MyISAM' tables, is in many cases faster than page locks, row locks or versioning. The drawback however is that if one doesn't take into account how table locks work, a single long-running query can block a table for updates for a long time. This can usable be avoided when designing the application. If not, one can always switch the trouble table to use one of the transactional table types. *Note Table locking::. * With UDF (user defined functions) one can extend MySQL with both normal SQL functions and aggregates, but this is not yet as easy or as flexible as in PostgreSQL. *Note Adding functions::. * Updates and deletes that run over multiple tables is harder to do in MySQL. This will, hoever, be fixed in MySQL 4.0 with multi-table `DELETE' and multi-table `UPDATE' and in MySQL 4.1 with subselects. PostgreSQL currently offers the following advantages over MySQL: Note that because we know the MySQL road map, we have included in the following table the version when MySQL should support this feature. Unfortunately we couldn't do this for previous comparison, because we don't know the PostgreSQL roadmap. *Feature* *MySQL version* Subselects 4.1 Foreign keys 4.0 and 4.1 Views 4.2 Stored procedures 4.1 Extensible type system Not planned Unions 4.0 Full join 4.0 or 4.1 Triggers 4.1 Constrainst 4.1 Cursors 4.1 or 4.2 Extensible index types like R-trees R-trees are planned for 4.2 Inherited tables Not planned Other reasons to use PostgreSQL: * Standard usage in PostgreSQL is closer to ANSI SQL in some cases. * One can speed up PostgreSQL by coding things as stored procedures. * PostgreSQL has a bigger team of developers that contribute to the server. Drawbacks with PostgreSQL compared to MySQL: * `VACUUM()' makes PostgreSQL hard to use in a 24/7 environment. * Only transactional tables. * Much slower `INSERT', `DELETE', and `UPDATE'. For a complete list of drawbacks, you should also examine the first table in this section. Benchmarking MySQL and PostgreSQL ................................. The only open source benchmark that we know of that can be used to benchmark MySQL and PostgreSQL (and other databases) is our own. It can be found at `http://www.mysql.com/information/benchmarks.html'. We have many times asked the PostgreSQL developers and some PostgreSQL users to help us extend this benchmark to make it the definitive benchmark for databases, but unfortunately we haven't gotten any feedback for this. We the MySQL developers have, because of this, spent a lot of hours to get maximum performance from PostgreSQL for the benchmarks, but because we don't know PostgreSQL intimately, we are sure that there are things that we have missed. We have on the benchmark page documented exactly how we did run the benchmark so that it should be easy for anyone to repeat and verify our results. The benchmarks are usually run with and without the `--fast' option. When run with `--fast' we are trying to use every trick the server can do to get the code to execute as fast as possible. The idea is that the normal run should show how the server would work in a default setup and the `--fast' run shows how the server would do if the application developer would use extensions in the server to make his application run faster. When running with PostgreSQL and `--fast' we do a `VACUUM()' after every major table `UPDATE' and `DROP TABLE' to make the database in perfect shape for the following `SELECT's. The time for `VACUUM()' is measured separately. When running with PostgreSQL 7.1.1 we could, however, not run with `--fast' because during the `INSERT' test, the postmaster (the PostgreSQL deamon) died and the database was so corrupted that it was impossible to restart postmaster. After this happened twice, we decided to postpone the `--fast' test until next PostgreSQL release. The details about the machine we run the benchmark can be found on the benchmark page. Before going to the other benchmarks we know of, we would like to give some background on benchmarks: It's very easy to write a test that shows ANY database to be best database in the world, by just restricting the test to something the database is very good at and not test anything that the database is not good at. If one after this publishes the result with a single figure, things are even easier. This would be like us measuring the speed of MySQL compared to PostgreSQL by looking at the summary time of the MySQL benchmarks on our web page. Based on this MySQL would be more than 40 times faster than PostgreSQL, something that is of course not true. We could make things even worse by just taking the test where PostgreSQL performs worst and claim that MySQL is more than 2000 times faster than PostgreSQL. The case is that MySQL does a lot of optimizations that PostgreSQL doesn't do. This is of course also true the other way around. An SQL optimizer is a very complex thing, and a company could spend years on just making the optimizer faster and faster. When looking at the benchmark results you should look for things that you do in your application and just use these results to decide which database would be best suited for your application. The benchmark results also shows things a particular database is not good at and should give you a notion about things to avoid and what you may have to do in other ways. We know of two benchmark tests that claims that PostgreSQL performs better than MySQL. These both where multi-user tests, a test that we here at MySQL AB haven't had time to write and include in the benchmark suite, mainly because it's a big task to do this in a manner that is fair against all databases. One is the benchmark paid for by Great Bridge, which you can read about at: `http://www.greatbridge.com/about/press.php?content_id=4'. This is the probably worst benchmark we have ever seen anyone conduct. This was not only tuned to only test what PostgreSQL is absolutely best at, it was also totally unfair against every other database involved in the test. *NOTE*: We know that not even some of the main PostgreSQL developers did like the way Great Bridge conducted the benchmark, so we don't blame them for the way the benchmark was made. This benchmark has been condemned in a lot of postings and newsgroups so we will here just shortly repeat some things that where wrong with it. * The tests were run with an expensive commercial tool, that makes it impossible for an open source company like us to verify the benchmarks, or even check how the benchmarks were really done. The tool is not even a true benchmark tool, but an application/setup testing tool. To refer this as a "standard" benchmark tool is to stretch the truth a long way. * Great Bridge admitted that they had optimized the PostgreSQL database (with `VACUUM()' before the test) and tuned the startup for the tests, something they hadn't done for any of the other databases involved. To say "This process optimizes indexes and frees up disk space a bit. The optimized indexes boost performance by some margin." Our benchmarks clearly indicate that the difference in running a lot of selects on a database with and without `VACUUM()' can easily differ by a factor of ten. * The test results were also strange. The AS3AP test documentation mentions that the test does "selections, simple joins, projections, aggregates, one-tuple updates, and bulk updates". PostgreSQL is good at doing `SELECT's and `JOIN's (especially after a `VACUUM()'), but doesn't perform as well on `INSERT's or `UPDATE's. The benchmarks seem to indicate that only `SELECT's were done (or very few updates). This could easily explain they good results for PostgreSQL in this test. The bad results for MySQL will be obvious a bit down in this document. * They did run the so-called benchmark from a Windows machine against a Linux machine over ODBC, a setup that no normal database user would ever do when running a heavy multi-user application. This tested more the ODBC driver and the Windows protocol used between the clients than the database itself. * When running the database against Oracle and MS-SQL (Great Bridge has indirectly indicated that the databases they used in the test), they didn't use the native protocol but instead ODBC. Anyone that has ever used Oracle knows that all real application uses the native interface instead of ODBC. Doing a test through ODBC and claiming that the results had anything to do with using the database in a real-world situation can't be regarded as fair. They should have done two tests with and without ODBC to provide the right facts (after having got experts to tune all involved databases of course). * They refer to the TPC-C tests, but they don't mention anywhere that the test they did was not a true TPC-C test and they were not even allowed to call it a TPC-C test. A TPC-C test can only be conducted by the rules approved by the TPC Council (`http://www.tpc.org'). Great Bridge didn't do that. By doing this they have both violated the TPC trademark and miscredited their own benchmarks. The rules set by the TPC Council are very strict to ensure that no one can produce false results or make unprovable statements. Apparently Great Bridge wasn't interested in doing this. * After the first test, we contacted Great Bridge and mentioned to them some of the obvious mistakes they had done with MySQL: - Running with a debug version of our ODBC driver - Running on a Linux system that wasn't optimized for threads - Using an old MySQL version when there was a recommended newer one available - Not starting MySQL with the right options for heavy multi-user use (the default installation of MySQL is tuned for minimal resource use). Great Bridge did run a new test, with our optimized ODBC driver and with better startup options for MySQL, but refused to either use our updated glibc library or our standard binary (used by 80% of our users), which was statically linked with a fixed glibc library. According to what we know, Great Bridge did nothing to ensure that the other databases were set up correctly to run well in their test environment. We are sure however that they didn't contact Oracle or Microsoft to ask for their advice in this matter ;) * The benchmark was paid for by Great Bridge, and they decided to publish only partial, chosen results (instead of publishing it all). Tim Perdue, a long time PostgreSQL fan and a reluctant MySQL user published a comparison on phpbuider (http://www.phpbuilder.com/columns/tim20001112.php3). When we became aware of the comparison, we phoned Tim Perdue about this because there were a lot of strange things in his results. For example, he claimed that MySQL had a problem with five users in his tests, when we know that there are users with similar machines as his that are using MySQL with 2000 simultaneous connections doing 400 queries per second. (In this case the limit was the web bandwidth, not the database.) It sounded like he was using a Linux kernel that either had some problems with many threads, such as kernels before 2.4, which had a problem with many threads on multi-CPU machines. We have documented in this manual how to fix this and Tim should be aware of this problem. The other possible problem could have been an old glibc library and that Tim didn't use a MySQL binary from our site, which is linked with a corrected glibc library, but had compiled a version of his own with. In any of the above cases, the symptom would have been exactly what Tim had measured. We asked Tim if we could get access to his data so that we could repeat the benchmark and if he could check the MySQL version on the machine to find out what was wrong and he promised to come back to us about this. He has not done that yet. Because of this we can't put any trust in this benchmark either :( Over time things also changes and the above benchmarks are not that relevant anymore. MySQL now have a couple of different table handlers with different speed/concurrency tradeoffs. *Note Table types::. It would be interesting to see how the above tests would run with the different transactional table types in MySQL. PostgreSQL has of course also got new features since the test was made. As the above test are not publicly available there is no way for us to know how the database would preform in the same tests today. Conclusion: The only benchmarks that exist today that anyone can download and run against MySQL and PostgreSQL is the MySQL benchmarks. We here at MySQL believe that open source databases should be tested with open source tools! This is the only way to ensure that no one does tests that nobody can reproduce and use this to claim that a database is better than another. Without knowing all the facts it's impossible to answer the claims of the tester. The thing we find strange is that every test we have seen about PostgreSQL, that is impossible to reproduce, claims that PostgreSQL is better in most cases while our tests, which anyone can reproduce, clearly shows otherwise. With this we don't want to say that PostgreSQL isn't good at many things (it is!) or that it isn't faster than MySQL under certain conditions. We would just like to see a fair test where they are very good so that we could get some friendly competition going! For more information about our benchmarks suite *Note MySQL Benchmarks::. We are working on an even better benchmark suite, including multi user tests, and a better documentation of what the individual tests really do and how to add more tests to the suite. MySQL and the future (The TODO) =============================== This appendix lists the features that we plan to implement in MySQL. Everything in this list is approximately in the order it will be done. If you want to affect the priority order, please register a license or support us and tell us what you want to have done more quickly. *Note Licensing and Support::. The plan is that we in the future will support the full ANSI SQL99 standard, but with a lot of useful extensions. The challenge is to do this without sacrifying the speed or compromise the code. Things that should be in 4.0 ---------------------------- We plan to make MySQL Version 4.0 a "quick" release where we only add some new stuff to enable others to help us with developing new features into Version 4.1. The MySQL 4.0 version should only take us about a month to make after which we want to stabilize it and start working on Version 4.1. Version 4.0 should have the following new features: The news section for 4.0 includes a list of the features we have already implemented in the 4.0 tree. *Note News-4.0.x::. * New table definition file format (`.frm' files) This will enable us to not run out of bits when adding more table options. One will still be able to use the old `.frm' file format with 4.0. All newly created tables will, however, use the new format. The new file format will enable us to add new column types, more options for keys and `FOREIGN KEY' support. * `mysqld' as a library. This will have the same interface as the standard MySQL client (with an extra function to just set up startup parameters) but will be faster (no TCP/IP or socket overhead), smaller and much easier to use for embedded products. One will be able to define at link time if one wants to use the client/server model or a stand-alone application just by defining which library to link with. The `mysqld' will support all standard MySQL features and one can use it in a threaded client to run different queries in each thread. * Replication should work with `RAND()' and user variables `@var'. * Online backup with very low performance penalty. The online backup will make it easy to add a new replication slave without taking down the master. * `DELETE FROM table_name' will return the number of deleted rows. For fast execution one should use `TRUNCATE table_name'. * Allow `DELETE' on `MyISAM' tables to use the record cache. To do this, we need to update the threads record cache when we update the `.MYD' file. * Better replication. * More functions for full-text search. *Note Fulltext Features to Appear in MySQL 4.0::. * Character set casts and syntax for handling multiple character sets. * Allow users to change startup options without taking down the server. * Help for all commands from the client. * Secure connections (with SSL). * Extend the optimizer to be able to optimize some `ORDER BY key_name DESC' queries. * `SHOW COLUMNS FROM table_name' (used by `mysql' client to allow expansions of column names) should not open the table, but only the definition file. This will require less memory and be much faster. * New key cache * When using `SET CHARACTER SET' we should translate the whole query at once and not only strings. This will enable users to use the translated characters in database, table and column names. * Add a portable interface over `gethostbyaddr_r()' so that we can change `ip_to_hostname()' to not block other threads while doing DNS lookups. * Add `record_in_range()' method to `MERGE' tables to be able to choose the right index when there is many to choose from. We should also extend the info interface to get the key distribution for each index, of `analyze' is run on all sub tables. * `SET SQL_DEFAULT_TABLE_TYPE=[MyISAM | INNODB | BDB | HEAP]'. Things that must be done in the real near future ------------------------------------------------ * Fail safe replication. * Subqueries. `select id from t where grp in (select grp from g where u > 100)' * Derived tables. select a.col1, b.col2 from (select max(col1) as col1 from root_table ) a, other_table b where a.col1=b.col1 This could be done by automatically creating temporary tables for the derived tables for the duration of the query. * Add `PREPARE' of statements and sending of parameters to `mysqld'. * Extend the server/client protocol to support warnings. * Add options to the server/protocol protocol to get progress notes for long running commands. * Add database and real table name (in case of alias) to the MYSQL_FIELD structure. * Don't allow more than a defined number of threads to run MyISAM recover at the same time. * Change `INSERT ... SELECT' to optionally use concurrent inserts. * Implement `RENAME DATABASE'. To make this safe for all table handlers, it should work as follows: * Create the new database. * For every table do a rename of the table to another database, as we do with the `RENAME' command. * Drop the old database. * Return the original field types() when doing `SELECT MIN(column) ... GROUP BY'. * Multiple result sets. * Change the protocol to allow binary transfer of values. To do this efficiently, we need to add an API to allow binding of variables. * Make it possible to specify `long_query_time' with a granularity in microseconds. * Add a configurable prompt to the `mysql' command line client, with options like database in use, time and date... * Add range checking to `MERGE' tables. * Link the `myisampack' code into the server. * Port of MySQL to BeOS. * Port of the MySQL clients to LynxOS. * Add a temporary key buffer cache during `INSERT/DELETE/UPDATE' so that we can gracefully recover if the index file gets full. * If you perform an `ALTER TABLE' on a table that is symlinked to another disk, create temporary tables on this disk. * Implement a `DATE/DATETIME' type that handles time zone information properly, so that dealing with dates in different time zones is easier. * FreeBSD and MIT-pthreads; Do sleeping threads take CPU? * Check if locked threads take any CPU. * Fix configure so that one can compile all libraries (like `MyISAM') without threads. * Add an option to periodically flush key pages for tables with delayed keys if they haven't been used in a while. * Allow join on key parts (optimization issue). * `INSERT SQL_CONCURRENT' and `mysqld --concurrent-insert' to do a concurrent insert at the end of the file if the file is read-locked. * Remember `FOREIGN' key definitions in the `.frm' file. * Cascading `DELETE' * Server side cursors. * Check if `lockd' works with modern Linux kernels; If not, we have to fix `lockd'! To test this, start `mysqld' with `--enable-locking' and run the different fork* test suits. They shouldn't give any errors if `lockd' works. * Allow SQL variables in `LIMIT', like in `LIMIT @a,@b'. * Allow update of variables in `UPDATE' statements. For example: `UPDATE TABLE foo SET @a=a+b,a=@a, b=@a+c' * Change when user variables are updated so that one can use them with `GROUP BY', as in the following example: `SELECT id, @a:=count(*), sum(sum_col)/@a FROM table_name GROUP BY id'. * Don't add automatic `DEFAULT' values to columns. Give an error when using an `INSERT' that doesn't contain a column that doesn't have a `DEFAULT'. * Caching of queries and results. This should be done as a separated module that examines each query and if this is query is in the cache the cached result should be returned. When one updates a table one should remove as few queries as possible from the cache. This should give a big speed bost on machines with much RAM where queries are often repeated (like WWW applications). One idea would be to only cache queries of type: `SELECT CACHED ...' * Fix `libmysql.c' to allow two `mysql_query()' commands in a row without reading results or give a nice error message when one does this. * Optimize `BIT' type to take 1 bit (now `BIT' takes 1 char). * Check why MIT-pthreads `ctime()' doesn't work on some FreeBSD systems. * Add an `IMAGE' option to `LOAD DATA INFILE' to not update `TIMESTAMP' and `AUTO_INCREMENT' fields. * Added `LOAD DATE INFILE.. UPDATE' syntax. * For tables with primary keys, if the data contains the primary key, entries matching that primary key are updated from the remainder of the columns. However, columns MISSING from the incoming data feed are not touched. * For tables tables with primary keys that are missing some part of the key in the incoming data stream, or that have no primary key, the feed is treated as a `LOAD DATA INFILE ... REPLACE INTO' is now. * Make `LOAD DATA INFILE' understand syntax like: LOAD DATA INFILE 'file_name.txt' INTO TABLE tbl_name TEXT_FIELDS (text_field1, text_field2, text_field3) SET table_field1=concatenate(text_field1, text_field2), table_field3=23 IGNORE text_field3 This can be used to skip over extra columns in the text file, or update columns based on expressions of the read data... * `LOAD DATA INFILE 'file_name' INTO TABLE 'table_name' ERRORS TO err_table_name' This would cause any errors and warnings to be logged into the err_table_name table. That table would have a structure like: line_number - line number in data file error_message - the error/warning message and maybe data_line - the line from the data file * Add true `VARCHAR' support (There is already support for this in MyISAM). * Automatic output from `mysql' to netscape. * `LOCK DATABASES'. (with various options) * Change sort to allocate memory in "hunks" to get better memory utilization. * `DECIMAL' and `NUMERIC' types can't read exponential numbers; `Field_decimal::store(const char *from,uint len)' must be recoded to fix this. * Functions: ADD_TO_SET(value,set) and REMOVE_FROM_SET(value,set) * Add use of `t1 JOIN t2 ON ...' and `t1 JOIN t2 USING ...' Currently, you can only use this syntax with `LEFT JOIN'. * Add full support for `unsigned long long' type. * Many more variables for `show status'. Counts for: `INSERT'/`DELETE'/`UPDATE' statements. Records reads and updated. Selects on 1 table and selects with joins. Mean number of tables in select. Number of `ORDER BY' and `GROUP BY' queries. * If you abort `mysql' in the middle of a query, you should open another connection and kill the old running query. Alternatively, an attempt should be made to detect this in the server. * Add a handler interface for table information so you can use it as a system table. This would be a bit slow if you requested information about all tables, but very flexible. `SHOW INFO FROM tbl_name' for basic table information should be implemented. * Add support for UNICODE. * `NATURAL JOIN' and `UNION JOIN' * Allow `select a from crash_me left join crash_me2 using (a)'; In this case a is assumed to come from the crash_me table. * Fix that `ON' and `USING' works with the `JOIN' join type. * Oracle like `CONNECT BY PRIOR ...' to search hierarchy structures. * `mysqladmin copy database new-database'. - Requires COPY command to be added to `mysqld' * Processlist should show number of queries/thread. * `SHOW HOSTS' for printing information about the hostname cache. * `DELETE' and `REPLACE' options to the `UPDATE' statement (this will delete rows when one gets a duplicate key error while updating). * Change the format of `DATETIME' to store fractions of seconds. * Add all missing ANSI92 and ODBC 3.0 types. * Change table names from empty strings to `NULL' for calculated columns. * Don't use 'Item_copy_string' on numerical values to avoid number->string->number conversion in case of: `SELECT COUNT(*)*(id+0) FROM table_name GROUP BY id' * Make it possible to use the new GNU regexp library instead of the current one (The GNU library should be much faster than the old one). * Change that `ALTER TABLE' doesn't abort clients that executes `INSERT DELAYED'. * Fix that when columns referenced in an `UPDATE' clause contains the old values before the update started. * `myisamchk', `REPAIR' and `OPTIMIZE TABLE' should be able to handle cases where the data and/or index files are symbolic links. * Add simulation of `pread()'/`pwrite()' on Windows to enable concurrent inserts. * A logfile analyzer that could parsed out information about which tables are hit most often, how often multi-table joins are executed, etc. It should help users identify areas or table design that could be optimized to execute much more efficient queries. * Add `SUM(DISTINCT)' * Add `ANY()',`EVERY()' and `SOME()' group functions. In ANSI SQL these only works on boolean columns, but we can extend these to work on any columns/expressions by applying: value == 0 -> FALSE and value <> 0 -> TRUE. * Fix that the type for `MAX(column)' is the same as the column type. create table t1 (a DATE); insert into t1 values (now()); create table t2 select max(a) from t1; show columns from t2; * Come up with a nice syntax for a statement that will `UPDATE' the row if it exists and `INSERT' a new row if the row didn't exist. (Like `REPLACE' works with `INSERT' / `DELETE') Things that have to be done sometime ------------------------------------ * Implement function: `get_changed_tables(timeout,table1,table2,...)' * Atomic multi-table updates, eg `update items,month set items.price=month.price where items.id=month.id;'; * Change reading through tables to use memmap when possible. Now only compressed tables use memmap. * Add a new privilege *'Show_priv'* for `SHOW' commands. * Make the automatic timestamp code nicer. Add timestamps to the update log with `SET TIMESTAMP=#;' * Use read/write mutex in some places to get more speed. * Full foreign key support. One probably wants to implement a procedural language first. * Simple views (first on one table, later on any expression). * Automatically close some tables if a table, temporary table or temporary files gets error 23 (not enough open files). * When one finds a field=#, change all occurrences of field to #. Now this is only done for some simple cases. * Change all const expressions with calculated expressions if possible. * Optimize key = expression. At the moment only key = field or key = constant are optimized. * Join some of the copy functions for nicer code. * Change `sql_yacc.yy' to an inline parser to reduce its size and get better error messages (5 days). * Change the parser to use only one rule per different number of arguments in function. * Use of full calculation names in the order part. (For ACCESS97) * `UNION', `MINUS', `INTERSECT' and `FULL OUTER JOIN'. (Currently only `LEFT OUTER JOIN' is supported) * Allow `UNIQUE' on fields that can be `NULL'. * `SQL_OPTION MAX_SELECT_TIME=#' to put a time limit on a query. * Make the update log to a database. * Negative `LIMIT' to retrieve data from the end. * Alarm around client connect/read/write functions. * Please note the changes to `safe_mysqld': according to FSSTND (which Debian tries to follow) PID files should go into `/var/run/<progname>.pid' and log files into `/var/log'. It would be nice if you could put the "DATADIR" in the first declaration of "pidfile" and "log", so the placement of these files can be changed with a single statement. * Allow a client to request logging. * Add use of `zlib()' for `gzip'-ed files to `LOAD DATA INFILE'. * Fix sorting and grouping of `BLOB' columns (partly solved now). * Stored procedures. This is currently not regarded to be very important as stored procedures are not very standardized yet. Another problem is that true stored procedures make it much harder for the optimizer and in many cases the result is slower than before We will, on the other hand, add a simple (atomic) update language that can be used to write loops and such in the MySQL server. * Change to use semaphores when counting threads. One should first implement a semaphore library to MIT-pthreads. * Don't assign a new `AUTO_INCREMENT' value when one sets a column to 0. Use `NULL' instead. * Add full support for `JOIN' with parentheses. * As an alternative for one thread / connection manage a pool of threads to handle the queries. * Allow one to get more than one lock with `GET_LOCK'. When doing this, one must also handle the possible deadlocks this change will introduce. Time is given according to amount of work, not real time. Some things we don't have any plans to do ----------------------------------------- * Nothing; In the long run we plan to be fully ANSI 92 / ANSI 99 compliant. MySQL Installation ****************** This chapter describes how to obtain and install MySQL: * For a list of sites from which you can obtain MySQL, see *Note Getting MySQL: Getting MySQL. * To see which platforms are supported, see *Note Which OS::. Please note that not all supported system are equally good for running MySQL on them. On some it is much more robust and efficient than others - see *Note Which OS:: for details. * Several versions of MySQL are available in both binary and source distributions. We also provide public access to our current source tree for those who want to see our most recent developments and help us test new code. To determine which version and type of distribution you should use, see *Note Which version::. When in doubt, use the binary distribution. * Installation instructions for binary and source distributions are described in *Note Installing binary:: and *Note Installing source::. Each set of instructions includes a section on system-specific problems you may run into. * For post-installation procedures, see *Note Post-installation::. These procedures apply whether you install MySQL using a binary or source distribution. Quick Standard Installation of MySQL ==================================== Installing MySQL on Linux ------------------------- The recommended way to install MySQL on Linux is by using an RPM file. The MySQL RPMs are currently being built on a RedHat Version 6.2 system but should work on other versions of Linux that support `rpm' and use `glibc'. If you have problems with an RPM file, for example, if you receive the error "`Sorry, the host 'xxxx' could not be looked up'", see *Note Binary notes-Linux::. The RPM files you may want to use are: * `MySQL-VERSION.i386.rpm' The MySQL server. You will need this unless you only want to connect to a MySQL server running on another machine. * `MySQL-client-VERSION.i386.rpm' The standard MySQL client programs. You probably always want to install this package. * `MySQL-bench-VERSION.i386.rpm' Tests and benchmarks. Requires Perl and msql-mysql-modules RPMs. * `MySQL-devel-VERSION.i386.rpm' Libraries and include files needed if you want to compile other MySQL clients, such as the Perl modules. * `MySQL-VERSION.src.rpm' This contains the source code for all of the above packages. It can also be used to try to build RPMs for other architectures (for example, Alpha or SPARC). To see all files in an RPM package, run: shell> rpm -qpl MySQL-VERSION.i386.rpm To perform a standard minimal installation, run: shell> rpm -i MySQL-VERSION.i386.rpm MySQL-client-VERSION.i386.rpm To install just the client package, run: shell> rpm -i MySQL-client-VERSION.i386.rpm The RPM places data in `/var/lib/mysql'. The RPM also creates the appropriate entries in `/etc/rc.d/' to start the server automatically at boot time. (This means that if you have performed a previous installation, you may want to make a copy of your previously installed MySQL startup file if you made any changes to it, so you don't lose your changes.) After installing the RPM file(s), the `mysqld' daemon should be running and you should now be able to start using MySQL. *Note Post-installation::. If something goes wrong, you can find more information in the binary installation chapter. *Note Installing binary::. Installing MySQL on Windows --------------------------- The following instructions apply to precompiled binary distributions. If you download a source distribution, you will have to compile and install it yourself. If you don't have a copy of the MySQL distribution, you should first download one from `http://www.mysql.com/downloads/mysql-3.23.html'. If you plan to connect to MySQL from some other program, you will probably also need the *MyODBC* driver. You can find this at the *MyODBC* download page (`http://www.mysql.com/downloads/api-myodbc.html'). To install either distribution, unzip it in some empty directory and run the `Setup.exe' program. By default, MySQL-Windows is configured to be installed in `C:\mysql'. If you want to install MySQL elsewhere, install it in `C:\mysql' first, then move the installation to where you want it. If you do move MySQL, you must indicate where everything is located by supplying a `--basedir' option when you start the server. For example, if you have moved the MySQL distribution to `D:\programs\mysql', you must start `mysqld' like this: C:\> D:\programs\mysql\bin\mysqld --basedir D:\programs\mysql Use `mysqld --help' to display all the options that `mysqld' understands! With all newer MySQL versions, you can also create a `C:\my.cnf' file that holds any default options for the MySQL server. Copy the file `\mysql\my-xxxxx.cnf' to `C:\my.cnf' and edit it to suit your setup. Note that you should specify all paths with `/' instead of `\'. If you use `\', you need to specify it twice, because `\' is the escape character in MySQL. *Note Option files::. Starting with MySQL 3.23.38, the Windows distribution includes both the normal and the *MySQL-Max* binaries. The main benefit of using the normal `mysqld.exe' binary is that it's a little faster and uses less resources. Here is a list of the different MySQL servers you can use: `mysqld' Compiled with full debugging and automatic memory allocation checking, symbolic links, BDB and InnoDB tables. `mysqld-opt' Optimized binary with no support for transactional tables. `mysqld-nt' Optimized binary for NT with support for named pipes. You can run this version on Win98, but in this case no named pipes are created and you must have TCP/IP installed. `mysqld-max' Optimized binary with support for symbolic links, BDB and InnoDB tables. `mysqld-max-nt' Like `mysqld-max', but compiled with support for named pipes. All of the above binaries are optimized for the Pentium Pro processor but should work on any Intel processor >= i386. NOTE: If you want to use InnoDB tables, there are certain startup options that must be specified in your `my.ini' file! *Note InnoDB start::. General Installation Issues =========================== How to Get MySQL ---------------- Check the MySQL home page (http://www.mysql.com/) for information about the current version and for downloading instructions. Our main download mirror is located at: `http://download.sourceforge.net/mirrors/mysql/' If you are interested in becoming a MySQL mirror site, you may anonymously rsync with: `rsync://download.sourceforge.net/mysql/'. Please send e-mail to <webmaster@mysql.com> notifying us of your mirror to be added to the list below. If you have problems downloading from our main site, try using one of the mirrors listed below. Please report bad or out-of-date mirrors to <webmaster@mysql.com>. *Europe:* * Austria [Univ. of Technology/Vienna] WWW (http://gd.tuwien.ac.at/db/mysql/) FTP (ftp://gd.tuwien.ac.at/db/mysql/) * Bulgaria [online.bg/Sofia] WWW (http://mysql.online.bg/) FTP (ftp://mysql.online.bg/) * Czech Republic [Masaryk University in Brno] WWW (http://mysql.linux.cz/index.html) FTP (ftp://ftp.fi.muni.cz/pub/mysql/) * Czech Republic [www.sopik.cz] WWW (http://www.mysql.cz/) * Czech Republic [www.gin.cz] WWW (http://mysql.gin.cz/) FTP (ftp://ftp.gin.cz/pub/MIRRORS/www.mysql.com/) * Denmark [Borsen] WWW (http://mysql.borsen.dk/) * Denmark [SunSITE] WWW (http://mirrors.sunsite.dk/mysql/) FTP (ftp://sunsite.dk/mirrors/mysql/) * Estonia [OKinteractive] WWW (http://mysql.mirror.ok.ee) * France [mtesa.net] WWW (http://mysql.mtesa.net/) * France [fastorama.com, Chatenois] WWW (http://mysql.fastorama.com/) FTP (ftp://ftp.fastorama.com/unix/mysql/) * Finland [tonnikala.net] WWW (http://mysql.tonnikala.org/) * Germany [Kernelnotes.de, Bonn] WWW (http://www.kernelnotes.de/mysql/) FTP (ftp://ftp.kernelnotes.de/pub/mirror/mysql.org/) * Germany [Wolfenbuettel] WWW (http://www.fh-wolfenbuettel.de/ftp/pub/database/mysql/) FTP (ftp://ftp.fh-wolfenbuettel.de/pub/database/mysql/) * Greece [NTUA, Athens] WWW (http://www.ntua.gr/mysql/) FTP (ftp://ftp.ntua.gr/pub/databases/mysql/) * Hungary [Xenia] WWW (http://mysql.sote.hu/) FTP (ftp://xenia.sote.hu/pub/mirrors/www.mysql.com/) * Hungary [TiszaneT] WWW (http://mysql.tiszanet.hu/) FTP (ftp://mysql.tiszanet.hu/pub/mirrors/mysql/) * Hungary [stop.hu] WWW (http://mysql.mirror.stop.hu/) * Iceland [GM] WWW (http://mysql.gm.is/) FTP (ftp://ftp.gm.is/pub/mysql/) * Italy [feelinglinux.com] WWW (http://mysql.feelinglinux.com/) * Italy [Teta Srl] WWW (http://www.teta.it/mysql/) * Italy [tzone.it] WWW (http://mysql.tzone.it/) * Ireland [Esat Net] WWW (http://ftp.esat.net/mirrors/download.sourceforge.net/pub/mirrors/mysql/) FTP (ftp://ftp.esat.net/mirrors/download.sourceforge.net/pub/mirrors/mysql/) * Latvia [linux.lv] FTP (ftp://ftp.linux.lv/pub/software/mysql/) * Netherlands [Silverpoint] WWW (http://mysql.silverpoint.nl/) * Netherlands [Widexs BV] WWW (http://mysql.widexs.nl/) FTP (ftp://mysql.widexs.nl/pub/mysql/) * Netherlands [ProServe] WWW (http://mysql.proserve.nl/) * Poland [Sunsite] WWW (http://sunsite.icm.edu.pl/mysql/) FTP (ftp://sunsite.icm.edu.pl/pub/unix/mysql/) * Poland [ncservice.com/Gdansk] WWW (http://mysql.service.net.pl/) * Portugal [Netc] WWW (http://ftp.netc.pt/pub/mysql/) FTP (ftp://ftp.netc.pt/pub/mysql/) * Romania [roedu.net/Bucharest] FTP (ftp://ftp.roedu.net/pub/mirrors/ftp.mysql.com/) * Russia [DirectNet] WWW (http://mysql.directnet.ru/) FTP (ftp://ftp.dn.ru/pub/MySQL/) * Russia [Scientific Center/Chernogolovka] FTP (ftp://ftp.chg.ru/pub/databases/mysql/) * Sweden [Sunet] WWW (http://ftp.sunet.se/pub/unix/databases/relational/mysql/) FTP (ftp://ftp.sunet.se/pub/unix/databases/relational/mysql/) * Switzerland [Sunsite] WWW (http://sunsite.cnlab-switch.ch/ftp/mirror/mysql/) FTP (ftp://sunsite.cnlab-switch.ch/mirror/mysql/) * UK [PLiG/UK] WWW (http://ftp.plig.org/pub/mysql/) FTP (ftp://ftp.plig.org/pub/mysql/) * Ukraine [PACO] WWW (http://mysql.paco.net.ua) FTP (ftp://mysql.paco.net.ua/) * Ukraine [ISP Alkar Teleport/Dnepropetrovsk] WWW (http://mysql.dp.ua/) * Yugoslavia [bolex.co.yu] WWW (http://mysql.boa.org.yu/) FTP (ftp://ftp.linux.org.yu/pub/MySQL/) *North America:* * Canada [Tryc] WWW (http://web.tryc.on.ca/mysql/) * USA [Hurricane Electric/San Jose] WWW (http://mysql.he.net/) * USA [ValueClick, Los Angeles CA] WWW (http://mysql.valueclick.com/) FTP (ftp://mysql.valueclick.com/pub/mysql/Downloads/) * USA [Wisconsin University/Wisconsin] WWW (http://mirror.sit.wisc.edu/mysql/) FTP (ftp://mirror.sit.wisc.edu/mirrors/mysql/) * USA [LinuxWired/Scottsdale, AZ] WWW (http://mysql.linuxwired.net/) FTP (ftp://ftp.linuxwired.net/pub/mirrors/mysql/) * USA [adgrafix.com/Boston, MA] WWW (http://mysql.adgrafix.com/) * USA [netNumina/Cambridge, MA] WWW (http://mysql.mirrors.netnumina.com/) * USA [Ahaza Systems/Seattle, WA] WWW (http://mysql.mirrortree.com/) FTP (ftp://mysql.mirrortree.com/pub/mysql/) *South America:* * Argentina [bannerlandia.com] WWW (http://mysql.bannerlandia.com.ar/) FTP (ftp://mysql.bannerlandia.com.ar/mirrors/mysql/) * Chile [Vision] WWW (http://mysql.vision.cl/) * Chile [PSINet] WWW (http://mysql.psinet.cl/) FTP (ftp://ftp.psinet.cl/pub/database/mysql/) * Chile [Tecnoera] WWW (http://mysql.tecnoera.com/) *Asia:* * China [linuxforum.net] WWW (http://www2.linuxforum.net/mirror/mysql/) * China [HKLPG/Hong Kong] WWW (http://mysql.hklpg.org) * China [Gremlins/Hong Kong] WWW (http://mysql.gremlins.com.hk/) FTP (ftp://ftp.mirrors.gremlins.com.hk/mysql/) * China [shellhung.org/Hong Kong] WWW (http://mysql.shellhung.org/) FTP (ftp://ftp.shellhung.org/pub/Mirror/mysql/) * Indonesia [incaf.net] WWW (http://mysql.incaf.net/) * Indonesia [web.id] WWW (http://mysql.itb.web.id/) FTP (ftp://mysql.itb.web.id/pub/MySQL/) * Japan [Soft Agency] WWW (http://www.softagency.co.jp/MySQL) * Japan [u-aizu.ac.jp/Aizu] FTP (ftp://ftp.u-aizu.ac.jp/ftp/pub/dbms/mysql/mysql.com) * South Korea [Webiiz] WWW (http://mysql.webiiz.com/) * South Korea [PanworldNet] WWW (http://mysql.holywar.net/) * Singapore [HJC] WWW (http://mysql.hjc.edu.sg) FTP (ftp://ftp.hjc.edu.sg/mysql) * Taiwan [TTN] WWW (http://mysql.ttn.net) * Taiwan [nctu.edu/HsinChu] WWW (http://mysql.nctu.edu.tw/) *Africa:* * South-Africa [Mweb] WWW (http://www.mysql.mweb.co.za/) * South Africa [The Internet Solution/Johannesburg] FTP (ftp://ftp.is.co.za/linux/mysql/) Operating Systems Supported by MySQL ------------------------------------ We use GNU Autoconf, so it is possible to port MySQL to all modern systems with working Posix threads and a C++ compiler. (To compile only the client code, a C++ compiler is required but not threads.) We use and develop the software ourselves primarily on Sun Solaris (Versions 2.5 - 2.7) and SuSE Linux Version 7.x. Note that for many operating systems, the native thread support works only in the latest versions. MySQL has been reported to compile successfully on the following operating system/thread package combinations: * AIX 4.x with native threads. *Note IBM-AIX::. * Amiga. * BSDI 2.x with the included MIT-pthreads package. *Note BSDI::. * BSDI 3.0, 3.1 and 4.x with native threads. *Note BSDI::. * DEC Unix 4.x with native threads. *Note Alpha-DEC-UNIX::. * FreeBSD 2.x with the included MIT-pthreads package. *Note FreeBSD::. * FreeBSD 3.x and 4.x with native threads. *Note FreeBSD::. * HP-UX 10.20 with the included MIT-pthreads package. *Note HP-UX 10.20::. * HP-UX 11.x with the native threads. *Note HP-UX 11.x::. * Linux 2.0+ with LinuxThreads 0.7.1+ or `glibc' 2.0.7+. *Note Linux::. * Mac OS X Server. *Note Mac OS X::. * NetBSD 1.3/1.4 Intel and NetBSD 1.3 Alpha (Requires GNU make). *Note NetBSD::. * OpenBSD > 2.5 with native therads. OpenBSD < 2.5 with the included MIT-pthreads package. *Note OpenBSD::. * OS/2 Warp 3, FixPack 29 and OS/2 Warp 4, FixPack 4. *Note OS/2::. * SGI Irix 6.x with native threads. *Note SGI-Irix::. * Solaris 2.5 and above with native threads on SPARC and x86. *Note Solaris::. * SunOS 4.x with the included MIT-pthreads package. *Note Solaris::. * SCO OpenServer with a recent port of the FSU Pthreads package. *Note SCO::. * SCO UnixWare 7.0.1. *Note SCO Unixware::. * Tru64 Unix * Win95, Win98, NT, and Win2000. *Note Windows::. Note that not all platforms are suited equally well for running MySQL. How well a certain platform is suited for a high-load mission critical MySQL server is determined by the following factors: * General stability of the thread library. A platform may have excellent reputation otherwise, but if the thread library is unstable in the code that is called by MySQL, even if everything else is perfect, MySQL will be only as stable as the thread library. * The ability of the kernel and/or thread library to take advantage of *SMP* on multi-processor systems. In other words, when a process creates a thread, it should be possible for that thread to run on a different CPU than the original process. * The ability of the kernel and/or the thread library to run many threads which acquire/release a mutex over a short critical region frequently without excessive context switches. In other words, if the implementation of `pthread_mutex_lock()' is too anxious to yield CPU, this will hurt MySQL tremendously. If this issue is not taken care of, adding extra CPUs will actually make MySQL slower. * General file system stability/performance. * Ability of the file system to deal with large files at all and deal with them efficiently, if your tables are big. * Our level of expertise here at MySQL AB with the platform. If we know a platform well, we introduce platform-specific optimizations/fixes enabled at compile time. We can also provide advice on configuring your system optimally for MySQL. * The amount of testing of similar configurations we have done internally. * The number of users that have successfully run MySQL on that platform in similar configurations. If this number is high, the chances of hitting some platform-specific surprise are much smaller. Based on the above criteria, the best platforms for running MySQL at this point are x86 with SuSE Linux 7.1, 2.4 kernel and ReiserFS (or any similar Linux distribution) and Sparc with Solaris 2.7 or 2.8. FreeBSD comes third, but we really hope it will join the top club once the thread library is improved. We also hope that at some point we will be able to include all other platforms on which MySQL compiles, runs ok, but not quite with the same level of stability and performance, into the top category. This will require some effort on our part in cooperation with the developers of the OS/library components MySQL depends upon. If you are interested in making one of those components better, are in a position to influence their development, and need more detailed instructions on what MySQL needs to run better, send an e-mail to <internals@lists.mysql.com>. Please note that the comparison above is not to say that one OS is better or worse than the other in general. We are talking about choosing a particular OS for a dedicated purpose - running MySQL, and compare platforms in that regard only. With this in mind, the result of this comparison would be different if we included more issues into it. And in some cases, the reason one OS is better than the other could simply be that we have put forth more effort into testing on and optimizing for that particular platform. We are just stating our observations to help you make a decision on which platform to use MySQL on in your setup. Which MySQL Version to Use -------------------------- The first decision to make is whether you want to use the latest development release or the last stable release: * Normally, if you are beginning to use MySQL for the first time or trying to port it to some system for which there is no binary distribution, we recommend going with the stable release (currently Version 3.23.44. Note that all MySQL releases are checked with the MySQL benchmarks and an extensive test suite before each release. * Otherwise, if you are running an old system and want to upgrade, but don't want to take chances with a non-seamless upgrade, you should upgrade to the latest in the same branch you are using (where only the last version number is newer than yours). We have tried to fix only fatal bugs and make small, relatively safe changes to that version. The second decision to make is whether you want to use a source distribution or a binary distribution. In most cases you should probably use a binary distribution, if one exists for your platform, as this generally will be easier to install than a source distribution. In the following cases you probably will be better off with a source installation: * If you want to install MySQL at some explicit location. (The standard binary distributions are "ready to run" at any place, but you may want to get even more flexibility). * To be able to satisfy different user requirements, we are providing two different binary versions; One compiled with the non-transactional table handlers, (a small, fast binary), and one configured with the most important extended options like transaction-safe tables. Both versions are compiled from the same source distribution. All native `MySQL' clients can connect to both MySQL versions. The extended MySQL binary distribution is marked with the `-max' suffix and is configured with the same options as `mysqld-max'. *Note `mysqld-max': mysqld-max. If you want to use the `MySQL-Max' RPM, you must first install the standard `MySQL' RPM. * If you want to configure `mysqld' with some extra features that are NOT in the standard binary distributions. Here is a list of the most common extra options that you may want to use: * `--with-berkeley-db' * `--with-innodb' * `--with-raid' * `--with-libwrap' * `--with-named-z-lib (This is done for some of the binaries)' * `--with-debug[=full]' * The default binary distribution is normally compiled with support for all characters sets and should work on a variety of processors from the same processor family. If you want a faster MySQL server you may want to recompile it with support for only the character sets you need, use a better compiler (like `pgcc') or use compiler options that are better optimized for your processor. * If you have found a bug and reported it to the MySQL development team you will probably receive a patch that you need to apply to the source distribution to get the bug fixed. * If you want to read (and/or modify) the C and C++ code that makes up MySQL, you should get a source distribution. The source code is always the ultimate manual. Source distributions also contain more tests and examples than binary distributions. The MySQL naming scheme uses release numbers that consist of three numbers and a suffix. For example, a release name like `mysql-3.21.17-beta' is interpreted like this: * The first number (`3') describes the file format. All Version 3 releases have the same file format. * The second number (`21') is the release level. Normally there are two to choose from. One is the release/stable branch (currently `23') and the other is the development branch (currently `4.0'). Normally both are stable, but the development version may have quirks, missing documentation on new features, or may fail to compile on some systems. * The third number (`17') is the version number within the release level. This is incremented for each new distribution. Usually you want the latest version for the release level you have chosen. * The suffix (`beta') indicates the stability level of the release. The possible suffixes are: - `alpha' indicates that the release contains some large section of new code that hasn't been 100% tested. Known bugs (usually there are none) should be documented in the News section. *Note News::. There are also new commands and extensions in most alpha releases. Active development that may involve major code changes can occur on an alpha release, but everything will be tested before doing a release. There should be no known bugs in any MySQL release. - `beta' means that all new code has been tested. No major new features that could cause corruption on old code are added. There should be no known bugs. A version changes from alpha to beta when there haven't been any reported fatal bugs within an alpha version for at least a month and we don't plan to add any features that could make any old command more unreliable. - `gamma' is a beta that has been around a while and seems to work fine. Only minor fixes are added. This is what many other companies call a release. - If there is no suffix, it means that the version has been run for a while at many different sites with no reports of bugs other than platform-specific bugs. Only critical bug fixes are applied to the release. This is what we call a stable release. All versions of MySQL are run through our standard tests and benchmarks to ensure that they are relatively safe to use. Because the standard tests are extended over time to check for all previously found bugs, the test suite keeps getting better. Note that all releases have been tested at least with: An internal test suite This is part of a production system for a customer. It has many tables with hundreds of megabytes of data. The MySQL benchmark suite This runs a range of common queries. It is also a test to see whether the latest batch of optimizations actually made the code faster. *Note MySQL Benchmarks::. The `crash-me' test This tries to determine what features the database supports and what its capabilities and limitations are. *Note MySQL Benchmarks::. Another test is that we use the newest MySQL version in our internal production environment, on at least one machine. We have more than 100 gigabytes of data to work with. Installation Layouts -------------------- This section describes the default layout of the directories created by installing binary and source distributions. A binary distribution is installed by unpacking it at the installation location you choose (typically `/usr/local/mysql') and creates the following directories in that location: *Directory* *Contents of directory* `bin' Client programs and the `mysqld' server `data' Log files, databases `include' Include (header) files `lib' Libraries `scripts' `mysql_install_db' `share/mysql' Error message files `sql-bench' Benchmarks A source distribution is installed after you configure and compile it. By default, the installation step installs files under `/usr/local', in the following subdirectories: *Directory* *Contents of directory* `bin' Client programs and scripts `include/mysql' Include (header) files `info' Documentation in Info format `lib/mysql' Libraries `libexec' The `mysqld' server `share/mysql' Error message files `sql-bench' Benchmarks and `crash-me' test `var' Databases and log files Within an installation directory, the layout of a source installation differs from that of a binary installation in the following ways: * The `mysqld' server is installed in the `libexec' directory rather than in the `bin' directory. * The data directory is `var' rather than `data'. * `mysql_install_db' is installed in the `/usr/local/bin' directory rather than in `/usr/local/mysql/scripts'. * The header file and library directories are `include/mysql' and `lib/mysql' rather than `include' and `lib'. You can create your own binary installation from a compiled source distribution by executing the script `scripts/make_binary_distribution'. How and When Updates Are Released --------------------------------- MySQL is evolving quite rapidly here at MySQL AB and we want to share this with other MySQL users. We try to make a release when we have very useful features that others seem to have a need for. We also try to help out users who request features that are easy to implement. We take note of what our licensed users want to have, and we especially take note of what our extended e-mail supported customers want and try to help them out. No one has to download a new release. The News section will tell you if the new release has something you really want. *Note News::. We use the following policy when updating MySQL: * For each minor update, the last number in the version string is incremented. When there are major new features or minor incompatibilities with previous versions, the second number in the version string is incremented. When the file format changes, the first number is increased. * Stable tested releases are meant to appear about 1-2 times a year, but if small bugs are found, a release with only bug fixes will be released. * Working releases are meant to appear about every 1-8 weeks. * Binary distributions for some platforms will be made by us for major releases. Other people may make binary distributions for other systems but probably less frequently. * We usually make patches available as soon as we have located and fixed small bugs. * For non-critical but annoying bugs, we will make patches available if they are sent to us. Otherwise we will combine many of them into a larger patch. * If there is, by any chance, a fatal bug in a release we will make a new release as soon as possible. We would like other companies to do this, too. The current stable release is Version 3.23; We have already moved active development to Version 4.0. Bugs will still be fixed in the stable version. We don't believe in a complete freeze, as this also leaves out bug fixes and things that "must be done." "Somewhat frozen" means that we may add small things that "almost surely will not affect anything that's already working." MySQL Binaries Compiled by MySQL AB ----------------------------------- As a service, we at MySQL AB provide a set of binary distributions of MySQL that are compiled at our site or at sites where customers kindly have given us access to their machines. These distributions are generated with `scripts/make_binary_distribution' and are configured with the following compilers and options: SunOS 4.1.4 2 sun4c with `gcc' 2.7.2.1 `CC=gcc CXX=gcc CXXFLAGS="-O3 -felide-constructors" ./configure --prefix=/usr/local/mysql --disable-shared --with-extra-charsets=complex --enable-assembler' SunOS 5.5.1 (and above) sun4u with `egcs' 1.0.3a or 2.90.27 or gcc 2.95.2 and newer `CC=gcc CFLAGS="-O3" CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex --enable-assembler' SunOS 5.6 i86pc with `gcc' 2.8.1 `CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex' Linux 2.0.33 i386 with `pgcc' 2.90.29 (`egcs' 1.0.3a) `CFLAGS="-O3 -mpentium -mstack-align-double" CXX=gcc CXXFLAGS="-O3 -mpentium -mstack-align-double -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --with-extra-charsets=complex' Linux 2.2.x with x686 with `gcc' 2.95.2 `CFLAGS="-O3 -mpentiumpro" CXX=gcc CXXFLAGS="-O3 -mpentiumpro -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-extra-charset=complex' SCO 3.2v5.0.4 i386 with `gcc' 2.7-95q4 `CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex' AIX 2 4 with `gcc' 2.7.2.2 `CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex' OSF1 V4.0 564 alpha with `gcc' 2.8.1 `CC=gcc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex' Irix 6.3 IP32 with `gcc' 2.8.0 `CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex' BSDI BSD/OS 3.1 i386 with `gcc' 2.7.2.1 `CC=gcc CXX=gcc CXXFLAGS=-O ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex' BSDI BSD/OS 2.1 i386 with `gcc' 2.7.2 `CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex' Anyone who has more optimal options for any of the configurations listed above can always mail them to the developer's mailing list at <internals@lists.mysql.com>. RPM distributions prior to MySQL Version 3.22 are user-contributed. Beginning with Version 3.22, the RPMs are generated by us at MySQL AB. If you want to compile a debug version of MySQL, you should add `--with-debug' or `--with-debug=full' to the above configure lines and remove any `-fomit-frame-pointer' options. Installing a MySQL Source Distribution ====================================== Before you proceed with the source installation, check first to see if our binary is available for your platform and if it will work for you. We put in a lot of effort into making sure that our binaries are built with the best possible options. You need the following tools to build and install MySQL from source: * GNU `gunzip' to uncompress the distribution. * A reasonable `tar' to unpack the distribution. GNU `tar' is known to work. Sun `tar' is known to have problems. * A working ANSI C++ compiler. `gcc' >= 2.95.2, `egcs' >= 1.0.2 or `egcs 2.91.66', SGI C++, and SunPro C++ are some of the compilers that are known to work. `libg++' is not needed when using `gcc'. `gcc' 2.7.x has a bug that makes it impossible to compile some perfectly legal C++ files, such as `sql/sql_base.cc'. If you only have `gcc' 2.7.x, you must upgrade your `gcc' to be able to compile MySQL. `gcc' 2.8.1 is also known to have problems on some platforms so it should be avoided if there exists a new compiler for the platform.. `gcc' >= 2.95.2 is recommended when compiling MySQL Version 3.23.x. * A good `make' program. GNU `make' is always recommended and is sometimes required. If you have problems, we recommend trying GNU `make' 3.75 or newer. If you are using a recent version of *gcc*, recent enough to understand `-fno-exceptions' option, it is *VERY IMPORTANT* that you use it. Otherwise, you may compile a binary that crashes randomly. We also recommend that you use `-felide-contructors' and `-fno-rtti' along with `-fno-exceptions'. When in doubt, do the following: CFLAGS="-O3" CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static On most systems this will give you a fast and stable binary. If you run into problems, *PLEASE ALWAYS USE `mysqlbug'* when posting questions to <mysql@lists.mysql.com>. Even if the problem isn't a bug, `mysqlbug' gathers system information that will help others solve your problem. By not using `mysqlbug', you lessen the likelihood of getting a solution to your problem! You will find `mysqlbug' in the `scripts' directory after you unpack the distribution. *Note Bug reports::. Quick Installation Overview --------------------------- The basic commands you must execute to install a MySQL source distribution are: shell> groupadd mysql shell> useradd -g mysql mysql shell> gunzip < mysql-VERSION.tar.gz | tar -xvf - shell> cd mysql-VERSION shell> ./configure --prefix=/usr/local/mysql shell> make shell> make install shell> scripts/mysql_install_db shell> chown -R root /usr/local/mysql shell> chown -R mysql /usr/local/mysql/var shell> chgrp -R mysql /usr/local/mysql shell> cp support-files/my-medium.cnf /etc/my.cnf shell> /usr/local/mysql/bin/safe_mysqld --user=mysql & If you want have support for InnoDB tables, you should edit the `/etc/my.cnf' file and remove the `#' character before the parameters that starts with `innodb_...'. *Note Option files::. *Note InnoDB start::. If you start from a source RPM, then do the following: shell> rpm --rebuild MySQL-VERSION.src.rpm This will make a binary RPM that you can install. You can add new users using the `bin/mysql_setpermission' script if you install the `DBI' and `Msql-Mysql-modules' Perl modules. A more detailed description follows. To install a source distribution, follow the steps below, then proceed to *Note Post-installation::, for post-installation initialization and testing: 1. Pick the directory under which you want to unpack the distribution, and move into it. 2. Obtain a distribution file from one of the sites listed in *Note Getting MySQL: Getting MySQL. 3. If you are interested in using Berkeley DB tables with MySQL, you will need to obtain a patched version of the Berkeley DB source code. Please read the chapter on Berkeley DB tables before proceeding. *Note BDB::. MySQL source distributions are provided as compressed `tar' archives and have names like `mysql-VERSION.tar.gz', where `VERSION' is a number like 3.23.44. 4. Add a user and group for `mysqld' to run as: shell> groupadd mysql shell> useradd -g mysql mysql These commands add the `mysql' group, and the `mysql' user. The syntax for `useradd' and `groupadd' may differ slightly on different versions of Unix. They may also be called `adduser' and `addgroup'. You may wish to call the user and group something else instead of `mysql'. 5. Unpack the distribution into the current directory: shell> gunzip < /path/to/mysql-VERSION.tar.gz | tar xvf - This command creates a directory named `mysql-VERSION'. 6. Change into the top-level directory of the unpacked distribution: shell> cd mysql-VERSION Note that currently you must configure and build MySQL from this top-level directory. You can not build it in a different directory. 7. Configure the release and compile everything: shell> ./configure --prefix=/usr/local/mysql shell> make When you run `configure', you might want to specify some options. Run `./configure --help' for a list of options. *Note `configure' options: configure options, discusses some of the more useful options. If `configure' fails, and you are going to send mail to <mysql@lists.mysql.com> to ask for assistance, please include any lines from `config.log' that you think can help solve the problem. Also include the last couple of lines of output from `configure' if `configure' aborts. Post the bug report using the `mysqlbug' script. *Note Bug reports::. If the compile fails, see *Note Compilation problems::, for help with a number of common problems. 8. Install everything: shell> make install You might need to run this command as `root'. 9. Create the MySQL grant tables (necessary only if you haven't installed MySQL before): shell> scripts/mysql_install_db Note that MySQL versions older than Version 3.22.10 started the MySQL server when you run `mysql_install_db'. This is no longer true! 10. Change ownership of binaries to `root' and ownership of the data directory to the user that you will run `mysqld' as: shell> chown -R root /usr/local/mysql shell> chown -R mysql /usr/local/mysql/var shell> chgrp -R mysql /usr/local/mysql The first command changes the `owner' attribute of the files to the `root' user, the second one changes the `owner' attribute of the data directory to the `mysql' user, and the third one changes the `group' attribute to the `mysql' group. 11. If you want to install support for the Perl `DBI'/`DBD' interface, see *Note Perl support::. 12. If you would like MySQL to start automatically when you boot your machine, you can copy `support-files/mysql.server' to the location where your system has its startup files. More information can be found in the `support-files/mysql.server' script itself and in *Note Automatic start::. After everything has been installed, you should initialize and test your distribution: shell> /usr/local/mysql/bin/safe_mysqld --user=mysql & If that command fails immediately with `mysqld daemon ended' then you can find some information in the file `mysql-data-directory/'hostname'.err'. The likely reason is that you already have another `mysqld' server running. *Note Multiple servers::. *Note Post-installation::. Applying Patches ---------------- Sometimes patches appear on the mailing list or are placed in the patches area (http://www.mysql.com/Downloads/Patches) of the MySQL Web site. To apply a patch from the mailing list, save the message in which the patch appears in a file, change into the top-level directory of your MySQL source tree, and run these commands: shell> patch -p1 < patch-file-name shell> rm config.cache shell> make clean Patches from the FTP site are distributed as plain text files or as files compressed with `gzip'. Apply a plain patch as shown above for mailing list patches. To apply a compressed patch, change into the top-level directory of your MySQL source tree and run these commands: shell> gunzip < patch-file-name.gz | patch -p1 shell> rm config.cache shell> make clean After applying a patch, follow the instructions for a normal source install, beginning with the `./configure' step. After running the `make install' step, restart your MySQL server. You may need to bring down any currently running server before you run `make install'. (Use `mysqladmin shutdown' to do this.) Some systems do not allow you to install a new version of a program if it replaces the version that is currently executing. Typical `configure' Options --------------------------- The `configure' script gives you a great deal of control over how you configure your MySQL distribution. Typically you do this using options on the `configure' command line. You can also affect `configure' using certain environment variables. *Note Environment variables::. For a list of options supported by `configure', run this command: shell> ./configure --help Some of the more commonly-used `configure' options are described below: * To compile just the MySQL client libraries and client programs and not the server, use the `--without-server' option: shell> ./configure --without-server If you don't have a C++ compiler, `mysql' will not compile (it is the one client program that requires C++). In this case, you can remove the code in `configure' that tests for the C++ compiler and then run `./configure' with the `--without-server' option. The compile step will still try to build `mysql', but you can ignore any warnings about `mysql.cc'. (If `make' stops, try `make -k' to tell it to continue with the rest of the build even if errors occur.) * If you don't want your log files and database directories located under `/usr/local/var', use a `configure' command, something like one of these: shell> ./configure --prefix=/usr/local/mysql shell> ./configure --prefix=/usr/local \ --localstatedir=/usr/local/mysql/data The first command changes the installation prefix so that everything is installed under `/usr/local/mysql' rather than the default of `/usr/local'. The second command preserves the default installation prefix, but overrides the default location for database directories (normally `/usr/local/var') and changes it to `/usr/local/mysql/data'. * If you are using Unix and you want the MySQL socket located somewhere other than the default location (normally in the directory `/tmp' or `/var/run') use a `configure' command like this: shell> ./configure --with-unix-socket-path=/usr/local/mysql/tmp/mysql.sock Note that the given file must be an absolute pathname! You can also later change the location `mysql.sock' by using the MySQL option files. *Note Problems with mysql.sock::. * If you want to compile statically linked programs (for example, to make a binary distribution, to get more speed, or to work around problems with some RedHat Linux distributions), run `configure' like this: shell> ./configure --with-client-ldflags=-all-static \ --with-mysqld-ldflags=-all-static * If you are using `gcc' and don't have `libg++' or `libstdc++' installed, you can tell `configure' to use `gcc' as your C++ compiler: shell> CC=gcc CXX=gcc ./configure When you use `gcc' as your C++ compiler, it will not attempt to link in `libg++' or `libstdc++'. Here is some common environment variables to set depending on the compiler you are using: gcc 2.7.2.1 CC=gcc CXX=gcc CXXFLAGS="-O3 -felide-constructors" egcs 1.0.3a CC=gcc CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" gcc 2.95.2 CFLAGS="-O3 -mpentiumpro" CXX=gcc CXXFLAGS="-O3 -mpentiumpro -felide-constructors -fno-exceptions -fno-rtti" pgcc 2.90.29 CFLAGS="-O3 -mpentiumpro -mstack-align-double" or newer CXX=gcc CXXFLAGS="-O3 -mpentiumpro -mstack-align-double -felide-constructors -fno-exceptions -fno-rtti" In most cases you can get a reasonably optimal MySQL binary by using the options from the above and adding the following options to the configure line: --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static The full configure line would in other words be something like the following for all recent gcc versions: CFLAGS="-O3 -mpentiumpro" CXX=gcc CXXFLAGS="-O3 -mpentiumpro -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static The binaries we provide on the MySQL Web site at `http://www.mysql.com' are all compiled with full optimization and should be perfect for most users. *Note MySQL binaries::. There are some things you can tweak to make an even faster binary, but this is only for advanced users. *Note Compile and link options::. If the build fails and produces errors about your compiler or linker not being able to create the shared library `libmysqlclient.so.#' (`#' is a version number), you can work around this problem by giving the `--disable-shared' option to `configure'. In this case, `configure' will not build a shared `libmysqlclient.so.#' library. * You can configure MySQL not to use `DEFAULT' column values for non-`NULL' columns (that is, columns that are not allowed to be `NULL'). This causes `INSERT' statements to generate an error unless you explicitly specify values for all columns that require a non-`NULL' value. To suppress use of default values, run `configure' like this: shell> CXXFLAGS=-DDONT_USE_DEFAULT_FIELDS ./configure * By default, MySQL uses the ISO-8859-1 (Latin1) character set. To change the default set, use the `--with-charset' option: shell> ./configure --with-charset=CHARSET `CHARSET' may be one of `big5', `cp1251', `cp1257', `czech', `danish', `dec8', `dos', `euc_kr', `gb2312', `gbk', `german1', `hebrew', `hp8', `hungarian', `koi8_ru', `koi8_ukr', `latin1', `latin2', `sjis', `swe7', `tis620', `ujis', `usa7', or `win1251ukr'. *Note Character sets::. If you want to convert characters between the server and the client, you should take a look at the `SET OPTION CHARACTER SET' command. *Note `SET OPTION': SET OPTION. *Warning:* If you change character sets after having created any tables, you will have to run `myisamchk -r -q' on every table. Your indexes may be sorted incorrectly otherwise. (This can happen if you install MySQL, create some tables, then reconfigure MySQL to use a different character set and reinstall it.) With the option `--with-extra-charset=LIST' you can define which additional character sets should be incompiled in the server. Here `LIST' is either a list of character set separated with space, `complex' to include all characters that can't be dynamically loaded or `all' to include all character sets into the binaries. * To configure MySQL with debugging code, use the `--with-debug' option: shell> ./configure --with-debug This causes a safe memory allocator to be included that can find some errors and that provides output about what is happening. *Note Debugging server::. * If your client programs are using threads, you need to also compile a thread-safe version of the MySQL client library with the `--enable-thread-safe-client' configure options. This will create a `libmysqlclient_r' library with which you should link your threaded applications. *Note Threaded clients::. * Options that pertain to particular systems can be found in the system-specific section of this manual. *Note Operating System Specific Notes::. Installing from the Development Source Tree ------------------------------------------- *CAUTION:* You should read this section only if you are interested in helping us test our new code. If you just want to get MySQL up and running on your system, you should use a standard release distribution (either a source or binary distribution will do). To obtain our most recent development source tree, use these instructions: 1. Download *BitKeeper* from `http://www.bitmover.com/cgi-bin/download.cgi'. You will need *Bitkeeper* 2.0 or newer to access our repository. 2. Follow the instructions to install it. 3. After *BitKeeper* is installed, use this command if you want to clone the MySQL 3.23 branch: shell> bk clone bk://work.mysql.com:7000 mysql To clone the 4.0 branch, use this command instead: shell> bk clone bk://work.mysql.com:7001 mysql-4.0 The initial download of the source tree may take a while, depending on the speed of your connection; be patient. 4. You will need GNU `autoconf', `automake', `libtool', and `m4' to run the next set of commands. If you get some strange error during this stage, check that you really have `libtool' installed! shell> cd mysql shell> bk -r edit shell> aclocal; autoheader; autoconf; automake; shell> ./configure # Add your favorite options here shell> make A collection of our standard configure scripts is located in the `BUILD/' subdirectory. If you are lazy, you can use `BUILD/compile-pentium-debug'. To compile on a different architecture, modify the script removing flags that are Pentium-specific. 5. When the build is done, run `make install'. Be careful with this on a production machine; the command may overwrite your live release installation. If you have another installation of MySQL, we recommand that you run `./configure' with different values for the `prefix', `tcp-port', and `unix-socket-path' options than those used for your production server. 6. Play hard with your new installation and try to make the new features crash. Start by running `make test'. *Note MySQL test suite::. 7. If you have gotten to the `make' stage and the distribution does not compile, please report it to <bugs@lists.mysql.com>. If you have installed the latest versions of the required GNU tools, and they crash trying to process our configuration files, please report that also. However, if you execute `aclocal' and get a `command not found' error or a similar problem, do not report it. Instead, make sure all the necessary tools are installed and that your `PATH' variable is set correctly so your shell can find them. 8. After the initial `bk clone' operation to get the source tree, you should run `bk pull' periodically to get the updates. 9. You can examine the change history for the tree with all the diffs by using `bk sccstool'. If you see some funny diffs or code that you have a question about, do not hesitate to send e-mail to <internals@lists.mysql.com>. Also, if you think you have a better idea on how to do something, send an email to the same address with a patch. `bk diffs' will produce a patch for you after you have made changes to the source. If you do not have the time to code your idea, just send a description. 10. *BitKeeper* has a nice help utility that you can access via `bk helptool'. Problems Compiling? ------------------- All MySQL programs compile cleanly for us with no warnings on Solaris using `gcc'. On other systems, warnings may occur due to differences in system include files. See *Note MIT-pthreads:: for warnings that may occur when using MIT-pthreads. For other problems, check the list below. The solution to many problems involves reconfiguring. If you do need to reconfigure, take note of the following: * If `configure' is run after it already has been run, it may use information that was gathered during its previous invocation. This information is stored in `config.cache'. When `configure' starts up, it looks for that file and reads its contents if it exists, on the assumption that the information is still correct. That assumption is invalid when you reconfigure. * Each time you run `configure', you must run `make' again to recompile. However, you may want to remove old object files from previous builds first, because they were compiled using different configuration options. To prevent old configuration information or object files from being used, run these commands before rerunning `configure': shell> rm config.cache shell> make clean Alternatively, you can run `make distclean'. The list below describes some of the problems compiling MySQL that have been found to occur most often: * If you get errors when compiling `sql_yacc.cc', such as the ones shown below, you have probably run out of memory or swap space: Internal compiler error: program cc1plus got fatal signal 11 or Out of virtual memory or Virtual memory exhausted The problem is that `gcc' requires huge amounts of memory to compile `sql_yacc.cc' with inline functions. Try running `configure' with the `--with-low-memory' option: shell> ./configure --with-low-memory This option causes `-fno-inline' to be added to the compile line if you are using `gcc' and `-O0' if you are using something else. You should try the `--with-low-memory' option even if you have so much memory and swap space that you think you can't possibly have run out. This problem has been observed to occur even on systems with generous hardware configurations, and the `--with-low-memory' option usually fixes it. * By default, `configure' picks `c++' as the compiler name and GNU `c++' links with `-lg++'. If you are using `gcc', that behavior can cause problems during configuration such as this: configure: error: installation or configuration problem: C++ compiler cannot create executables. You might also observe problems during compilation related to `g++', `libg++', or `libstdc++'. One cause of these problems is that you may not have `g++', or you may have `g++' but not `libg++', or `libstdc++'. Take a look at the `config.log' file. It should contain the exact reason why your c++ compiler didn't work! To work around these problems, you can use `gcc' as your C++ compiler. Try setting the environment variable `CXX' to `"gcc -O3"'. For example: shell> CXX="gcc -O3" ./configure This works because `gcc' compiles C++ sources as well as `g++' does, but does not link in `libg++' or `libstdc++' by default. Another way to fix these problems, of course, is to install `g++', `libg++' and `libstdc++'. * If your compile fails with errors, such as any of the following, you must upgrade your version of `make' to GNU `make': making all in mit-pthreads make: Fatal error in reader: Makefile, line 18: Badly formed macro assignment or make: file `Makefile' line 18: Must be a separator (: or pthread.h: No such file or directory Solaris and FreeBSD are known to have troublesome `make' programs. GNU `make' Version 3.75 is known to work. * If you want to define flags to be used by your C or C++ compilers, do so by adding the flags to the `CFLAGS' and `CXXFLAGS' environment variables. You can also specify the compiler names this way using `CC' and `CXX'. For example: shell> CC=gcc shell> CFLAGS=-O3 shell> CXX=gcc shell> CXXFLAGS=-O3 shell> export CC CFLAGS CXX CXXFLAGS See *Note MySQL binaries::, for a list of flag definitions that have been found to be useful on various systems. * If you get an error message like this, you need to upgrade your `gcc' compiler: client/libmysql.c:273: parse error before `__attribute__' `gcc' 2.8.1 is known to work, but we recommend using `gcc' 2.95.2 or `egcs' 1.0.3a instead. * If you get errors such as those shown below when compiling `mysqld', `configure' didn't correctly detect the type of the last argument to `accept()', `getsockname()', or `getpeername()': cxx: Error: mysqld.cc, line 645: In this statement, the referenced type of the pointer value "&length" is "unsigned long", which is not compatible with "int". new_sock = accept(sock, (struct sockaddr *)&cAddr, &length); To fix this, edit the `config.h' file (which is generated by `configure'). Look for these lines: /* Define as the base type of the last arg to accept */ #define SOCKET_SIZE_TYPE XXX Change `XXX' to `size_t' or `int', depending on your operating system. (Note that you will have to do this each time you run `configure', because `configure' regenerates `config.h'.) * The `sql_yacc.cc' file is generated from `sql_yacc.yy'. Normally the build process doesn't need to create `sql_yacc.cc', because MySQL comes with an already-generated copy. However, if you do need to re-create it, you might encounter this error: "sql_yacc.yy", line xxx fatal: default action causes potential... This is a sign that your version of `yacc' is deficient. You probably need to install `bison' (the GNU version of `yacc') and use that instead. * If you need to debug `mysqld' or a MySQL client, run `configure' with the `--with-debug' option, then recompile and link your clients with the new client library. *Note Debugging client::. MIT-pthreads Notes ------------------ This section describes some of the issues involved in using MIT-pthreads. Note that on Linux you should NOT use MIT-pthreads but install LinuxThreads! *Note Linux::. If your system does not provide native thread support, you will need to build MySQL using the MIT-pthreads package. This includes older FreeBSD systems, SunOS 4.x, Solaris 2.4 and earlier, and some others. *Note Which OS::. * On most systems, you can force MIT-pthreads to be used by running `configure' with the `--with-mit-threads' option: shell> ./configure --with-mit-threads Building in a non-source directory is not supported when using MIT-pthreads, because we want to minimize our changes to this code. * The checks that determine whether or not to use MIT-pthreads occur only during the part of the configuration process that deals with the server code. If you have configured the distribution using `--without-server' to build only the client code, clients will not know whether or not MIT-pthreads is being used and will use Unix socket connections by default. Because Unix sockets do not work under MIT-pthreads, this means you will need to use `-h' or `--host' when you run client programs. * When MySQL is compiled using MIT-pthreads, system locking is disabled by default for performance reasons. You can tell the server to use system locking with the `--use-locking' option. * Sometimes the pthread `bind()' command fails to bind to a socket without any error message (at least on Solaris). The result is that all connections to the server fail. For example: shell> mysqladmin version mysqladmin: connect to server at '' failed; error: 'Can't connect to mysql server on localhost (146)' The solution to this is to kill the `mysqld' server and restart it. This has only happened to us when we have forced the server down and done a restart immediately. * With MIT-pthreads, the `sleep()' system call isn't interruptible with `SIGINT' (break). This is only noticeable when you run `mysqladmin --sleep'. You must wait for the `sleep()' call to terminate before the interrupt is served and the process stops. * When linking, you may receive warning messages like these (at least on Solaris); they can be ignored: ld: warning: symbol `_iob' has differing sizes: (file /my/local/pthreads/lib/libpthread.a(findfp.o) value=0x4; file /usr/lib/libc.so value=0x140); /my/local/pthreads/lib/libpthread.a(findfp.o) definition taken ld: warning: symbol `__iob' has differing sizes: (file /my/local/pthreads/lib/libpthread.a(findfp.o) value=0x4; file /usr/lib/libc.so value=0x140); /my/local/pthreads/lib/libpthread.a(findfp.o) definition taken * Some other warnings also can be ignored: implicit declaration of function `int strtoll(...)' implicit declaration of function `int strtoul(...)' * We haven't gotten `readline' to work with MIT-pthreads. (This isn't needed, but may be interesting for someone.) Post-installation Setup and Testing =================================== Once you've installed MySQL (from either a binary or source distribution), you need to initialize the grant tables, start the server, and make sure that the server works okay. You may also wish to arrange for the server to be started and stopped automatically when your system starts up and shuts down. Normally you install the grant tables and start the server like this for installation from a source distribution: shell> ./scripts/mysql_install_db shell> cd mysql_installation_directory shell> ./bin/safe_mysqld --user=mysql & For a binary distribution (not RPM or pkg packages), do this: shell> cd mysql_installation_directory shell> ./bin/mysql_install_db shell> ./bin/safe_mysqld --user=mysql & This creates the `mysql' database which will hold all database privileges, the `test' database which you can use to test MySQL and also privilege entries for the user that run `mysql_install_db' and a `root' user (without any passwords). This also starts the `mysqld' server. `mysql_install_db' will not overwrite any old privilege tables, so it should be safe to run in any circumstances. If you don't want to have the `test' database you can remove it with `mysqladmin -u root drop test'. Testing is most easily done from the top-level directory of the MySQL distribution. For a binary distribution, this is your installation directory (typically something like `/usr/local/mysql'). For a source distribution, this is the main directory of your MySQL source tree. In the commands shown below in this section and in the following subsections, `BINDIR' is the path to the location in which programs like `mysqladmin' and `safe_mysqld' are installed. For a binary distribution, this is the `bin' directory within the distribution. For a source distribution, `BINDIR' is probably `/usr/local/bin', unless you specified an installation directory other than `/usr/local' when you ran `configure'. `EXECDIR' is the location in which the `mysqld' server is installed. For a binary distribution, this is the same as `BINDIR'. For a source distribution, `EXECDIR' is probably `/usr/local/libexec'. Testing is described in detail below: 1. If necessary, start the `mysqld' server and set up the initial MySQL grant tables containing the privileges that determine how users are allowed to connect to the server. This is normally done with the `mysql_install_db' script: shell> scripts/mysql_install_db Typically, `mysql_install_db' needs to be run only the first time you install MySQL. Therefore, if you are upgrading an existing installation, you can skip this step. (However, `mysql_install_db' is quite safe to use and will not update any tables that already exist, so if you are unsure of what to do, you can always run `mysql_install_db'.) `mysql_install_db' creates six tables (`user', `db', `host', `tables_priv', `columns_priv', and `func') in the `mysql' database. A description of the initial privileges is given in *Note Default privileges::. Briefly, these privileges allow the MySQL `root' user to do anything, and allow anybody to create or use databases with a name of `'test'' or starting with `'test_''. If you don't set up the grant tables, the following error will appear in the log file when you start the server: mysqld: Can't find file: 'host.frm' The above may also happen with a binary MySQL distribution if you don't start MySQL by executing exactly `./bin/safe_mysqld'! *Note `safe_mysqld': safe_mysqld. You might need to run `mysql_install_db' as `root'. However, if you prefer, you can run the MySQL server as an unprivileged (non-`root') user, provided that user can read and write files in the database directory. Instructions for running MySQL as an unprivileged user are given in *Note Changing MySQL user: Changing MySQL user. If you have problems with `mysql_install_db', see *Note `mysql_install_db': mysql_install_db. There are some alternatives to running the `mysql_install_db' script as it is provided in the MySQL distribution: * You may want to edit `mysql_install_db' before running it, to change the initial privileges that are installed into the grant tables. This is useful if you want to install MySQL on a lot of machines with the same privileges. In this case you probably should need only to add a few extra `INSERT' statements to the `mysql.user' and `mysql.db' tables! * If you want to change things in the grant tables after installing them, you can run `mysql_install_db', then use `mysql -u root mysql' to connect to the grant tables as the MySQL `root' user and issue SQL statements to modify the grant tables directly. * It is possible to re-create the grant tables completely after they have already been created. You might want to do this if you've already installed the tables but then want to re-create them after editing `mysql_install_db'. For more information about these alternatives, see *Note Default privileges::. 2. Start the MySQL server like this: shell> cd mysql_installation_directory shell> bin/safe_mysqld & If you have problems starting the server, see *Note Starting server::. 3. Use `mysqladmin' to verify that the server is running. The following commands provide a simple test to check that the server is up and responding to connections: shell> BINDIR/mysqladmin version shell> BINDIR/mysqladmin variables The output from `mysqladmin version' varies slightly depending on your platform and version of MySQL, but should be similar to that shown below: shell> BINDIR/mysqladmin version mysqladmin Ver 8.14 Distrib 3.23.32, for linux on i586 Copyright (C) 2000 MySQL AB & MySQL Finland AB & TCX DataKonsult AB This software comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to modify and redistribute it under the GPL license Server version 3.23.32-debug Protocol version 10 Connection Localhost via Unix socket TCP port 3306 UNIX socket /tmp/mysql.sock Uptime: 16 sec Threads: 1 Questions: 9 Slow queries: 0 Opens: 7 Flush tables: 2 Open tables: 0 Queries per second avg: 0.000 Memory in use: 132K Max memory used: 16773K To get a feeling for what else you can do with `BINDIR/mysqladmin', invoke it with the `--help' option. 4. Verify that you can shut down the server: shell> BINDIR/mysqladmin -u root shutdown 5. Verify that you can restart the server. Do this using `safe_mysqld' or by invoking `mysqld' directly. For example: shell> BINDIR/safe_mysqld --log & If `safe_mysqld' fails, try running it from the MySQL installation directory (if you are not already there). If that doesn't work, see *Note Starting server::. 6. Run some simple tests to verify that the server is working. The output should be similar to what is shown below: shell> BINDIR/mysqlshow +-----------+ | Databases | +-----------+ | mysql | +-----------+ shell> BINDIR/mysqlshow mysql Database: mysql +--------------+ | Tables | +--------------+ | columns_priv | | db | | func | | host | | tables_priv | | user | +--------------+ shell> BINDIR/mysql -e "select host,db,user from db" mysql +------+--------+------+ | host | db | user | +------+--------+------+ | % | test | | | % | test_% | | +------+--------+------+ There is also a benchmark suite in the `sql-bench' directory (under the MySQL installation directory) that you can use to compare how MySQL performs on different platforms. The `sql-bench/Results' directory contains the results from many runs against different databases and platforms. To run all tests, execute these commands: shell> cd sql-bench shell> run-all-tests If you don't have the `sql-bench' directory, you are probably using an RPM for a binary distribution. (Source distribution RPMs include the benchmark directory.) In this case, you must first install the benchmark suite before you can use it. Beginning with MySQL Version 3.22, there are benchmark RPM files named `mysql-bench-VERSION-i386.rpm' that contain benchmark code and data. If you have a source distribution, you can also run the tests in the `tests' subdirectory. For example, to run `auto_increment.tst', do this: shell> BINDIR/mysql -vvf test < ./tests/auto_increment.tst The expected results are shown in the `./tests/auto_increment.res' file. Problems Running `mysql_install_db' ----------------------------------- The purpose of the `mysql_install_db' script is to generate new MySQL privilege tables. It will not affect any other data! It will also not do anything if you already have MySQL privilege tables installed! If you want to re-create your privilege tables, you should take down the `mysqld' server, if it's running, and then do something like: mv mysql-data-directory/mysql mysql-data-directory/mysql-old mysql_install_db This section lists problems you might encounter when you run `mysql_install_db': *`mysql_install_db' doesn't install the grant tables* You may find that `mysql_install_db' fails to install the grant tables and terminates after displaying the following messages: starting mysqld daemon with databases from XXXXXX mysql daemon ended In this case, you should examine the log file very carefully! The log should be located in the directory `XXXXXX' named by the error message, and should indicate why `mysqld' didn't start. If you don't understand what happened, include the log when you post a bug report using `mysqlbug'! *Note Bug reports::. *There is already a `mysqld' daemon running* In this case, you probably don't have to run `mysql_install_db' at all. You have to run `mysql_install_db' only once, when you install MySQL the first time. *Installing a second `mysqld' daemon doesn't work when one daemon is running* This can happen when you already have an existing MySQL installation, but want to put a new installation in a different place (for example, for testing, or perhaps you simply want to run two installations at the same time). Generally the problem that occurs when you try to run the second server is that it tries to use the same socket and port as the old one. In this case you will get the error message: `Can't start server: Bind on TCP/IP port: Address already in use' or `Can't start server : Bind on unix socket...'. *Note Installing many servers::. *You don't have write access to `/tmp'* If you don't have write access to create a socket file at the default place (in `/tmp') or permission to create temporary files in `/tmp,' you will get an error when running `mysql_install_db' or when starting or using `mysqld'. You can specify a different socket and temporary directory as follows: shell> TMPDIR=/some_tmp_dir/ shell> MYSQL_UNIX_PORT=/some_tmp_dir/mysqld.sock shell> export TMPDIR MYSQL_UNIX_PORT *Note Problems with mysql.sock::. `some_tmp_dir' should be the path to some directory for which you have write permission. *Note Environment variables::. After this you should be able to run `mysql_install_db' and start the server with these commands: shell> scripts/mysql_install_db shell> BINDIR/safe_mysqld & *`mysqld' crashes immediately* If you are running RedHat Version 5.0 with a version of `glibc' older than 2.0.7-5, you should make sure you have installed all `glibc' patches! There is a lot of information about this in the MySQL mail archives. Links to the mail archives are available online at `http://www.mysql.com/documentation/'. Also, see *Note Linux::. You can also start `mysqld' manually using the `--skip-grant-tables' option and add the privilege information yourself using `mysql': shell> BINDIR/safe_mysqld --skip-grant-tables & shell> BINDIR/mysql -u root mysql From `mysql', manually execute the SQL commands in `mysql_install_db'. Make sure you run `mysqladmin flush-privileges' or `mysqladmin reload' afterward to tell the server to reload the grant tables. Problems Starting the MySQL Server ---------------------------------- If you are going to use tables that support transactions (BDB, InnoDB), you should first create a my.cnf file and set startup options for the table types you plan to use. *Note Table types::. Generally, you start the `mysqld' server in one of three ways: * By invoking `mysql.server'. This script is used primarily at system startup and shutdown, and is described more fully in *Note Automatic start::. * By invoking `safe_mysqld', which tries to determine the proper options for `mysqld' and then runs it with those options. *Note `safe_mysqld': safe_mysqld. * On NT you should install `mysqld' as a service as follows: bin\mysqld-nt --install # Install MySQL as a service You can now start/stop `mysqld' as follows: NET START mysql NET STOP mysql Note that in this case you can't use any other options for `mysqld'! You can remove the service as follows: bin\mysqld-nt --remove # remove MySQL as a service * By invoking `mysqld' directly. When the `mysqld' daemon starts up, it changes directory to the data directory. This is where it expects to write log files and the pid (process ID) file, and where it expects to find databases. The data directory location is hardwired in when the distribution is compiled. However, if `mysqld' expects to find the data directory somewhere other than where it really is on your system, it will not work properly. If you have problems with incorrect paths, you can find out what options `mysqld' allows and what the default path settings are by invoking `mysqld' with the `--help' option. You can override the defaults by specifying the correct pathnames as command-line arguments to `mysqld'. (These options can be used with `safe_mysqld' as well.) Normally you should need to tell `mysqld' only the base directory under which MySQL is installed. You can do this with the `--basedir' option. You can also use `--help' to check the effect of changing path options (note that `--help' _must_ be the final option of the `mysqld' command). For example: shell> EXECDIR/mysqld --basedir=/usr/local --help Once you determine the path settings you want, start the server without the `--help' option. Whichever method you use to start the server, if it fails to start up correctly, check the log file to see if you can find out why. Log files are located in the data directory (typically `/usr/local/mysql/data' for a binary distribution, `/usr/local/var' for a source distribution, `\mysql\data\mysql.err' on Windows.) Look in the data directory for files with names of the form `host_name.err' and `host_name.log' where `host_name' is the name of your server host. Then check the last few lines of these files: shell> tail host_name.err shell> tail host_name.log If you find something like the following in the log file: 000729 14:50:10 bdb: Recovery function for LSN 1 27595 failed 000729 14:50:10 bdb: warning: ./test/t1.db: No such file or directory 000729 14:50:10 Can't init databases This means that you didn't start `mysqld' with `--bdb-no-recover' and Berkeley DB found something wrong with its log files when it tried to recover your databases. To be able to continue, you should move away the old Berkeley DB log file from the database directory to some other place, where you can later examine these. The log files are named `log.0000000001', where the number will increase over time. If you are running `mysqld' with BDB table support and `mysqld' core dumps at start this could be because of some problems with the BDB recover log. In this case you can try starting `mysqld' with `--bdb-no-recover'. If this helps, then you should remove all `log.*' files from the data directory and try starting `mysqld' again. If you get the following error, it means that some other program (or another `mysqld' server) is already using the TCP/IP port or socket `mysqld' is trying to use: Can't start server: Bind on TCP/IP port: Address already in use or Can't start server : Bind on unix socket... Use `ps' to make sure that you don't have another `mysqld' server running. If you can't find another server running, you can try to execute the command `telnet your-host-name tcp-ip-port-number' and press `RETURN' a couple of times. If you don't get an error message like `telnet: Unable to connect to remote host: Connection refused', something is using the TCP/IP port `mysqld' is trying to use. See *Note mysql_install_db:: and *Note Multiple servers::. If `mysqld' is currently running, you can find out what path settings it is using by executing this command: shell> mysqladmin variables or shell> mysqladmin -h 'your-host-name' variables If `safe_mysqld' starts the server but you can't connect to it, you should make sure you have an entry in `/etc/hosts' that looks like this: 127.0.0.1 localhost This problem occurs only on systems that don't have a working thread library and for which MySQL must be configured to use MIT-pthreads. If you can't get `mysqld' to start you can try to make a trace file to find the problem. *Note Making trace files::. If you are using InnoDB tables, refer to the InnoDB-specific startup options. *Note InnoDB start::. If you are using BDB (Berkeley DB) tables, you should familiarize yourself with the different BDB specific startup options. *Note BDB start::. Starting and Stopping MySQL Automatically ----------------------------------------- The `mysql.server' and `safe_mysqld' scripts can be used to start the server automatically at system startup time. `mysql.server' can also be used to stop the server. The `mysql.server' script can be used to start or stop the server by invoking it with `start' or `stop' arguments: shell> mysql.server start shell> mysql.server stop `mysql.server' can be found in the `share/mysql' directory under the MySQL installation directory or in the `support-files' directory of the MySQL source tree. Before `mysql.server' starts the server, it changes directory to the MySQL installation directory, then invokes `safe_mysqld'. You might need to edit `mysql.server' if you have a binary distribution that you've installed in a non-standard location. Modify it to `cd' into the proper directory before it runs `safe_mysqld'. If you want the server to run as some specific user, add an appropriate `user' line to the `/etc/my.cnf' file, as shown later in this section. `mysql.server stop' brings down the server by sending a signal to it. You can take down the server manually by executing `mysqladmin shutdown'. You might want to add these start and stop commands to the appropriate places in your `/etc/rc*' files when you start using MySQL for production applications. Note that if you modify `mysql.server', then upgrade MySQL sometime, your modified version will be overwritten, so you should make a copy of your edited version that you can reinstall. If your system uses `/etc/rc.local' to start external scripts, you should append the following to it: /bin/sh -c 'cd /usr/local/mysql ; ./bin/safe_mysqld --user=mysql &' You can also add options for `mysql.server' in a global `/etc/my.cnf' file. A typical `/etc/my.cnf' file might look like this: [mysqld] datadir=/usr/local/mysql/var socket=/var/tmp/mysql.sock port=3306 user=mysql [mysql.server] basedir=/usr/local/mysql The `mysql.server' script understands the following options: `datadir', `basedir', and `pid-file'. The following table shows which option groups each of the startup scripts read from option files: *Script* *Option groups* `mysqld' `mysqld' and `server' `mysql.server' `mysql.server', `mysqld', and `server' `safe_mysqld' `mysql.server', `mysqld', and `server' *Note Option files::. Upgrading/Downgrading MySQL =========================== You can always move the MySQL form and data files between different versions on the same architecture as long as you have the same base version of MySQL. The current base version is 3. If you change the character set when running MySQL (which may also change the sort order), you must run `myisamchk -r -q' on all tables. Otherwise your indexes may not be ordered correctly. If you are afraid of new versions, you can always rename your old `mysqld' to something like `mysqld'-'old-version-number'. If your new `mysqld' then does something unexpected, you can simply shut it down and restart with your old `mysqld'! When you do an upgrade you should also back up your old databases, of course. If after an upgrade, you experience problems with recompiled client programs, like `Commands out of sync' or unexpected core dumps, you probably have used an old header or library file when compiling your programs. In this case you should check the date for your `mysql.h' file and `libmysqlclient.a' library to verify that they are from the new MySQL distribution. If not, please recompile your programs! If you get some problems that the new `mysqld' server doesn't want to start or that you can't connect without a password, check that you don't have some old `my.cnf' file from your old installation! You can check this with: `program-name --print-defaults'. If this outputs anything other than the program name, you have an active `my.cnf' file that will affect things! It is a good idea to rebuild and reinstall the `Msql-Mysql-modules' distribution whenever you install a new release of MySQL, particularly if you notice symptoms such as all your `DBI' scripts dumping core after you upgrade MySQL. Upgrading From Version 3.22 to Version 3.23 ------------------------------------------- MySQL Version 3.23 supports tables of the new `MyISAM' type and the old `ISAM' type. You don't have to convert your old tables to use these with Version 3.23. By default, all new tables will be created with type `MyISAM' (unless you start `mysqld' with the `--default-table-type=isam' option). You can change an `ISAM' table to a `MyISAM' table with `ALTER TABLE table_name TYPE=MyISAM' or the Perl script `mysql_convert_table_format'. Version 3.22 and 3.21 clients will work without any problems with a Version 3.23 server. The following lists tell what you have to watch out for when upgrading to Version 3.23: * All tables that uses the `tis620' character set must be fixed with `myisamchk -r' or `REPAIR TABLE'. * If you do a `DROP DATABASE' on a symbolic linked database, both the link and the original database is deleted. (This didn't happen in 3.22 because configure didn't detect the `readlink' system call). * `OPTIMIZE TABLE' now only works for *MyISAM* tables. For other table types, you can use `ALTER TABLE' to optimize the table. During `OPTIMIZE TABLE' the table is now locked from other threads. * The MySQL client `mysql' is now by default started with the option `--no-named-commands (-g)'. This option can be disabled with `--enable-named-commands (-G)'. This may cause incompatibility problems in some cases, for example in SQL scripts that use named commands without a semicolon! Long format commands still work from the first line. * If you are using the `german' character sort order, you must repair all your tables with `isamchk -r', as we have made some changes in the sort order! * The default return type of `IF' will now depend on both arguments and not only the first argument. * `AUTO_INCREMENT' will not work with negative numbers. The reason for this is that negative numbers caused problems when wrapping from -1 to 0. `AUTO_INCREMENT' is now for MyISAM tables handled at a lower level and is much faster than before. For MyISAM tables old numbers are also not reused anymore, even if you delete some rows from the table. * `CASE', `DELAYED', `ELSE', `END', `FULLTEXT', `INNER', `RIGHT', `THEN' and `WHEN' are now reserved words. * `FLOAT(X)' is now a true floating-point type and not a value with a fixed number of decimals. * When declaring `DECIMAL(length,dec)' the length argument no longer includes a place for the sign or the decimal point. * A `TIME' string must now be of one of the following formats: `[[[DAYS] [H]H:]MM:]SS[.fraction]' or `[[[[[H]H]H]H]MM]SS[.fraction]' * `LIKE' now compares strings using the same character comparison rules as `'=''. If you require the old behavior, you can compile MySQL with the `CXXFLAGS=-DLIKE_CMP_TOUPPER' flag. * `REGEXP' is now case insensitive for normal (not binary) strings. * When you check/repair tables you should use `CHECK TABLE' or `myisamchk' for `MyISAM' tables (`.MYI') and `isamchk' for ISAM (`.ISM') tables. * If you want your `mysqldump' files to be compatible between MySQL Version 3.22 and Version 3.23, you should not use the `--opt' or `--full' option to `mysqldump'. * Check all your calls to `DATE_FORMAT()' to make sure there is a `%' before each format character. (Later MySQL Version 3.22 did allow this syntax.) * `mysql_fetch_fields_direct' is now a function (it was a macro) and it returns a pointer to a `MYSQL_FIELD' instead of a `MYSQL_FIELD'. * `mysql_num_fields()' can no longer be used on a `MYSQL*' object (it's now a function that takes `MYSQL_RES*' as an argument. You should now use `mysql_field_count()' instead. * In MySQL Version 3.22, the output of `SELECT DISTINCT ...' was almost always sorted. In Version 3.23, you must use `GROUP BY' or `ORDER BY' to obtain sorted output. * `SUM()' now returns `NULL', instead of 0, if there is no matching rows. This is according to ANSI SQL. * An `AND' or `OR' with `NULL' values will now return `NULL' instead of 0. This mostly affects queries that use `NOT' on an `AND/OR' expression as `NOT NULL' = `NULL'. `LPAD()' and `RPAD()' will shorten the result string if it's longer than the length argument. Upgrading from Version 3.21 to Version 3.22 ------------------------------------------- Nothing that affects compatibility has changed between Version 3.21 and 3.22. The only pitfall is that new tables that are created with `DATE' type columns will use the new way to store the date. You can't access these new fields from an old version of `mysqld'. After installing MySQL Version 3.22, you should start the new server and then run the `mysql_fix_privilege_tables' script. This will add the new privileges that you need to use the `GRANT' command. If you forget this, you will get `Access denied' when you try to use `ALTER TABLE', `CREATE INDEX', or `DROP INDEX'. If your MySQL root user requires a password, you should give this as an argument to `mysql_fix_privilege_tables'. The C API interface to `mysql_real_connect()' has changed. If you have an old client program that calls this function, you must place a `0' for the new `db' argument (or recode the client to send the `db' element for faster connections). You must also call `mysql_init()' before calling `mysql_real_connect()'! This change was done to allow the new `mysql_options()' function to save options in the `MYSQL' handler structure. The `mysqld' variable `key_buffer' has changed names to `key_buffer_size', but you can still use the old name in your startup files. Upgrading from Version 3.20 to Version 3.21 ------------------------------------------- If you are running a version older than Version 3.20.28 and want to switch to Version 3.21, you need to do the following: You can start the `mysqld' Version 3.21 server with `safe_mysqld --old-protocol' to use it with clients from a Version 3.20 distribution. In this case, the new client function `mysql_errno()' will not return any server error, only `CR_UNKNOWN_ERROR' (but it works for client errors), and the server uses the old `password()' checking rather than the new one. If you are *NOT* using the `--old-protocol' option to `mysqld', you will need to make the following changes: * All client code must be recompiled. If you are using ODBC, you must get the new *MyODBC* 2.x driver. * The script `scripts/add_long_password' must be run to convert the `Password' field in the `mysql.user' table to `CHAR(16)'. * All passwords must be reassigned in the `mysql.user' table (to get 62-bit rather than 31-bit passwords). * The table format hasn't changed, so you don't have to convert any tables. MySQL Version 3.20.28 and above can handle the new `user' table format without affecting clients. If you have a MySQL version earlier than Version 3.20.28, passwords will no longer work with it if you convert the `user' table. So to be safe, you should first upgrade to at least Version 3.20.28 and then upgrade to Version 3.21. The new client code works with a 3.20.x `mysqld' server, so if you experience problems with 3.21.x, you can use the old 3.20.x server without having to recompile the clients again. If you are not using the `--old-protocol' option to `mysqld', old clients will issue the error message: ERROR: Protocol mismatch. Server Version = 10 Client Version = 9 The new Perl `DBI'/`DBD' interface also supports the old `mysqlperl' interface. The only change you have to make if you use `mysqlperl' is to change the arguments to the `connect()' function. The new arguments are: `host', `database', `user', `password' (the `user' and `password' arguments have changed places). *Note Perl `DBI' Class: Perl DBI Class. The following changes may affect queries in old applications: * `HAVING' must now be specified before any `ORDER BY' clause. * The parameters to `LOCATE()' have been swapped. * There are some new reserved words. The most notable are `DATE', `TIME', and `TIMESTAMP'. Upgrading to Another Architecture --------------------------------- If you are using MySQL Version 3.23, you can copy the `.frm', `.MYI', and `.MYD' files between different architectures that support the same floating-point format. (MySQL takes care of any byte swapping issues.) The MySQL `ISAM' data and index files (`.ISD' and `*.ISM', respectively) are architecture-dependent and in some cases OS-dependent. If you want to move your applications to another machine that has a different architecture or OS than your current machine, you should not try to move a database by simply copying the files to the other machine. Use `mysqldump' instead. By default, `mysqldump' will create a file full of SQL statements. You can then transfer the file to the other machine and feed it as input to the `mysql' client. Try `mysqldump --help' to see what options are available. If you are moving the data to a newer version of MySQL, you should use `mysqldump --opt' with the newer version to get a fast, compact dump. The easiest (although not the fastest) way to move a database between two machines is to run the following commands on the machine on which the database is located: shell> mysqladmin -h 'other hostname' create db_name shell> mysqldump --opt db_name \ | mysql -h 'other hostname' db_name If you want to copy a database from a remote machine over a slow network, you can use: shell> mysqladmin create db_name shell> mysqldump -h 'other hostname' --opt --compress db_name \ | mysql db_name You can also store the result in a file, then transfer the file to the target machine and load the file into the database there. For example, you can dump a database to a file on the source machine like this: shell> mysqldump --quick db_name | gzip > db_name.contents.gz (The file created in this example is compressed.) Transfer the file containing the database contents to the target machine and run these commands there: shell> mysqladmin create db_name shell> gunzip < db_name.contents.gz | mysql db_name You can also use `mysqldump' and `mysqlimport' to accomplish the database transfer. For big tables, this is much faster than simply using `mysqldump'. In the commands shown below, `DUMPDIR' represents the full pathname of the directory you use to store the output from `mysqldump'. First, create the directory for the output files and dump the database: shell> mkdir DUMPDIR shell> mysqldump --tab=DUMPDIR db_name Then transfer the files in the `DUMPDIR' directory to some corresponding directory on the target machine and load the files into MySQL there: shell> mysqladmin create db_name # create database shell> cat DUMPDIR/*.sql | mysql db_name # create tables in database shell> mysqlimport db_name DUMPDIR/*.txt # load data into tables Also, don't forget to copy the `mysql' database, because that's where the grant tables (`user', `db', `host') are stored. You may have to run commands as the MySQL `root' user on the new machine until you have the `mysql' database in place. After you import the `mysql' database on the new machine, execute `mysqladmin flush-privileges' so that the server reloads the grant table information. Operating System Specific Notes =============================== Linux Notes (All Linux Versions) -------------------------------- The notes below regarding *glibc* apply only to the situation when you build MySQL yourself. If you are running Linux on an x86 machine, in most cases it is much better for you to just use our binary. We link our binaries against the best patched version of *glibc* we can come up with and with the best compiler options, in an attempt to make it suitable for a high-load server. So if you read the text below, and are in doubt about what you should do, try our binary first to see if it meets your needs, and worry about your own build only after you have discovered that our binary is not good enough. In that case, we would appreciate a note about it, so we can build a better binary next time. For a typical user, even for setups with a lot of concurrent connections and/or tables exceeding 2GB limit, our binary in most cases is the best choice. MySQL uses LinuxThreads on Linux. If you are using an old Linux version that doesn't have `glibc2', you must install LinuxThreads before trying to compile MySQL. You can get LinuxThreads at `http://www.mysql.com/Downloads/Linux'. *NOTE:* We have seen some strange problems with Linux 2.2.14 and MySQL on SMP systems; If you have a SMP system, we recommend you to upgrade to Linux 2.4 ASAP! Your system will be faster and more stable by doing this! Note that `glibc' versions before and including Version 2.1.1 have a fatal bug in `pthread_mutex_timedwait' handling, which is used when you do `INSERT DELAYED'. We recommend you to not use `INSERT DELAYED' before upgrading glibc. If you plan to have 1000+ concurrent connections, you will need to make some changes to LinuxThreads, recompile it, and relink MySQL against the new `libpthread.a'. Increase `PTHREAD_THREADS_MAX' in `sysdeps/unix/sysv/linux/bits/local_lim.h' to 4096 and decrease `STACK_SIZE' in `linuxthreads/internals.h' to 256 KB. The paths are relative to the root of `glibc' Note that MySQL will not be stable with around 600-1000 connections if `STACK_SIZE' is the default of 2 MB. If you have a problem with that MySQL can't open enough files, or connections, it may be that you haven't configured Linux to handle enough files. In Linux 2.2 and forwards, you can check the number of allocated file handlers by doing: cat /proc/sys/fs/file-max cat /proc/sys/fs/dquot-max cat /proc/sys/fs/super-max If you have more than 16M of memory, you should add something like the following in your boot script (`/etc/rc/boot.local' on SuSE): echo 65536 > /proc/sys/fs/file-max echo 8192 > /proc/sys/fs/dquot-max echo 1024 > /proc/sys/fs/super-max You can also run the above from the command line as root, but in this case your old limits will be used next time your computer reboots. You should also add /etc/my.cnf: [safe_mysqld] open-files-limit=8192 The above should allow MySQL to create up to 8192 connections + files. The `STACK_SIZE' constant in LinuxThreads controls the spacing of thread stacks in the address space. It needs to be large enough so that there will be plenty of room for the stack of each individual thread, but small enough to keep the stack of some thread from running into the global `mysqld' data. Unfortunately, the Linux implementation of `mmap()', as we have experimentally discovered, will successfully unmap an already mapped region if you ask it to map out an address already in use, zeroing out the data on the entire page, instead of returning an error. So, the safety of `mysqld' or any other threaded application depends on the "gentleman" behavior of the code that creates threads. The user must take measures to make sure the number of running threads at any time is sufficiently low for thread stacks to stay away from the global heap. With `mysqld', you should enforce this "gentleman" behavior by setting a reasonable value for the `max_connections' variable. If you build MySQL yourself and do not want to mess with patching LinuxThreads, you should set `max_connections' to a value no higher than 500. It should be even less if you have a large key buffer, large heap tables, or some other things that make `mysqld' allocate a lot of memory or if you are running a 2.2 kernel with a 2GB patch. If you are using our binary or RPM version 3.23.25 or later, you can safely set `max_connections' at 1500, assuming no large key buffer or heap tables with lots of data. The more you reduce `STACK_SIZE' in LinuxThreads the more threads you can safely create. We recommend the values between 128K and 256K. If you use a lot of concurrent connections, you may suffer from a "feature" in the 2.2 kernel that penalizes a process for forking or cloning a child in an attempt to prevent a fork bomb attack. This will cause MySQL not to scale well as you increase the number of concurrent clients. On single CPU systems, we have seen this manifested in a very slow thread creation, which means it may take a long time to connect to MySQL (as long as 1 minute), and it may take just as long to shut it down. On multiple CPU systems, we have observed a gradual drop in query speed as the number of clients increases. In the process of trying to find a solution, we have received a kernel patch from one of our users, who claimed it made a lot of difference for his site. The patch is available here (`http://www.mysql.com/Downloads/Patches/linux-fork.patch'). We have now done rather extensive testing of this patch on both development and production systems. It has significantly improved `MySQL' performance without causing any problems and we now recommend it to our users who are still running high-load servers on 2.2 kernels. This issue has been fixed in the 2.4 kernel, so if you are not satisfied with the current performance of your system, rather than patching your 2.2 kernel, it might be easier to just upgrade to 2.4, which will also give you a nice SMP boost in addition to fixing this fairness bug. We have tested MySQL on the 2.4 kernel on a 2 CPU machine and found MySQL scales MUCH better - there was virtually no slowdown on query throughput all the way up to 1000 clients, and MySQL scaling factor ( computed as the ratio of maximum throughput to the throughput with one client) was 180%. We have observed similar results on a 4-CPU system - virtually no slowdown as the number of clients was increased up to 1000, and 300% scaling factor. So for a high-load SMP server we would definitely recommend the 2.4 kernel at this point. We have discovered that it is essential to run `mysqld' process with the highest possible priority on the 2.4 kernel to achieve maximum performance. This can be done by adding `renice -20 $$' command to `safe_mysqld'. In our testing on a 4-CPU machine, increasing the priority gave 60% increase in throughput with 400 clients. We are currently also trying to collect more info on how well `MySQL' performs on 2.4 kernel on 4-way and 8-way systems. If you have access such a system and have done some benchmarks, please send a mail to <docs@mysql.com> with the results - we will include them in the manual. There is another issue that greatly hurts MySQL performance, especially on SMP systems. The implementation of mutex in LinuxThreads in *glibc-2.1* is very bad for programs with many threads that only hold the mutex for a short time. On an SMP system, ironic as it is, if you link MySQL against unmodified *LinuxThreads*, removing processors from the machine improves MySQL performance in many cases. We have made a patch available for *glibc 2.1.3*, linuxthreads-2.1-patch (http://www.mysql.com/Downloads/Linux/linuxthreads-2.1-patch) to correct this behavior. With *glibc-2.2.2* MySQL version 3.23.36 will use the adaptive mutex, which is much better than even the patched one in *glibc-2.1.3*. Be warned, however, that under some conditions, the current mutex code in *glibc-2.2.2* overspins, which hurts MySQL performance. The chance of this condition can be reduced by renicing `mysqld' process to the highest priority. We have also been able to correct the overspin behavior with a patch, available here (http://www.mysql.com/Downloads/Linux/linuxthreads-2.2.2.patch). It combines the correction of overspin, maximum number of threads, and stack spacing all in one. You will need to apply it in the `linuxthreads' directory with `patch -p0 </tmp/linuxthreads-2.2.2.patch'. We hope it will be included in some form in to the future releases of `glibc-2.2'. In any case, if you link against `glibc-2.2.2' you still need to correct `STACK_SIZE' and `PTHREAD_THREADS_MAX'. We hope that the defaults will be corrected to some more acceptable values for high-load MySQL setup in the future, so that your own build can be reduced to `./configure; make; make install'. We recommend that you use the above patches to build a special static version of `libpthread.a' and use it only for statically linking against `MySQL'. We know that the patches are safe for `MySQL' and significantly improve its performance, but we cannot say anything about other applications. If you link other applications against the patched version of the library, or build a patched shared version and install it on your system, you are doing it at your own risk with regard to other applications that depend on `LinuxThreads'. If you experience any strange problems during the installation of MySQL, or with some common utilties hanging, it is very likely that they are either library or compiler related. If this is the case, using our binary will resolve them. One known problem with the binary distribution is that with older Linux systems that use `libc' (like RedHat 4.x or Slackware), you will get some non-fatal problems with hostname resolution. *Note Binary notes-Linux::. When using LinuxThreads you will see a minimum of three processes running. These are in fact threads. There will be one thread for the LinuxThreads manager, one thread to handle connections, and one thread to handle alarms and signals. Note that the Linux kernel and the LinuxThread library can by default only have 1024 threads. This means that you can only have up to 1021 connections to MySQL on an unpatched system. The page `http://www.volano.com/linuxnotes.html' contains information how to go around this limit. If you see a dead `mysqld' daemon process with `ps', this usually means that you have found a bug in MySQL or you have a corrupted table. *Note Crashing::. To get a core dump on Linux if `mysqld' dies with a SIGSEGV signal, you can start `mysqld' with the `--core-file' option. Note that you also probably need to raise the `core file size' by adding `ulimit -c 1000000' to `safe_mysqld' or starting `safe_mysqld' with `--core-file-sizes=1000000'. *Note `safe_mysqld': safe_mysqld. To get a core dump on Linux if `mysqld' dies with a SIGSEGV signal, you can start `mysqld' with the `--core-file' option. Note that you also probably need to raise the `core file size' by adding `ulimit -c 1000000' to `safe_mysqld' or starting `safe_mysqld' with `--core-file-sizes=1000000'. *Note `safe_mysqld': safe_mysqld. If you are linking your own MySQL client and get the error: ld.so.1: ./my: fatal: libmysqlclient.so.4: open failed: No such file or directory When executing them, the problem can be avoided by one of the following methods: * Link the client with the following flag (instead of `-Lpath'): `-Wl,r/path-libmysqlclient.so'. * Copy `libmysqclient.so' to `/usr/lib'. * Add the pathname of the directory where `libmysqlclient.so' is located to the `LD_RUN_PATH' environment variable before running your client. If you are using the Fujitsu compiler `(fcc / FCC)' you will have some problems compiling MySQL because the Linux header files are very `gcc' oriented. The following `configure' line should work with `fcc/FCC': CC=fcc CFLAGS="-O -K fast -K lib -K omitfp -Kpreex -D_GNU_SOURCE -DCONST=const -DNO_STRTOLL_PROTO" CXX=FCC CXXFLAGS="-O -K fast -K lib -K omitfp -K preex --no_exceptions --no_rtti -D_GNU_SOURCE -DCONST=const -Dalloca=__builtin_alloca -DNO_STRTOLL_PROTO '-D_EXTERN_INLINE=static __inline'" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-low-memory Linux Notes for Binary Distributions .................................... MySQL needs at least Linux Version 2.0. The binary release is linked with `-static', which means you do not normally need to worry about which version of the system libraries you have. You need not install LinuxThreads, either. A program linked with `-static' is slightly bigger than a dynamically linked program but also slightly faster (3-5%). One problem, however, is that you can't use user-definable functions (UDFs) with a statically linked program. If you are going to write or use UDF functions (this is something only for C or C++ programmers), you must compile MySQL yourself, using dynamic linking. If you are using a `libc'-based system (instead of a `glibc2' system), you will probably get some problems with hostname resolving and `getpwnam()' with the binary release. (This is because `glibc' unfortunately depends on some external libraries to resolve hostnames and `getpwent()', even when compiled with `-static'). In this case you probably get the following error message when you run `mysql_install_db': Sorry, the host 'xxxx' could not be looked up or the following error when you try to run `mysqld' with the `--user' option: getpwnam: No such file or directory You can solve this problem in one of the following ways: * Get a MySQL source distribution (an RPM or the `tar.gz' distribution) and install this instead. * Execute `mysql_install_db --force'; This will not execute the `resolveip' test in `mysql_install_db'. The downside is that you can't use host names in the grant tables; you must use IP numbers instead (except for `localhost'). If you are using an old MySQL release that doesn't support `--force', you have to remove the `resolveip' test in `mysql_install' with an editor. * Start `mysqld' with `su' instead of using `--user'. The Linux-Intel binary and RPM releases of MySQL are configured for the highest possible speed. We are always trying to use the fastest stable compiler available. MySQL Perl support requires Version Perl 5.004_03 or newer. On some Linux 2.2 versions, you may get the error `Resource temporarily unavailable' when you do a lot of new connections to a `mysqld' server over TCP/IP. The problem is that Linux has a delay between when you close a TCP/IP socket and until this is actually freed by the system. As there is only room for a finite number of TCP/IP slots, you will get the above error if you try to do too many new TCP/IP connections during a small time, like when you run the MySQL `test-connect' benchmark over TCP/IP. We have mailed about this problem a couple of times to different Linux mailing lists but have never been able to resolve this properly. The only known 'fix' to this problem is to use persistent connections in your clients or use sockets, if you are running the database server and clients on the same machine. We hope that the `Linux 2.4' kernel will fix this problem in the future. Linux x86 Notes ............... MySQL requires `libc' Version 5.4.12 or newer. It's known to work with `libc' 5.4.46. `glibc' Version 2.0.6 and later should also work. There have been some problems with the `glibc' RPMs from RedHat, so if you have problems, check whether or not there are any updates! The `glibc' 2.0.7-19 and 2.0.7-29 RPMs are known to work. On some older Linux distributions, `configure' may produce an error like this: Syntax error in sched.h. Change _P to __P in the /usr/include/sched.h file. See the Installation chapter in the Reference Manual. Just do what the error message says and add an extra underscore to the `_P' macro that has only one underscore, then try again. You may get some warnings when compiling; those shown below can be ignored: mysqld.cc -o objs-thread/mysqld.o mysqld.cc: In function `void init_signals()': mysqld.cc:315: warning: assignment of negative value `-1' to `long unsigned int' mysqld.cc: In function `void * signal_hand(void *)': mysqld.cc:346: warning: assignment of negative value `-1' to `long unsigned int' In Debian GNU/Linux, if you want MySQL to start automatically when the system boots, do the following: shell> cp support-files/mysql.server /etc/init.d/mysql.server shell> /usr/sbin/update-rc.d mysql.server defaults 99 `mysql.server' can be found in the `share/mysql' directory under the MySQL installation directory or in the `support-files' directory of the MySQL source tree. If `mysqld' always core dumps when it starts up, the problem may be that you have an old `/lib/libc.a'. Try renaming it, then remove `sql/mysqld' and do a new `make install' and try again. This problem has been reported on some Slackware installations. If you get the following error when linking `mysqld', it means that your `libg++.a' is not installed correctly: /usr/lib/libc.a(putc.o): In function `_IO_putc': putc.o(.text+0x0): multiple definition of `_IO_putc' You can avoid using `libg++.a' by running `configure' like this: shell> CXX=gcc ./configure Linux SPARC Notes ................. In some implementations, `readdir_r()' is broken. The symptom is that `SHOW DATABASES' always returns an empty set. This can be fixed by removing `HAVE_READDIR_R' from `config.h' after configuring and before compiling. Some problems will require patching your Linux installation. The patch can be found at `http://www.mysql.com/Downloads/patches/Linux-sparc-2.0.30.diff'. This patch is against the Linux distribution `sparclinux-2.0.30.tar.gz' that is available at `vger.rutgers.edu' (a version of Linux that was never merged with the official 2.0.30). You must also install LinuxThreads Version 0.6 or newer. Linux Alpha Notes ................. MySQL Version 3.23.12 is the first MySQL version that is tested on Linux-Alpha. If you plan to use MySQL on Linux-Alpha, you should ensure that you have this version or newer. We have tested MySQL on Alpha with our benchmarks and test suite, and it appears to work nicely. The main thing we haven't yet had time to test is how things works with many concurrent users. When we compiled the standard MySQL binary we are using SuSE 6.4, kernel 2.2.13-SMP, Compaq C compiler (V6.2-504) and Compaq C++ compiler (V6.3-005) on a Comaq DS20 machine with an Alpha EV6 processor. You can find the above compilers at `http://www.support.compaq.com/alpha-tools'). By using these compilers, instead of gcc, we get about 9-14 % better performance with MySQL. Note that the configure line optimized the binary for the current CPU; This means you can only use our binary if you have an Alpha EV6 processor. We also compile statically to avoid library problems. CC=ccc CFLAGS="-fast" CXX=cxx CXXFLAGS="-fast -noexceptions -nortti" ./configure --prefix=/usr/local/mysql --disable-shared --with-extra-charsets=complex --enable-thread-safe-client --with-mysqld-ldflags=-non_shared --with-client-ldflags=-non_shared If you want to use egcs the following configure line worked for us: CFLAGS="-O3 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --disable-shared Some known problems when running MySQL on Linux-Alpha: * Debugging threaded applications like MySQL will not work with `gdb 4.18'. You should download and use gdb 5.0 instead! * If you try linking `mysqld' statically when using `gcc', the resulting image will core dump at start. In other words, *DON'T* use `--with-mysqld-ldflags=-all-static' with `gcc'. Linux PowerPC Notes ................... MySQL should work on MkLinux with the newest `glibc' package (tested with `glibc' 2.0.7). Linux MIPS Notes ................ To get MySQL to work on Qube2, (Linux Mips), you need the newest `glibc' libraries (`glibc-2.0.7-29C2' is known to work). You must also use the `egcs' C++ compiler (`egcs-1.0.2-9', `gcc 2.95.2' or newer). Linux IA64 Notes ................ To get MySQL to compile on Linux Ia64, we use the following compile line: Using `gcc-2.96': CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql "--with-comment=Official MySQL binary" --with-extra-charsets=complex On Ia64 the MySQL client binaries are using shared libraries. This means that if you install our binary distribution in some other place than `/usr/local/mysql' you need to either modify `/etc/ld.so.conf' or add the path to the directory where you have `libmysqlclient.so' to the `LD_LIBRARY_PATH' environment variable. *Note Link errors::. Windows Notes ------------- This section describes installation and use of MySQL on Windows. This information is also provided in the `README' file that comes with the MySQL Windows distribution. Starting MySQL on Windows 95 or Windows 98 .......................................... MySQL uses TCP/IP to connect a client to a server. (This will allow any machine on your network to connect to your MySQL server.) Because of this, you must install TCP/IP on your machine before starting MySQL. You can find TCP/IP on your Windows CD-ROM. Note that if you are using an old Win95 release (for example OSR2), it's likely that you have an old Winsock package! MySQL requires Winsock 2! You can get the newest Winsock from `http://www.microsoft.com/'. Win98 has the new Winsock 2 library, so the above doesn't apply for Win98. To start the `mysqld' server, you should start an MS-DOS window and type: C:\> C:\mysql\bin\mysqld This will start `mysqld' in the background without a window. You can kill the MySQL server by executing: C:\> C:\mysql\bin\mysqladmin -u root shutdown Note that Win95 and Win98 don't support creation of named pipes. On Win95 and Win98, you can only use named pipes to connect to a remote MySQL server running on a Windows NT server host. (The MySQL server must also support named pipes, of course. For example, using `mysqld-opt' under NT will not allow named pipe connections. You should use either `mysqld-nt' or `mysqld-max-nt'.) If `mysqld' doesn't start, please check the `\mysql\data\mysql.err' file to see if the server wrote any message there to indicate the cause of the problem. You can also try to start the server with `mysqld --standalone'; In this case, you may get some useful information on the screen that may help solve the problem. The last option is to start `mysqld' with `--standalone --debug'. In this case `mysqld' will write a log file `C:\mysqld.trace' that should contain the reason why `mysqld' doesn't start. *Note Making trace files::. Starting MySQL on Windows NT or Windows 2000 ............................................ The Win95/Win98 section also applies to MySQL on NT/Win2000, with the following differences: To get MySQL to work with TCP/IP on NT, you must install service pack 3 (or newer)! Note that everything in the following that applies for NT also applies for Win2000! For NT/Win2000, the server name is `mysqld-nt'. Normally you should install MySQL as a service on NT/Win2000: C:\> C:\mysql\bin\mysqld-nt --install or C:\> C:\mysql\bin\mysqld-max-nt --install (Under Windows NT, you can actually install any of the server binaries as a service, but only those having names that end with `-nt.exe' provide support for named pipes.) You can start and stop the MySQL service with these commands: C:\> NET START mysql C:\> NET STOP mysql Note that in this case you can't use any other options for `mysqld-nt'! You can also run `mysqld-nt' as a stand-alone program on NT if you need to start `mysqld-nt' with any options! If you start `mysqld-nt' without options on NT, `mysqld-nt' tries to start itself as a service with the default service options. If you have stopped `mysqld-nt', you have to start it with `NET START mysql'. The service is installed with the name `MySQL'. Once installed, it must be started using the Services Control Manager (SCM) Utility found in the Control Panel, or by using the `NET START MySQL' command. If any options are desired, they must be specified as "Startup parameters" in the SCM utility before you start the MySQL service. Once running, `mysqld-nt' can be stopped using `mysqladmin', or from the SCM utility or by using the command `NET STOP MySQL'. If you use SCM to stop `mysqld-nt', there is a strange message from SCM about `mysqld shutdown normally'. When run as a service, `mysqld-nt' has no access to a console and so no messages can be seen. On NT you can get the following service error messages: Permission Denied Means that it cannot find `mysqld-nt.exe'. Cannot Register Means that the path is incorrect. Failed to install Means that the service is already installed or service. that the Service Control Manager is in bad state. If you have problems installing `mysqld-nt' as a service, try starting it with the full path: C:\> C:\mysql\bin\mysqld-nt --install If this doesn't work, you can get `mysqld-nt' to start properly by fixing the path in the registry! If you don't want to start `mysqld-nt' as a service, you can start it as follows: C:\> C:\mysql\bin\mysqld-nt --standalone or C:\> C:\mysql\bin\mysqld --standalone --debug The last version gives you a debug trace in `C:\mysqld.trace'. *Note Making trace files::. Running MySQL on Windows ........................ MySQL supports TCP/IP on all Windows platforms and named pipes on NT. The default is to use named pipes for local connections on NT and TCP/IP for all other cases if the client has TCP/IP installed. The host name specifies which protocol is used: *Host name* *Protocol* NULL (none) On NT, try named pipes first; if that doesn't work, use TCP/IP. On Win95/Win98, TCP/IP is used. . Named pipes localhost TCP/IP to current host hostname TCP/IP You can force a MySQL client to use named pipes by specifying the `--pipe' option or by specifying `.' as the host name. Use the `--socket' option to specify the name of the pipe. You can test whether or not MySQL is working by executing the following commands: C:\> C:\mysql\bin\mysqlshow C:\> C:\mysql\bin\mysqlshow -u root mysql C:\> C:\mysql\bin\mysqladmin version status proc C:\> C:\mysql\bin\mysql test If `mysqld' is slow to answer to connections on Win95/Win98, there is probably a problem with your DNS. In this case, start `mysqld' with `--skip-name-resolve' and use only `localhost' and IP numbers in the MySQL grant tables. You can also avoid DNS when connecting to a `mysqld-nt' MySQL server running on NT by using the `--pipe' argument to specify use of named pipes. This works for most MySQL clients. There are two versions of the MySQL command-line tool: `mysql' Compiled on native Windows, which offers very limited text editing capabilities. `mysqlc' Compiled with the Cygnus GNU compiler and libraries, which offers `readline' editing. If you want to use `mysqlc.exe', you must copy `C:\mysql\lib\cygwinb19.dll' to your Windows system directory (`\windows\system' or similar place). The default privileges on Windows give all local users full privileges to all databases without specifying a password. To make MySQL more secure, you should set a password for all users and remove the row in the `mysql.user' table that has `Host='localhost'' and `User='''. You should also add a password for the `root' user. The following example starts by removing the anonymous user that can be used by anyone to access the `test' database, then sets a `root' user password: C:\> C:\mysql\bin\mysql mysql mysql> DELETE FROM user WHERE Host='localhost' AND User=''; mysql> QUIT C:\> C:\mysql\bin\mysqladmin reload C:\> C:\mysql\bin\mysqladmin -u root password your_password After you've set the password, if you want to take down the `mysqld' server, you can do so using this command: C:\> mysqladmin --user=root --password=your_password shutdown If you are using the old shareware version of MySQL Version 3.21 under Windows, the above command will fail with an error: `parse error near 'SET OPTION password''. The fix is in to upgrade to the current MySQL version, which is freely available. With the current MySQL versions you can easily add new users and change privileges with `GRANT' and `REVOKE' commands. *Note GRANT::. Connecting to a Remote MySQL from Windows with SSH .................................................. Here is a note about how to connect to get a secure connection to remote MySQL server with SSH (by David Carlson <dcarlson@mplcomm.com>): * Install an SSH client on your Windows machine. As a user, the best non-free one I've found is from `SecureCRT' from `http://www.vandyke.com/'. Another option is `f-secure' from `http://www.f-secure.com/'. You can also find some free ones on *Google* at `http://directory.google.com/Top/Computers/Security/Products_and_Tools/Cryptography/SSH/Clients/Windows/'. * Start your Windows SSH client. Set `Host_Name = yourmysqlserver_URL_or_IP'. Set `userid=your_userid' to log in to your server (probably not the same as your MySQL login/password. * Set up port forwarding. Either do a remote forward (Set `local_port: 3306', `remote_host: yourmysqlservername_or_ip', `remote_port: 3306' ) or a local forward (Set `port: 3306', `host: localhost', `remote port: 3306'). * Save everything, otherwise you'll have to redo it the next time. * Log in to your server with SSH session you just created. * On your Windows machine, start some ODBC application (such as Access). * Create a new file in Windows and link to MySQL using the ODBC driver the same way you normally do, EXCEPT type in `localhost' for the MySQL host server -- not `yourmysqlservername'. You should now have an ODBC connection to MySQL, encrypted using SSH. Splitting Data Across Different Disks on Windows ................................................ Beginning with MySQL Version 3.23.16, the `mysqld-max' and `mysql-max-nt' servers in the MySQL distribution are compiled with the `-DUSE_SYMDIR' option. This allows you to put a database on different disk by adding a symbolic link to it (in a manner similar to the way that symbolic links work on Unix). On Windows, you make a symbolic link to a database by creating a file that contains the path to the destination directory and saving this in the `mysql_data' directory under the filename `database.sym'. Note that the symbolic link will be used only if the directory `mysql_data_dir\database' doesn't exist. For example, if the MySQL data directory is `C:\mysql\data' and you want to have database `foo' located at `D:\data\foo', you should create the file `C:\mysql\data\foo.sym' that contains the text `D:\data\foo\'. After that, all tables created in the database `foo' will be created in `D:\data\foo'. Note that because of the speed penalty you get when opening every table, we have not enabled this by default even if you have compiled MySQL with support for this. To enable symlinks you should put in your `my.cnf' or `my.ini' file the following entry: [mysqld] use-symbolic-links In MySQL 4.0 we will enable symlinks by default. Then you should instead use the `skip-symlink' option if you want to disable this. Compiling MySQL Clients on Windows .................................. In your source files, you should include `windows.h' before you include `mysql.h': #if defined(_WIN32) || defined(_WIN64) #include <windows.h> #endif #include <mysql.h> You can either link your code with the dynamic `libmysql.lib' library, which is just a wrapper to load in `libmysql.dll' on demand, or link with the static `mysqlclient.lib' library. Note that as the mysqlclient libraries are compiled as threaded libraries, you should also compile your code to be multi-threaded! MySQL-Windows Compared to Unix MySQL .................................... MySQL-Windows has by now proven itself to be very stable. This version of MySQL has the same features as the corresponding Unix version with the following exceptions: *Win95 and threads* Win95 leaks about 200 bytes of main memory for each thread creation. Each connection in MySQL creates a new thread, so you shouldn't run `mysqld' for an extended time on Win95 if your server handles many connections! WinNT and Win98 don't suffer from this bug. *Concurrent reads* MySQL depends on the `pread()' and `pwrite()' calls to be able to mix `INSERT' and `SELECT'. Currently we use mutexes to emulate `pread()'/`pwrite()'. We will, in the long run, replace the file level interface with a virtual interface so that we can use the `readfile()'/`writefile()' interface on NT to get more speed. The current implementation limits the number of open files MySQL can use to 1024, which means that you will not be able to run as many concurrent threads on NT as on Unix. *Blocking read* MySQL uses a blocking read for each connection. This means that: * A connection will not be disconnected automatically after 8 hours, as happens with the Unix version of MySQL. * If a connection hangs, it's impossible to break it without killing MySQL. * `mysqladmin kill' will not work on a sleeping connection. * `mysqladmin shutdown' can't abort as long as there are sleeping connections. We plan to fix this problem when our Windows developers have figured out a nice workaround. *UDF functions* For the moment, MySQL-Windows does not support user-definable functions. *`DROP DATABASE'* You can't drop a database that is in use by some thread. *Killing MySQL from the task manager* You can't kill MySQL from the task manager or with the shutdown utility in Win95. You must take it down with `mysqladmin shutdown'. *Case-insensitive names* Filenames are case insensitive on Windows, so database and table names are also case insensitive in MySQL for Windows. The only restriction is that database and table names must be specified using the same case throughout a given statement. *Note Name case sensitivity::. *The `\' directory character* Pathname components in Win95 are separated by the `\' character, which is also the escape character in MySQL. If you are using `LOAD DATA INFILE' or `SELECT ... INTO OUTFILE', you must double the `\' character: mysql> LOAD DATA INFILE "C:\\tmp\\skr.txt" INTO TABLE skr; mysql> SELECT * INTO OUTFILE 'C:\\tmp\\skr.txt' FROM skr; Alternatively, use Unix style filenames with `/' characters: mysql> LOAD DATA INFILE "C:/tmp/skr.txt" INTO TABLE skr; mysql> SELECT * INTO OUTFILE 'C:/tmp/skr.txt' FROM skr; *`Can't open named pipe' error* If you use a MySQL 3.22 version on NT with the newest mysql-clients you will get the following error: error 2017: can't open named pipe to host: . pipe... This is because the release version of MySQL uses named pipes on NT by default. You can avoid this error by using the `--host=localhost' option to the new MySQL clients or create an option file `C:\my.cnf' that contains the following information: [client] host = localhost *`Access denied for user' error* If you get the error `Access denied for user: 'some-user@unknown' to database 'mysql'' when accessing a MySQL server on the same machine, this means that MySQL can't resolve your host name properly. To fix this, you should create a file `\windows\hosts' with the following information: 127.0.0.1 localhost *`ALTER TABLE'* While you are executing an `ALTER TABLE' statement, the table is locked from usage by other threads. This has to do with the fact that on Windows, you can't delete a file that is in use by another threads. (In the future, we may find some way to work around this problem.) *`DROP TABLE' on a table that is in use by a `MERGE' table will not work* The `MERGE' handler does its table mapping hidden from MySQL. Because Windows doesn't allow you to drop files that are open, you first must flush all `MERGE' tables (with `FLUSH TABLES') or drop the `MERGE' table before dropping the table. We will fix this at the same time we introduce `VIEW's. Here are some open issues for anyone who might want to help us with the Windows release: * Make a single-user `MYSQL.DLL' server. This should include everything in a standard MySQL server, except thread creation. This will make MySQL much easier to use in applications that don't need a true client/server and don't need to access the server from other hosts. * Add some nice start and shutdown icons to the MySQL installation. * Create a tool to manage registry entries for the MySQL startup options. The registry entry reading is already coded into `mysqld.cc', but it should be recoded to be more parameter oriented. The tool should also be able to update the `C:\my.cnf' option file if the user prefers to use that instead of the registry. * When registering `mysqld' as a service with `--install' (on NT) it would be nice if you could also add default options on the command line. For the moment, the workaround is to list the parameters in the `C:\my.cnf' file instead. * It would be real nice to be able to kill `mysqld' from the task manager. For the moment, you must use `mysqladmin shutdown'. * Port `readline' to Windows for use in the `mysql' command line tool. * GUI versions of the standard MySQL clients (`mysql', `mysqlshow', `mysqladmin', and `mysqldump') would be nice. * It would be nice if the socket read and write functions in `net.c' were interruptible. This would make it possible to kill open threads with `mysqladmin kill' on Windows. * `mysqld' always starts in the "C" locale and not in the default locale. We would like to have `mysqld' use the current locale for the sort order. * Implement UDF functions with `.DLL's. * Add macros to use the faster thread-safe increment/decrement methods provided by Windows. Other Windows-specific issues are described in the `README' file that comes with the MySQL-Windows distribution. Solaris Notes ------------- On Solaris, you may run into trouble even before you get the MySQL distribution unpacked! Solaris `tar' can't handle long file names, so you may see an error like this when you unpack MySQL: x mysql-3.22.12-beta/bench/Results/ATIS-mysql_odbc-NT_4.0-cmp-db2,informix,ms-sql,mysql,oracle,solid,sybase, 0 bytes, 0 tape blocks tar: directory checksum error In this case, you must use GNU `tar' (`gtar') to unpack the distribution. You can find a precompiled copy for Solaris at `http://www.mysql.com/Downloads/'. Sun native threads work only on Solaris 2.5 and higher. For Version 2.4 and earlier, MySQL will automatically use MIT-pthreads. *Note MIT-pthreads::. If you get the following error from configure: checking for restartable system calls... configure: error can not run test programs while cross compiling This means that you have something wrong with your compiler installation! In this case you should upgrade your compiler to a newer version. You may also be able to solve this problem by inserting the following row into the `config.cache' file: ac_cv_sys_restartable_syscalls=${ac_cv_sys_restartable_syscalls='no'} If you are using Solaris on a SPARC, the recommended compiler is `gcc' 2.95.2. You can find this at `http://gcc.gnu.org/'. Note that `egcs' 1.1.1 and `gcc' 2.8.1 don't work reliably on SPARC! The recommended `configure' line when using `gcc' 2.95.2 is: CC=gcc CFLAGS="-O3" \ CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" \ ./configure --prefix=/usr/local/mysql --with-low-memory --enable-assembler If you have a ultra sparc, you can get 4 % more performance by adding "-mcpu=v8 -Wa,-xarch=v8plusa" to CFLAGS and CXXFLAGS. If you have Sun Workshop (Fortre) 5.3 (or newer) compiler, you can run `configure' like this: CC=cc CFLAGS="-Xa -fast -xO4 -native -xstrconst -mt" \ CXX=CC CXXFLAGS="-noex -xO4 -mt" \ ./configure --prefix=/usr/local/mysql --enable-assembler In the MySQL benchmarks, we got a 6 % speedup on an Ultrasparc when using Sun Workshop 5.3 compared to using gcc with -mcpu flags. If you get a problem with `fdatasync' or `sched_yield', you can fix this by adding `LIBS=-lrt' to the configure line The following paragraph is only relevant for older compilers than WorkShop 5.3: You may also have to edit the `configure' script to change this line: #if !defined(__STDC__) || __STDC__ != 1 to this: #if !defined(__STDC__) If you turn on `__STDC__' with the `-Xc' option, the Sun compiler can't compile with the Solaris `pthread.h' header file. This is a Sun bug (broken compiler or broken include file). If `mysqld' issues the error message shown below when you run it, you have tried to compile MySQL with the Sun compiler without enabling the multi-thread option (`-mt'): libc internal error: _rmutex_unlock: rmutex not held Add `-mt' to `CFLAGS' and `CXXFLAGS' and try again. If you get the following error when compiling MySQL with `gcc', it means that your `gcc' is not configured for your version of Solaris: shell> gcc -O3 -g -O2 -DDBUG_OFF -o thr_alarm ... ./thr_alarm.c: In function `signal_hand': ./thr_alarm.c:556: too many arguments to function `sigwait' The proper thing to do in this case is to get the newest version of `gcc' and compile it with your current `gcc' compiler! At least for Solaris 2.5, almost all binary versions of `gcc' have old, unusable include files that will break all programs that use threads (and possibly other programs)! Solaris doesn't provide static versions of all system libraries (`libpthreads' and `libdl'), so you can't compile MySQL with `--static'. If you try to do so, you will get the error: ld: fatal: library -ldl: not found or undefined reference to `dlopen' or cannot find -lrt If too many processes try to connect very rapidly to `mysqld', you will see this error in the MySQL log: Error in accept: Protocol error You might try starting the server with the `--set-variable back_log=50' option as a workaround for this. *Note Command-line options::. If you are linking your own MySQL client, you might get the following error when you try to execute it: ld.so.1: ./my: fatal: libmysqlclient.so.#: open failed: No such file or directory The problem can be avoided by one of the following methods: * Link the client with the following flag (instead of `-Lpath'): `-Wl,r/full-path-to-libmysqlclient.so'. * Copy `libmysqclient.so' to `/usr/lib'. * Add the pathname of the directory where `libmysqlclient.so' is located to the `LD_RUN_PATH' environment variable before running your client. When using the `--with-libwrap' configure option, you must also include the libraries that `libwrap.a' needs: --with-libwrap="/opt/NUtcpwrapper-7.6/lib/libwrap.a -lnsl -lsocket If you have problems with configure trying to link with `-lz' and you don't have `zlib' installed, you have two options: * If you want to be able to use the compressed communication protocol, you need to get and install zlib from ftp.gnu.org. * Configure with `--with-named-z-libs=no'. If you are using gcc and have problems with loading `UDF' functions into MySQL, try adding `-lgcc' to the link line for the `UDF' function. If you would like MySQL to start automatically, you can copy `support-files/mysql.server' to `/etc/init.d' and create a symbolic link to it named `/etc/rc3.d/S99mysql.server'. Solaris 2.7/2.8 Notes ..................... You can normally use a Solaris 2.6 binary on Solaris 2.7 and 2.8. Most of the Solaris 2.6 issues also apply for Solaris 2.7 and 2.8. Note that MySQL Version 3.23.4 and above should be able to autodetect new versions of Solaris and enable workarounds for the following problems! Solaris 2.7 / 2.8 has some bugs in the include files. You may see the following error when you use `gcc': /usr/include/widec.h:42: warning: `getwc' redefined /usr/include/wchar.h:326: warning: this is the location of the previous definition If this occurs, you can do the following to fix the problem: Copy `/usr/include/widec.h' to `.../lib/gcc-lib/os/gcc-version/include' and change line 41 from: #if !defined(lint) && !defined(__lint) to #if !defined(lint) && !defined(__lint) && !defined(getwc) Alternatively, you can edit `/usr/include/widec.h' directly. Either way, after you make the fix, you should remove `config.cache' and run `configure' again! If you get errors like this when you run `make', it's because `configure' didn't detect the `curses.h' file (probably because of the error in `/usr/include/widec.h'): In file included from mysql.cc:50: /usr/include/term.h:1060: syntax error before `,' /usr/include/term.h:1081: syntax error before `;' The solution to this is to do one of the following: * Configure with `CFLAGS=-DHAVE_CURSES_H CXXFLAGS=-DHAVE_CURSES_H ./configure'. * Edit `/usr/include/widec.h' as indicted above and rerun configure. * Remove the `#define HAVE_TERM' line from `config.h' file and run `make' again. If you get a problem that your linker can't find `-lz' when linking your client program, the problem is probably that your `libz.so' file is installed in `/usr/local/lib'. You can fix this by one of the following methods: * Add `/usr/local/lib' to `LD_LIBRARY_PATH'. * Add a link to `libz.so' from `/lib'. * If you are using Solaris 8, you can install the optional zlib from your Solaris 8 CD distribution. * Configure MySQL with the `--with-named-z-libs=no' option. Solaris x86 Notes ................. On Solaris 2.8 on x86, `mysqld' will core dump if you run 'strip' in. If you are using `gcc' or `egcs' on Solaris x86 and you experience problems with core dumps under load, you should use the following `configure' command: CC=gcc CFLAGS="-O3 -fomit-frame-pointer -DHAVE_CURSES_H" \ CXX=gcc \ CXXFLAGS="-O3 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti -DHAVE_CURSES_H" \ ./configure --prefix=/usr/local/mysql This will avoid problems with the `libstdc++' library and with C++ exceptions. If this doesn't help, you should compile a debug version and run it with a trace file or under `gdb'. *Note Using gdb on mysqld::. BSD Notes --------- FreeBSD Notes ............. FreeBSD 3.x is recommended for running MySQL since the thread package is much more integrated. The easiest and therefor the preferred way to install is to use the mysql-server and mysql-client ports available on `http://www.freebsd.org'. Using these gives you: * A working MySQL with all optimizations known to work on your version of FreeBSD enabled. * Automatic configuration and build. * Startup scripts installed in /usr/local/etc/rc.d. * Ability to see which files that are installed with pkg_info -L. And to remove them all with pkg_delete if you no longer want MySQL on that machine. It is recommended you use MIT-pthreads on FreeBSD 2.x and native threads on Versions 3 and up. It is possible to run with native threads on some late 2.2.x versions but you may encounter problems shutting down `mysqld'. The MySQL `Makefile's require GNU make (`gmake') to work. If you want to compile MySQL you need to install GNU `make' first. Be sure to have your name resolver setup correct. Otherwise you may experience resolver delays or failures when connecting to `mysqld'. Make sure that the `localhost' entry in the `/etc/hosts' file is correct (otherwise you will have problems connecting to the database). The `/etc/hosts' file should start with a line: 127.0.0.1 localhost localhost.your.domain If you notice that `configure' will use MIT-pthreads, you should read the MIT-pthreads notes. *Note MIT-pthreads::. If you get an error from `make install' that it can't find `/usr/include/pthreads', `configure' didn't detect that you need MIT-pthreads. This is fixed by executing these commands: shell> rm config.cache shell> ./configure --with-mit-threads FreeBSD is also known to have a very low default file handle limit. *Note Not enough file handles::. Uncomment the ulimit -n section in safe_mysqld or raise the limits for the `mysqld' user in /etc/login.conf (and rebuild it with cap_mkdb /etc/login.conf). Also be sure you set the appropriate class for this user in the password file if you are not using the default (use: chpass mysqld-user-name). *Note `safe_mysqld': safe_mysqld. If you get problems with the current date in MySQL, setting the `TZ' variable will probably help. *Note Environment variables::. To get a secure and stable system you should only use FreeBSD kernels that are marked `-STABLE'. NetBSD notes ............ To compile on NetBSD you need GNU `make'. Otherwise the compile will crash when `make' tries to run `lint' on C++ files. OpenBSD Notes ............. OpenBSD 2.5 Notes ................. On OpenBSD Version 2.5, you can compile MySQL with native threads with the following options: CFLAGS=-pthread CXXFLAGS=-pthread ./configure --with-mit-threads=no OpenBSD 2.8 Notes ................. Our users have reported that OpenBSD 2.8 has a threading bug which causes problems with MySQL. The OpenBSD Developers have fixed the problem, but as of January 25th, 2001, it's only available in the "-current" branch. The symptoms of this threading bug are: slow response, high load, high CPU usage, and crashes. BSD/OS Notes ............ BSD/OS Version 2.x Notes ........................ If you get the following error when compiling MySQL, your `ulimit' value for virtual memory is too low: item_func.h: In method `Item_func_ge::Item_func_ge(const Item_func_ge &)': item_func.h:28: virtual memory exhausted make[2]: *** [item_func.o] Error 1 Try using `ulimit -v 80000' and run `make' again. If this doesn't work and you are using `bash', try switching to `csh' or `sh'; some BSDI users have reported problems with `bash' and `ulimit'. If you are using `gcc', you may also use have to use the `--with-low-memory' flag for `configure' to be able to compile `sql_yacc.cc'. If you get problems with the current date in MySQL, setting the `TZ' variable will probably help. *Note Environment variables::. BSD/OS Version 3.x Notes ........................ Upgrade to BSD/OS Version 3.1. If that is not possible, install BSDIpatch M300-038. Use the following command when configuring MySQL: shell> env CXX=shlicc++ CC=shlicc2 \ ./configure \ --prefix=/usr/local/mysql \ --localstatedir=/var/mysql \ --without-perl \ --with-unix-socket-path=/var/mysql/mysql.sock The following is also known to work: shell> env CC=gcc CXX=gcc CXXFLAGS=-O3 \ ./configure \ --prefix=/usr/local/mysql \ --with-unix-socket-path=/var/mysql/mysql.sock You can change the directory locations if you wish, or just use the defaults by not specifying any locations. If you have problems with performance under heavy load, try using the `--skip-thread-priority' option to `mysqld'! This will run all threads with the same priority; on BSDI Version 3.1, this gives better performance (at least until BSDI fixes their thread scheduler). If you get the error `virtual memory exhausted' while compiling, you should try using `ulimit -v 80000' and run `make' again. If this doesn't work and you are using `bash', try switching to `csh' or `sh'; some BSDI users have reported problems with `bash' and `ulimit'. BSD/OS Version 4.x Notes ........................ BSDI Version 4.x has some thread-related bugs. If you want to use MySQL on this, you should install all thread-related patches. At least M400-023 should be installed. On some BSDI Version 4.x systems, you may get problems with shared libraries. The symptom is that you can't execute any client programs, for example, `mysqladmin'. In this case you need to reconfigure not to use shared libraries with the `--disable-shared' option to configure. Some customers have had problems on BSDI 4.0.1 that the `mysqld' binary after a while can't open tables. This is because some library/system related bug causes `mysqld' to change current directory without asking for this! The fix is to either upgrade to 3.23.34 or after running `configure' remove the line `#define HAVE_REALPATH' from `config.h' before running make. Note that the above means that you can't symbolic link a database directories to another database directory or symbolic link a table to another database on BSDI! (Making a symbolic link to another disk is ok). Mac OS X Notes -------------- Mac OS X Public Beta .................... MySQL should work without any problems on Mac OS X Public Beta (Darwin). You don't need the pthread patches for this OS! Mac OS X Server ............... Before trying to configure MySQL on Mac OS X server you must first install the pthread package from `http://www.prnet.de/RegEx/mysql.html'. Our binary for Mac OS X is compiled on Rhapsody 5.5 with the following configure line: CC=gcc CFLAGS="-O2 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O2 -fomit-frame-pointer" ./configure --prefix=/usr/local/mysql "--with-comment=Official MySQL binary" --with-extra-charsets=complex --disable-shared You might want to also add aliases to your shell's resource file to access `mysql' and `mysqladmin' from the command line: alias mysql '/usr/local/mysql/bin/mysql' alias mysqladmin '/usr/local/mysql/bin/mysqladmin' Other Unix Notes ---------------- HP-UX Notes for Binary Distributions .................................... Some of the binary distributions of MySQL for HP-UX is distributed as an HP depot file and as a tar file. To use the depot file you must be running at least HP-UX 10.x to have access to HP's software depot tools. The HP version of MySQL was compiled on an HP 9000/8xx server under HP-UX 10.20, and uses MIT-pthreads. It is known to work well under this configuration. MySQL Version 3.22.26 and newer can also be built with HP's native thread package. Other configurations that may work: * HP 9000/7xx running HP-UX 10.20+ * HP 9000/8xx running HP-UX 10.30 The following configurations almost definitely won't work: * HP 9000/7xx or 8xx running HP-UX 10.x where x < 2 * HP 9000/7xx or 8xx running HP-UX 9.x To install the distribution, use one of the commands below, where `/path/to/depot' is the full pathname of the depot file: * To install everything, including the server, client and development tools: shell> /usr/sbin/swinstall -s /path/to/depot mysql.full * To install only the server: shell> /usr/sbin/swinstall -s /path/to/depot mysql.server * To install only the client package: shell> /usr/sbin/swinstall -s /path/to/depot mysql.client * To install only the development tools: shell> /usr/sbin/swinstall -s /path/to/depot mysql.developer The depot places binaries and libraries in `/opt/mysql' and data in `/var/opt/mysql'. The depot also creates the appropriate entries in `/etc/init.d' and `/etc/rc2.d' to start the server automatically at boot time. Obviously, this entails being `root' to install. To install the HP-UX tar.gz distribution, you must have a copy of GNU `tar'. HP-UX Version 10.20 Notes ......................... There are a couple of small problems when compiling MySQL on HP-UX. We recommend that you use `gcc' instead of the HP-UX native compiler, because `gcc' produces better code! We recommend using gcc 2.95 on HP-UX. Don't use high optimization flags (like -O6) as this may not be safe on HP-UX. Note that MIT-pthreads can't be compiled with the HP-UX compiler because it can't compile `.S' (assembler) files. The following configure line should work: CFLAGS="-DHPUX -I/opt/dce/include" CXXFLAGS="-DHPUX -I/opt/dce/include -felide-constructors -fno-exceptions -fno-rtti" CXX=gcc ./configure --with-pthread --with-named-thread-libs='-ldce' --prefix=/usr/local/mysql --disable-shared If you are compiling `gcc' 2.95 yourself, you should NOT link it with the DCE libraries (`libdce.a' or `libcma.a') if you want to compile MySQL with MIT-pthreads. If you mix the DCE and MIT-pthreads packages you will get a `mysqld' to which you cannot connect. Remove the DCE libraries while you compile `gcc' 2.95! HP-UX Version 11.x Notes ........................ For HP-UX Version 11.x we recommend MySQL Version 3.23.15 or later. Because of some critical bugs in the standard HP-UX libraries, you should install the following patches before trying to run MySQL on HP-UX 11.0: PHKL_22840 Streams cumulative PHNE_22397 ARPA cumulative This will solve a problem that one gets `EWOULDBLOCK' from `recv()' and `EBADF' from `accept()' in threaded applications. If you are using `gcc' 2.95.1 on an unpatched HP-UX 11.x system, you will get the error: In file included from /usr/include/unistd.h:11, from ../include/global.h:125, from mysql_priv.h:15, from item.cc:19: /usr/include/sys/unistd.h:184: declaration of C function ... /usr/include/sys/pthread.h:440: previous declaration ... In file included from item.h:306, from mysql_priv.h:158, from item.cc:19: The problem is that HP-UX doesn't define `pthreads_atfork()' consistently. It has conflicting prototypes in `/usr/include/sys/unistd.h':184 and `/usr/include/sys/pthread.h':440 (details below). One solution is to copy `/usr/include/sys/unistd.h' into `mysql/include' and edit `unistd.h' and change it to match the definition in `pthread.h'. Here's the diff: 183,184c183,184 < extern int pthread_atfork(void (*prepare)(), void (*parent)(), < void (*child)()); --- > extern int pthread_atfork(void (*prepare)(void), void (*parent)(void), > void (*child)(void)); After this, the following configure line should work: CFLAGS="-fomit-frame-pointer -O3 -fpic" CXX=gcc CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -O3" ./configure --prefix=/usr/local/mysql --disable-shared Here is some information that a HP-UX Version 11.x user sent us about compiling MySQL with HP-UX:x compiler: Environment: proper compilers. setenv CC cc setenv CXX aCC flags setenv CFLAGS -D_REENTRANT setenv CXXFLAGS -D_REENTRANT setenv CPPFLAGS -D_REENTRANT % aCC -V aCC: HP ANSI C++ B3910B X.03.14.06 % cc -V /tmp/empty.c cpp.ansi: HP92453-01 A.11.02.00 HP C Preprocessor (ANSI) ccom: HP92453-01 A.11.01.00 HP C Compiler cc: "/tmp/empty.c", line 1: warning 501: Empty source file. configuration: ./configure --with-pthread \ --prefix=/source-control/mysql \ --with-named-thread-libs=-lpthread \ --with-low-memory added '#define _CTYPE_INCLUDED' to include/m_ctype.h. This symbol is the one defined in HP's /usr/include/ctype.h: /* Don't include std ctype.h when this is included */ #define _CTYPE_H #define __CTYPE_INCLUDED #define _CTYPE_INCLUDED #define _CTYPE_USING /* Don't put names in global namespace. */ * I had to use the compile-time flag `-D_REENTRANT' to get the compiler to recognize the prototype for `localtime_r'. Alternatively I could have supplied the prototype for `localtime_r'. But I wanted to catch other bugs without needing to run into them. I wasn't sure where I needed it, so I added it to all flags. * The optimization flags used by MySQL (-O3) are not recognized by HP's compilers. I did not change the flags. If you get the following error from `configure' checking for cc option to accept ANSI C... no configure: error: MySQL requires a ANSI C compiler (and a C++ compiler). Try gcc. See the Installation chapter in the Reference Manual. Check that you don't have the path to the K&R compiler before the path to the HP-UX C and C++ compiler. IBM-AIX notes ............. Automatic detection of `xlC' is missing from Autoconf, so a `configure' command something like this is needed when compiling MySQL (This example uses the IBM compiler): export CC="xlc_r -ma -O3 -qstrict -qoptimize=3 -qmaxmem=8192 " export CXX="xlC_r -ma -O3 -qstrict -qoptimize=3 -qmaxmem=8192" export CFLAGS="-I /usr/local/include" export LDLFAGS="-L /usr/local/lib" export CPPFLAGS=$CFLAGS export CXXFLAGS=$CFLAGS ./configure --prefix=/usr/local \ --localstatedir=/var/mysql \ --sysconfdir=/etc/mysql \ --sbindir='/usr/local/bin' \ --libexecdir='/usr/local/bin' \ --enable-thread-safe-client \ --enable-large-files Above are the options used to compile the MySQL distribution that can be found at `http://www-frec.bull.com/'. If you change the `-O3' to `-O2' in the above configure line, you must also remove the `-qstrict' option (this is a limitation in the IBM C compiler). If you are using `gcc' or `egcs' to compile MySQL, you *MUST* use the `-fno-exceptions' flag, as the exception handling in `gcc'/`egcs' is not thread safe! (This is tested with `egcs' 1.1.). There are also some known problems with IBM's assembler, which may cause it to generate bad code when used with gcc. We recommend the following `configure' line with `egcs' and `gcc 2.95' on AIX: CC="gcc -pipe -mcpu=power -Wa,-many" \ CXX="gcc -pipe -mcpu=power -Wa,-many" \ CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti" \ ./configure --prefix=/usr/local/mysql --with-low-memory The `-Wa,-many' is necessary for the compile to be successful. IBM is aware of this problem but is in to hurry to fix it because of the workaround available. We don't know if the `-fno-exceptions' is required with `gcc 2.95', but as MySQL doesn't use exceptions and the above option generates faster code, we recommend that you should always use this option with `egcs / gcc'. If you get a problem with assembler code try changing the -mcpu=xxx to match your cpu. Typically power2, power, or powerpc may need to be used, alternatively you might need to use 604 or 604e. I'm not positive but I would think using "power" would likely be safe most of the time, even on a power2 machine. If you don't know what your cpu is then do a "uname -m", this will give you back a string that looks like "000514676700", with a format of xxyyyyyymmss where xx and ss are always 0's, yyyyyy is a unique system id and mm is the id of the CPU Planar. A chart of these values can be found at `http://www.rs6000.ibm.com/doc_link/en_US/a_doc_lib/cmds/aixcmds5/uname.htm'. This will give you a machine type and a machine model you can use to determine what type of cpu you have. If you have problems with signals (MySQL dies unexpectedly under high load) you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with: shell> CFLAGS=-DDONT_USE_THR_ALARM CXX=gcc \ CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -DDONT_USE_THR_ALARM" \ ./configure --prefix=/usr/local/mysql --with-debug --with-low-memory This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are "sleeping" on a connection with `mysqladmin kill' or `mysqladmin shutdown'. Instead, the client will die when it issues its next command. On some versions of AIX, linking with `libbind.a' makes `getservbyname' core dump. This is an AIX bug and should be reported to IBM. For AIX 4.2.1 and gcc you have to do the following changes. After configuring, edit `config.h' and `include/my_config.h' and change the line that says #define HAVE_SNPRINTF 1 to #undef HAVE_SNPRINTF And finally, in `mysqld.cc' you need to add a prototype for initgoups. #ifdef _AIX41 extern "C" int initgroups(const char *,int); #endif SunOS 4 Notes ............. On SunOS 4, MIT-pthreads is needed to compile MySQL, which in turn means you will need GNU `make'. Some SunOS 4 systems have problems with dynamic libraries and `libtool'. You can use the following `configure' line to avoid this problem: shell> ./configure --disable-shared --with-mysqld-ldflags=-all-static When compiling `readline', you may get warnings about duplicate defines. These may be ignored. When compiling `mysqld', there will be some `implicit declaration of function' warnings. These may be ignored. Alpha-DEC-UNIX Notes (Tru64) ............................ If you are using egcs 1.1.2 on Digital Unix, you should upgrade to gcc 2.95.2, as egcs on DEC has some serious bugs! When compiling threaded programs under Digital Unix, the documentation recommends using the `-pthread' option for `cc' and `cxx' and the libraries `-lmach -lexc' (in addition to `-lpthread'). You should run `configure' something like this: CC="cc -pthread" CXX="cxx -pthread -O" \ ./configure --with-named-thread-libs="-lpthread -lmach -lexc -lc" When compiling `mysqld', you may see a couple of warnings like this: mysqld.cc: In function void handle_connections()': mysqld.cc:626: passing long unsigned int *' as argument 3 of accept(int,sockadddr *, int *)' You can safely ignore these warnings. They occur because `configure' can detect only errors, not warnings. If you start the server directly from the command line, you may have problems with it dying when you log out. (When you log out, your outstanding processes receive a `SIGHUP' signal.) If so, try starting the server like this: shell> nohup mysqld [options] & `nohup' causes the command following it to ignore any `SIGHUP' signal sent from the terminal. Alternatively, start the server by running `safe_mysqld', which invokes `mysqld' using `nohup' for you. *Note `safe_mysqld': safe_mysqld. If you get a problem when compiling mysys/get_opt.c, just remove the line #define _NO_PROTO from the start of that file! If you are using Compac's CC compiler, the following configure line should work: CC="cc -pthread" CFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed all -arch host" CXX="cxx -pthread" CXXFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed all -arch host" export CC CFLAGS CXX CXXFLAGS ./configure \ --prefix=/usr/local/mysql \ --with-low-memory \ --enable-large-files \ --enable-shared=yes \ --with-named-thread-libs="-lpthread -lmach -lexc -lc" gnumake If you get a problem with libtool, when compiling with shared libraries as above, when linking `mysql', you should be able to get around this by issuing: cd mysql /bin/sh ../libtool --mode=link cxx -pthread -O3 -DDBUG_OFF \ -O4 -ansi_alias -ansi_args -fast -inline speed \ -speculate all \ -arch host -DUNDEF_HAVE_GETHOSTBYNAME_R \ -o mysql mysql.o readline.o sql_string.o completion_hash.o \ ../readline/libreadline.a -lcurses \ ../libmysql/.libs/libmysqlclient.so -lm cd .. gnumake gnumake install scripts/mysql_install_db Alpha-DEC-OSF1 Notes .................... If you have problems compiling and have DEC `CC' and `gcc' installed, try running `configure' like this: CC=cc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 \ ./configure --prefix=/usr/local/mysql If you get problems with the `c_asm.h' file, you can create and use a 'dummy' `c_asm.h' file with: touch include/c_asm.h CC=gcc CFLAGS=-I./include \ CXX=gcc CXXFLAGS=-O3 \ ./configure --prefix=/usr/local/mysql Note that the following problems with the `ld' program can be fixed by downloading the latest DEC (Compaq) patch kit from: `http://ftp.support.compaq.com/public/unix/'. On OSF1 V4.0D and compiler "DEC C V5.6-071 on Digital Unix V4.0 (Rev. 878)" the compiler had some strange behavior (undefined `asm' symbols). `/bin/ld' also appears to be broken (problems with `_exit undefined' errors occuring while linking `mysqld'). On this system, we have managed to compile MySQL with the following `configure' line, after replacing `/bin/ld' with the version from OSF 4.0C: CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql With the Digital compiler "C++ V6.1-029", the following should work: CC=cc -pthread CFLAGS=-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all -arch host CXX=cxx -pthread CXXFLAGS=-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all -arch host -noexceptions -nortti export CC CFLAGS CXX CXXFLAGS ./configure --prefix=/usr/mysql/mysql --with-mysqld-ldflags=-all-static --disable-shared --with-named-thread-libs="-lmach -lexc -lc" In some versions of OSF1, the `alloca()' function is broken. Fix this by removing the line in `config.h' that defines `'HAVE_ALLOCA''. The `alloca()' function also may have an incorrect prototype in `/usr/include/alloca.h'. This warning resulting from this can be ignored. `configure' will use the following thread libraries automatically: `--with-named-thread-libs="-lpthread -lmach -lexc -lc"'. When using `gcc', you can also try running `configure' like this: shell> CFLAGS=-D_PTHREAD_USE_D4 CXX=gcc CXXFLAGS=-O3 ./configure .... If you have problems with signals (MySQL dies unexpectedly under high load), you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with: shell> CFLAGS=-DDONT_USE_THR_ALARM \ CXXFLAGS=-DDONT_USE_THR_ALARM \ ./configure ... This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are "sleeping" on a connection with `mysqladmin kill' or `mysqladmin shutdown'. Instead, the client will die when it issues its next command. With `gcc' 2.95.2, you will probably run into the following compile error: sql_acl.cc:1456: Internal compiler error in `scan_region', at except.c:2566 Please submit a full bug report. To fix this you should change to the `sql' directory and do a "cut and paste" of the last `gcc' line, but change `-O3' to `-O0' (or add `-O0' immediately after `gcc' if you don't have any `-O' option on your compile line.) After this is done you can just change back to the top-level directly and run `make' again. SGI Irix Notes .............. If you are using Irix Version 6.5.3 or newer `mysqld' will only be able to create threads if you run it as a user with `CAP_SCHED_MGT' privileges (like `root') or give the `mysqld' server this privilege with the following shell command: shell> chcap "CAP_SCHED_MGT+epi" /opt/mysql/libexec/mysqld You may have to undefine some things in `config.h' after running `configure' and before compiling. In some Irix implementations, the `alloca()' function is broken. If the `mysqld' server dies on some `SELECT' statements, remove the lines from `config.h' that define `HAVE_ALLOC' and `HAVE_ALLOCA_H'. If `mysqladmin create' doesn't work, remove the line from `config.h' that defines `HAVE_READDIR_R'. You may have to remove the `HAVE_TERM_H' line as well. SGI recommends that you install all of the patches on this page as a set: http://support.sgi.com/surfzone/patches/patchset/6.2_indigo.rps.html At the very minimum, you should install the latest kernel rollup, the latest `rld' rollup, and the latest `libc' rollup. You definitely need all the POSIX patches on this page, for pthreads support: `http://support.sgi.com/surfzone/patches/patchset/6.2_posix.rps.html' If you get the something like the following error when compiling `mysql.cc': "/usr/include/curses.h", line 82: error(1084): invalid combination of type Type the following in the top-level directory of your MySQL source tree: shell> extra/replace bool curses_bool < /usr/include/curses.h > include/curses.h shell> make There have also been reports of scheduling problems. If only one thread is running, things go slow. Avoid this by starting another client. This may lead to a 2-to-10-fold increase in execution speed thereafter for the other thread. This is a poorly understood problem with Irix threads; you may have to improvise to find solutions until this can be fixed. If you are compiling with `gcc', you can use the following `configure' command: CC=gcc CXX=gcc CXXFLAGS=-O3 \ ./configure --prefix=/usr/local/mysql --enable-thread-safe-client --with-named-thread-libs=-lpthread On Irix 6.5.11 with native Irix C and C++ compilers ver. 7.3.1.2, the following is reported to work CC=cc CXX=CC CFLAGS='-O3 -n32 -TARG:platform=IP22 -I/usr/local/include \ -L/usr/local/lib' CXXFLAGS='-O3 -n32 -TARG:platform=IP22 \ -I/usr/local/include -L/usr/local/lib' ./configure --prefix=/usr/local/mysql \ --with-berkeley-db --with-innodb \ --with-libwrap=/usr/local --with-named-curses-libs=/usr/local/lib/libncurses.a SCO Notes ......... The current port is tested only on a "sco3.2v5.0.4" and "sco3.2v5.0.5" system. There has also been a lot of progress on a port to "sco 3.2v4.2". For the moment the recommended compiler on OpenServer is gcc 2.95.2. With this you should be able to compile MySQL with just: CC=gcc CXX=gcc ./configure ... (options) 1. For OpenServer 5.0.X you need to use GDS in Skunkware 95 (95q4c). This is necessary because GNU `gcc' 2.7.2 in Skunkware 97 does not have GNU `as'. You can also use `egcs' 1.1.2 or newer `http://www.egcs.com/'. If you are using `egcs' 1.1.2 you have to execute the following command: shell> cp -p /usr/include/pthread/stdtypes.h /usr/local/lib/gcc-lib/i386-pc-sco3.2v5.0.5/egcs-2.91.66/include/pthread/ 2. You need the port of GCC 2.5.x for this product and the Development system. They are required on this version of SCO Unix. You cannot just use the GCC Dev system. 3. You should get the FSU Pthreads package and install it first. This can be found at `http://www.cs.wustl.edu/~schmidt/ACE_wrappers/FSU-threads.tar.gz'. You can also get a precompiled package from `http://www.mysql.com/Downloads/SCO/FSU-threads-3.5c.tar.gz'. 4. FSU Pthreads can be compiled with SCO Unix 4.2 with tcpip. Or OpenServer 3.0 or Open Desktop 3.0 (OS 3.0 ODT 3.0), with the SCO Development System installed using a good port of GCC 2.5.x ODT or OS 3.0 you will need a good port of GCC 2.5.x There are a lot of problems without a good port. The port for this product requires the SCO Unix Development system. Without it, you are missing the libraries and the linker that is needed. 5. To build FSU Pthreads on your system, do the following: a. Run `./configure' in the `threads/src' directory and select the SCO OpenServer option. This command copies `Makefile.SCO5' to `Makefile'. b. Run `make'. c. To install in the default `/usr/include' directory, login as root, then `cd' to the `thread/src' directory, and run `make install'. 6. Remember to use GNU `make' when making MySQL. 7. If you don't start `safe_mysqld' as root, you probably will get only the default 110 open files per process. `mysqld' will write a note about this in the log file. 8. With SCO 3.2V5.0.5, you should use FSU Pthreads version 3.5c or newer. You should also use gcc 2.95.2 or newer! The following `configure' command should work: shell> ./configure --prefix=/usr/local/mysql --disable-shared 9. With SCO 3.2V4.2, you should use FSU Pthreads version 3.5c or newer. The following `configure' command should work: shell> CFLAGS="-D_XOPEN_XPG4" CXX=gcc CXXFLAGS="-D_XOPEN_XPG4" \ ./configure \ --prefix=/usr/local/mysql \ --with-named-thread-libs="-lgthreads -lsocket -lgen -lgthreads" \ --with-named-curses-libs="-lcurses" You may get some problems with some include files. In this case, you can find new SCO-specific include files at `http://www.mysql.com/Downloads/SCO/SCO-3.2v4.2-includes.tar.gz'. You should unpack this file in the `include' directory of your MySQL source tree. SCO development notes: * MySQL should automatically detect FSU Pthreads and link `mysqld' with `-lgthreads -lsocket -lgthreads'. * The SCO development libraries are re-entrant in FSU Pthreads. SCO claims that its libraries' functions are re-entrant, so they must be reentrant with FSU Pthreads. FSU Pthreads on OpenServer tries to use the SCO scheme to make re-entrant libraries. * FSU Pthreads (at least the version at `http://www.mysql.com/') comes linked with GNU `malloc'. If you encounter problems with memory usage, make sure that `gmalloc.o' is included in `libgthreads.a' and `libgthreads.so'. * In FSU Pthreads, the following system calls are pthreads-aware: `read()', `write()', `getmsg()', `connect()', `accept()', `select()', and `wait()'. If you want to install DBI on SCO, you have to edit the `Makefile' in DBI-xxx and each subdirectory. Note that the following assumes gcc 2.95.2 or newer: OLD: NEW: CC = cc CC = gcc CCCDLFLAGS = -KPIC -W1,-Bexport CCCDLFLAGS = -fpic CCDLFLAGS = -wl,-Bexport CCDLFLAGS = LD = ld LD = gcc -G -fpic LDDLFLAGS = -G -L/usr/local/lib LDDLFLAGS = -L/usr/local/lib LDFLAGS = -belf -L/usr/local/lib LDFLAGS = -L/usr/local/lib LD = ld LD = gcc -G -fpic OPTIMISE = -Od OPTIMISE = -O1 OLD: CCCFLAGS = -belf -dy -w0 -U M_XENIX -DPERL_SCO5 -I/usr/local/include NEW: CCFLAGS = -U M_XENIX -DPERL_SCO5 -I/usr/local/include This is because the Perl dynaloader will not load the `DBI' modules if they were compiled with `icc' or `cc'. Perl works best when compiled with `cc'. SCO Unixware Version 7.0 Notes .............................. You must use a version of MySQL at least as recent as Version 3.22.13 because that version fixes some portability problems under Unixware. We have been able to compile MySQL with the following `configure' command on Unixware Version 7.0.1: CC=cc CXX=CC ./configure --prefix=/usr/local/mysql If you want to use `gcc', you must use `gcc' 2.95.2 or newer. OS/2 Notes ---------- MySQL uses quite a few open files. Because of this, you should add something like the following to your `CONFIG.SYS' file: SET EMXOPT=-c -n -h1024 If you don't do this, you will probably run into the following error: File 'xxxx' not found (Errcode: 24) When using MySQL with OS/2 Warp 3, FixPack 29 or above is required. With OS/2 Warp 4, FixPack 4 or above is required. This is a requirement of the Pthreads library. MySQL must be installed in a partition that supports long filenames such as HPFS, FAT32, etc. The `INSTALL.CMD' script must be run from OS/2's own `CMD.EXE' and may not work with replacement shells such as `4OS2.EXE'. The `scripts/mysql-install-db' script has been renamed. It is now called `install.cmd' and is a REXX script, which will set up the default MySQL security settings and create the WorkPlace Shell icons for MySQL. Dynamic module support is compiled in but not fully tested. Dynamic modules should be compiled using the Pthreads run-time library. gcc -Zdll -Zmt -Zcrtdll=pthrdrtl -I../include -I../regex -I.. \ -o example udf_example.cc -L../lib -lmysqlclient udf_example.def mv example.dll example.udf *Note:* Due to limitations in OS/2, UDF module name stems must not exceed 8 characters. Modules are stored in the `/mysql2/udf' directory; the `safe-mysqld.cmd' script will put this directory in the `BEGINLIBPATH' environment variable. When using UDF modules, specified extensions are ignored -- it is assumed to be `.udf'. For example, in Unix, the shared module might be named `example.so' and you would load a function from it like this: mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "example.so"; Is OS/2, the module would be named `example.udf', but you would not specify the module extension: mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "example"; BeOS Notes ---------- We are really interested in getting MySQL to work on BeOS, but unfortunately we don't have any person who knows BeOS or has time to do a port. We are interested in finding someone to do a port, and we will help them with any technical questions they may have while doing the port. We have previously talked with some BeOS developers that have said that MySQL is 80% ported to BeOS, but we haven't heard from them in a while. Novell Netware Notes -------------------- We are really interested in getting MySQL to work on Netware, but unfortunately we don't have any person who knows Netware or has time to do a port. We are interested in finding someone to do a port, and we will help them with any technical questions they may have while doing the port. Introduction to MySQL: A MySQL Tutorial *************************************** This chapter provides a tutorial introduction to MySQL by showing how to use the `mysql' client program to create and use a simple database. `mysql' (sometimes referred to as the "terminal monitor" or just "monitor") is an interactive program that allows you to connect to a MySQL server, run queries, and view the results. `mysql' may also be used in batch mode: you place your queries in a file beforehand, then tell `mysql' to execute the contents of the file. Both ways of using `mysql' are covered here. To see a list of options provided by `mysql', invoke it with the `--help' option: shell> mysql --help This chapter assumes that `mysql' is installed on your machine and that a MySQL server is available to which you can connect. If this is not true, contact your MySQL administrator. (If _you_ are the administrator, you will need to consult other sections of this manual.) This chapter describes the entire process of setting up and using a database. If you are interested only in accessing an already-existing database, you may want to skip over the sections that describe how to create the database and the tables it contains. Because this chapter is tutorial in nature, many details are necessarily left out. Consult the relevant sections of the manual for more information on the topics covered here. Connecting to and Disconnecting from the Server =============================================== To connect to the server, you'll usually need to provide a MySQL user name when you invoke `mysql' and, most likely, a password. If the server runs on a machine other than the one where you log in, you'll also need to specify a hostname. Contact your administrator to find out what connection parameters you should use to connect (that is, what host, user name, and password to use). Once you know the proper parameters, you should be able to connect like this: shell> mysql -h host -u user -p Enter password: ******** The `********' represents your password; enter it when `mysql' displays the `Enter password:' prompt. If that works, you should see some introductory information followed by a `mysql>' prompt: shell> mysql -h host -u user -p Enter password: ******** Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 459 to server version: 3.22.20a-log Type 'help' for help. mysql> The prompt tells you that `mysql' is ready for you to enter commands. Some MySQL installations allow users to connect as the anonymous (unnamed) user to the server running on the local host. If this is the case on your machine, you should be able to connect to that server by invoking `mysql' without any options: shell> mysql After you have connected successfully, you can disconnect any time by typing `QUIT' at the `mysql>' prompt: mysql> QUIT Bye You can also disconnect by pressing Control-D. Most examples in the following sections assume you are connected to the server. They indicate this by the `mysql>' prompt. Entering Queries ================ Make sure you are connected to the server, as discussed in the previous section. Doing so will not in itself select any database to work with, but that's okay. At this point, it's more important to find out a little about how to issue queries than to jump right in creating tables, loading data into them, and retrieving data from them. This section describes the basic principles of entering commands, using several queries you can try out to familiarize yourself with how `mysql' works. Here's a simple command that asks the server to tell you its version number and the current date. Type it in as shown below following the `mysql>' prompt and hit the RETURN key: mysql> SELECT VERSION(), CURRENT_DATE; +--------------+--------------+ | version() | CURRENT_DATE | +--------------+--------------+ | 3.22.20a-log | 1999-03-19 | +--------------+--------------+ 1 row in set (0.01 sec) mysql> This query illustrates several things about `mysql': * A command normally consists of a SQL statement followed by a semicolon. (There are some exceptions where a semicolon is not needed. `QUIT', mentioned earlier, is one of them. We'll get to others later.) * When you issue a command, `mysql' sends it to the server for execution and displays the results, then prints another `mysql>' to indicate that it is ready for another command. * `mysql' displays query output as a table (rows and columns). The first row contains labels for the columns. The rows following are the query results. Normally, column labels are the names of the columns you fetch from database tables. If you're retrieving the value of an expression rather than a table column (as in the example just shown), `mysql' labels the column using the expression itself. * `mysql' shows how many rows were returned and how long the query took to execute, which gives you a rough idea of server performance. These values are imprecise because they represent wall clock time (not CPU or machine time), and because they are affected by factors such as server load and network latency. (For brevity, the "rows in set" line is not shown in the remaining examples in this chapter.) Keywords may be entered in any lettercase. The following queries are equivalent: mysql> SELECT VERSION(), CURRENT_DATE; mysql> select version(), current_date; mysql> SeLeCt vErSiOn(), current_DATE; Here's another query. It demonstrates that you can use `mysql' as a simple calculator: mysql> SELECT SIN(PI()/4), (4+1)*5; +-------------+---------+ | SIN(PI()/4) | (4+1)*5 | +-------------+---------+ | 0.707107 | 25 | +-------------+---------+ The commands shown thus far have been relatively short, single-line statements. You can even enter multiple statements on a single line. Just end each one with a semicolon: mysql> SELECT VERSION(); SELECT NOW(); +--------------+ | version() | +--------------+ | 3.22.20a-log | +--------------+ +---------------------+ | NOW() | +---------------------+ | 1999-03-19 00:15:33 | +---------------------+ A command need not be given all on a single line, so lengthy commands that require several lines are not a problem. `mysql' determines where your statement ends by looking for the terminating semicolon, not by looking for the end of the input line. (In other words, `mysql' accepts free-format input: it collects input lines but does not execute them until it sees the semicolon.) Here's a simple multiple-line statement: mysql> SELECT -> USER() -> , -> CURRENT_DATE; +--------------------+--------------+ | USER() | CURRENT_DATE | +--------------------+--------------+ | joesmith@localhost | 1999-03-18 | +--------------------+--------------+ In this example, notice how the prompt changes from `mysql>' to `->' after you enter the first line of a multiple-line query. This is how `mysql' indicates that it hasn't seen a complete statement and is waiting for the rest. The prompt is your friend, because it provides valuable feedback. If you use that feedback, you will always be aware of what `mysql' is waiting for. If you decide you don't want to execute a command that you are in the process of entering, cancel it by typing `\c': mysql> SELECT -> USER() -> \c mysql> Here, too, notice the prompt. It switches back to `mysql>' after you type `\c', providing feedback to indicate that `mysql' is ready for a new command. The following table shows each of the prompts you may see and summarizes what they mean about the state that `mysql' is in: *Prompt**Meaning* `mysql>'Ready for new command. ` Waiting for next line of multiple-line command. ->' ` Waiting for next line, collecting a string that begins with a '>' single quote (`''). ` Waiting for next line, collecting a string that begins with a ">' double quote (`"'). Multiple-line statements commonly occur by accident when you intend to issue a command on a single line, but forget the terminating semicolon. In this case, `mysql' waits for more input: mysql> SELECT USER() -> If this happens to you (you think you've entered a statement but the only response is a `->' prompt), most likely `mysql' is waiting for the semicolon. If you don't notice what the prompt is telling you, you might sit there for a while before realizing what you need to do. Enter a semicolon to complete the statement, and `mysql' will execute it: mysql> SELECT USER() -> ; +--------------------+ | USER() | +--------------------+ | joesmith@localhost | +--------------------+ The `'>' and `">' prompts occur during string collection. In MySQL, you can write strings surrounded by either `'' or `"' characters (for example, `'hello'' or `"goodbye"'), and `mysql' lets you enter strings that span multiple lines. When you see a `'>' or `">' prompt, it means that you've entered a line containing a string that begins with a `'' or `"' quote character, but have not yet entered the matching quote that terminates the string. That's fine if you really are entering a multiple-line string, but how likely is that? Not very. More often, the `'>' and `">' prompts indicate that you've inadvertantly left out a quote character. For example: mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30; "> If you enter this `SELECT' statement, then hit RETURN and wait for the result, nothing will happen. Instead of wondering why this query takes so long, notice the clue provided by the `">' prompt. It tells you that `mysql' expects to see the rest of an unterminated string. (Do you see the error in the statement? The string `"Smith' is missing the second quote.) At this point, what do you do? The simplest thing is to cancel the command. However, you cannot just type `\c' in this case, because `mysql' interprets it as part of the string that it is collecting! Instead, enter the closing quote character (so `mysql' knows you've finished the string), then type `\c': mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30; "> "\c mysql> The prompt changes back to `mysql>', indicating that `mysql' is ready for a new command. It's important to know what the `'>' and `">' prompts signify, because if you mistakenly enter an unterminated string, any further lines you type will appear to be ignored by `mysql' -- including a line containing `QUIT'! This can be quite confusing, especially if you don't know that you need to supply the terminating quote before you can cancel the current command. Creating and Using a Database ============================= Now that you know how to enter commands, it's time to access a database. Suppose you have several pets in your home (your menagerie) and you'd like to keep track of various types of information about them. You can do so by creating tables to hold your data and loading them with the desired information. Then you can answer different sorts of questions about your animals by retrieving data from the tables. This section shows you how to: * Create a database * Create a table * Load data into the table * Retrieve data from the table in various ways * Use multiple tables The menagerie database will be simple (deliberately), but it is not difficult to think of real-world situations in which a similar type of database might be used. For example, a database like this could be used by a farmer to keep track of livestock, or by a veterinarian to keep track of patient records. A menagerie distribution containing some of the queries and sample data used in the following sections can be obtained from the MySQL Web site. It's available in either compressed `tar' format (http://www.mysql.com/Downloads/Contrib/Examples/menagerie.tar.gz) or Zip format (http://www.mysql.com/Downloads/Contrib/Examples/menagerie.zip). Use the `SHOW' statement to find out what databases currently exist on the server: mysql> SHOW DATABASES; +----------+ | Database | +----------+ | mysql | | test | | tmp | +----------+ The list of databases is probably different on your machine, but the `mysql' and `test' databases are likely to be among them. The `mysql' database is required because it describes user access privileges. The `test' database is often provided as a workspace for users to try things out. If the `test' database exists, try to access it: mysql> USE test Database changed Note that `USE', like `QUIT', does not require a semicolon. (You can terminate such statements with a semicolon if you like; it does no harm.) The `USE' statement is special in another way, too: it must be given on a single line. You can use the `test' database (if you have access to it) for the examples that follow, but anything you create in that database can be removed by anyone else with access to it. For this reason, you should probably ask your MySQL administrator for permission to use a database of your own. Suppose you want to call yours `menagerie'. The administrator needs to execute a command like this: mysql> GRANT ALL ON menagerie.* TO your_mysql_name; where `your_mysql_name' is the MySQL user name assigned to you. Creating and Selecting a Database --------------------------------- If the administrator creates your database for you when setting up your permissions, you can begin using it. Otherwise, you need to create it yourself: mysql> CREATE DATABASE menagerie; Under Unix, database names are case sensitive (unlike SQL keywords), so you must always refer to your database as `menagerie', not as `Menagerie', `MENAGERIE', or some other variant. This is also true for table names. (Under Windows, this restriction does not apply, although you must refer to databases and tables using the same lettercase throughout a given query.) Creating a database does not select it for use; you must do that explicitly. To make `menagerie' the current database, use this command: mysql> USE menagerie Database changed Your database needs to be created only once, but you must select it for use each time you begin a `mysql' session. You can do this by issuing a `USE' statement as shown above. Alternatively, you can select the database on the command line when you invoke `mysql'. Just specify its name after any connection parameters that you might need to provide. For example: shell> mysql -h host -u user -p menagerie Enter password: ******** Note that `menagerie' is not your password on the command just shown. If you want to supply your password on the command line after the `-p' option, you must do so with no intervening space (for example, as `-pmypassword', not as `-p mypassword'). However, putting your password on the command line is not recommended, because doing so exposes it to snooping by other users logged in on your machine. Creating a Table ---------------- Creating the database is the easy part, but at this point it's empty, as `SHOW TABLES' will tell you: mysql> SHOW TABLES; Empty set (0.00 sec) The harder part is deciding what the structure of your database should be: what tables you will need and what columns will be in each of them. You'll want a table that contains a record for each of your pets. This can be called the `pet' table, and it should contain, as a bare minimum, each animal's name. Because the name by itself is not very interesting, the table should contain other information. For example, if more than one person in your family keeps pets, you might want to list each animal's owner. You might also want to record some basic descriptive information such as species and sex. How about age? That might be of interest, but it's not a good thing to store in a database. Age changes as time passes, which means you'd have to update your records often. Instead, it's better to store a fixed value such as date of birth. Then, whenever you need age, you can calculate it as the difference between the current date and the birth date. MySQL provides functions for doing date arithmetic, so this is not difficult. Storing birth date rather than age has other advantages, too: * You can use the database for tasks such as generating reminders for upcoming pet birthdays. (If you think this type of query is somewhat silly, note that it is the same question you might ask in the context of a business database to identify clients to whom you'll soon need to send out birthday greetings, for that computer-assisted personal touch.) * You can calculate age in relation to dates other than the current date. For example, if you store death date in the database, you can easily calculate how old a pet was when it died. You can probably think of other types of information that would be useful in the `pet' table, but the ones identified so far are sufficient for now: name, owner, species, sex, birth, and death. Use a `CREATE TABLE' statement to specify the layout of your table: mysql> CREATE TABLE pet (name VARCHAR(20), owner VARCHAR(20), -> species VARCHAR(20), sex CHAR(1), birth DATE, death DATE); `VARCHAR' is a good choice for the `name', `owner', and `species' columns because the column values will vary in length. The lengths of those columns need not all be the same, and need not be `20'. You can pick any length from `1' to `255', whatever seems most reasonable to you. (If you make a poor choice and it turns out later that you need a longer field, MySQL provides an `ALTER TABLE' statement.) Animal sex can be represented in a variety of ways, for example, `"m"' and `"f"', or perhaps `"male"' and `"female"'. It's simplest to use the single characters `"m"' and `"f"'. The use of the `DATE' data type for the `birth' and `death' columns is a fairly obvious choice. Now that you have created a table, `SHOW TABLES' should produce some output: mysql> SHOW TABLES; +---------------------+ | Tables in menagerie | +---------------------+ | pet | +---------------------+ To verify that your table was created the way you expected, use a `DESCRIBE' statement: mysql> DESCRIBE pet; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | name | varchar(20) | YES | | NULL | | | owner | varchar(20) | YES | | NULL | | | species | varchar(20) | YES | | NULL | | | sex | char(1) | YES | | NULL | | | birth | date | YES | | NULL | | | death | date | YES | | NULL | | +---------+-------------+------+-----+---------+-------+ You can use `DESCRIBE' any time, for example, if you forget the names of the columns in your table or what types they are. Loading Data into a Table ------------------------- After creating your table, you need to populate it. The `LOAD DATA' and `INSERT' statements are useful for this. Suppose your pet records can be described as shown below. (Observe that MySQL expects dates in `YYYY-MM-DD' format; this may be different than what you are used to.) *name* *owner* *species* *sex* *birth* *death* Fluffy Harold cat f 1993-02-04 Claws Gwen cat m 1994-03-17 Buffy Harold dog f 1989-05-13 Fang Benny dog m 1990-08-27 Bowser Diane dog m 1998-08-31 1995-07-29 Chirpy Gwen bird f 1998-09-11 Whistler Gwen bird 1997-12-09 Slim Benny snake m 1996-04-29 Because you are beginning with an empty table, an easy way to populate it is to create a text file containing a row for each of your animals, then load the contents of the file into the table with a single statement. You could create a text file `pet.txt' containing one record per line, with values separated by tabs, and given in the order in which the columns were listed in the `CREATE TABLE' statement. For missing values (such as unknown sexes or death dates for animals that are still living), you can use `NULL' values. To represent these in your text file, use `\N'. For example, the record for Whistler the bird would look like this (where the whitespace between values is a single tab character): `Whistler' `Gwen' `bird' `\N' `1997-12-09' `\N' To load the text file `pet.txt' into the `pet' table, use this command: mysql> LOAD DATA LOCAL INFILE "pet.txt" INTO TABLE pet; You can specify the column value separator and end of line marker explicitly in the `LOAD DATA' statement if you wish, but the defaults are tab and linefeed. These are sufficient for the statement to read the file `pet.txt' properly. When you want to add new records one at a time, the `INSERT' statement is useful. In its simplest form, you supply values for each column, in the order in which the columns were listed in the `CREATE TABLE' statement. Suppose Diane gets a new hamster named Puffball. You could add a new record using an `INSERT' statement like this: mysql> INSERT INTO pet -> VALUES ('Puffball','Diane','hamster','f','1999-03-30',NULL); Note that string and date values are specified as quoted strings here. Also, with `INSERT', you can insert `NULL' directly to represent a missing value. You do not use `\N' like you do with `LOAD DATA'. From this example, you should be able to see that there would be a lot more typing involved to load your records initially using several `INSERT' statements rather than a single `LOAD DATA' statement. Retrieving Information from a Table ----------------------------------- The `SELECT' statement is used to pull information from a table. The general form of the statement is: SELECT what_to_select FROM which_table WHERE conditions_to_satisfy `what_to_select' indicates what you want to see. This can be a list of columns, or `*' to indicate "all columns." `which_table' indicates the table from which you want to retrieve data. The `WHERE' clause is optional. If it's present, `conditions_to_satisfy' specifies conditions that rows must satisfy to qualify for retrieval. Selecting All Data .................. The simplest form of `SELECT' retrieves everything from a table: mysql> SELECT * FROM pet; +----------+--------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +----------+--------+---------+------+------------+------------+ | Fluffy | Harold | cat | f | 1993-02-04 | NULL | | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | | Fang | Benny | dog | m | 1990-08-27 | NULL | | Bowser | Diane | dog | m | 1998-08-31 | 1995-07-29 | | Chirpy | Gwen | bird | f | 1998-09-11 | NULL | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | | Slim | Benny | snake | m | 1996-04-29 | NULL | | Puffball | Diane | hamster | f | 1999-03-30 | NULL | +----------+--------+---------+------+------------+------------+ This form of `SELECT' is useful if you want to review your entire table, for instance, after you've just loaded it with your initial dataset. As it happens, the output just shown reveals an error in your data file: Bowser appears to have been born after he died! Consulting your original pedigree papers, you find that the correct birth year is 1989, not 1998. There are are least a couple of ways to fix this: * Edit the file `pet.txt' to correct the error, then empty the table and reload it using `DELETE' and `LOAD DATA': mysql> SET AUTOCOMMIT=1; # Used for quick re-create of the table mysql> DELETE FROM pet; mysql> LOAD DATA LOCAL INFILE "pet.txt" INTO TABLE pet; However, if you do this, you must also re-enter the record for Puffball. * Fix only the erroneous record with an `UPDATE' statement: mysql> UPDATE pet SET birth = "1989-08-31" WHERE name = "Bowser"; As shown above, it is easy to retrieve an entire table. But typically you don't want to do that, particularly when the table becomes large. Instead, you're usually more interested in answering a particular question, in which case you specify some constraints on the information you want. Let's look at some selection queries in terms of questions about your pets that they answer. Selecting Particular Rows ......................... You can select only particular rows from your table. For example, if you want to verify the change that you made to Bowser's birth date, select Bowser's record like this: mysql> SELECT * FROM pet WHERE name = "Bowser"; +--------+-------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +--------+-------+---------+------+------------+------------+ | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | +--------+-------+---------+------+------------+------------+ The output confirms that the year is correctly recorded now as 1989, not 1998. String comparisons are normally case insensitive, so you can specify the name as `"bowser"', `"BOWSER"', etc. The query result will be the same. You can specify conditions on any column, not just `name'. For example, if you want to know which animals were born after 1998, test the `birth' column: mysql> SELECT * FROM pet WHERE birth >= "1998-1-1"; +----------+-------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+-------+ | Chirpy | Gwen | bird | f | 1998-09-11 | NULL | | Puffball | Diane | hamster | f | 1999-03-30 | NULL | +----------+-------+---------+------+------------+-------+ You can combine conditions, for example, to locate female dogs: mysql> SELECT * FROM pet WHERE species = "dog" AND sex = "f"; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+ The preceding query uses the `AND' logical operator. There is also an `OR' operator: mysql> SELECT * FROM pet WHERE species = "snake" OR species = "bird"; +----------+-------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+-------+ | Chirpy | Gwen | bird | f | 1998-09-11 | NULL | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | | Slim | Benny | snake | m | 1996-04-29 | NULL | +----------+-------+---------+------+------------+-------+ `AND' and `OR' may be intermixed. If you do that, it's a good idea to use parentheses to indicate how conditions should be grouped: mysql> SELECT * FROM pet WHERE (species = "cat" AND sex = "m") -> OR (species = "dog" AND sex = "f"); +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+ Selecting Particular Columns ............................ If you don't want to see entire rows from your table, just name the columns in which you're interested, separated by commas. For example, if you want to know when your animals were born, select the `name' and `birth' columns: mysql> SELECT name, birth FROM pet; +----------+------------+ | name | birth | +----------+------------+ | Fluffy | 1993-02-04 | | Claws | 1994-03-17 | | Buffy | 1989-05-13 | | Fang | 1990-08-27 | | Bowser | 1989-08-31 | | Chirpy | 1998-09-11 | | Whistler | 1997-12-09 | | Slim | 1996-04-29 | | Puffball | 1999-03-30 | +----------+------------+ To find out who owns pets, use this query: mysql> SELECT owner FROM pet; +--------+ | owner | +--------+ | Harold | | Gwen | | Harold | | Benny | | Diane | | Gwen | | Gwen | | Benny | | Diane | +--------+ However, notice that the query simply retrieves the `owner' field from each record, and some of them appear more than once. To minimize the output, retrieve each unique output record just once by adding the keyword `DISTINCT': mysql> SELECT DISTINCT owner FROM pet; +--------+ | owner | +--------+ | Benny | | Diane | | Gwen | | Harold | +--------+ You can use a `WHERE' clause to combine row selection with column selection. For example, to get birth dates for dogs and cats only, use this query: mysql> SELECT name, species, birth FROM pet -> WHERE species = "dog" OR species = "cat"; +--------+---------+------------+ | name | species | birth | +--------+---------+------------+ | Fluffy | cat | 1993-02-04 | | Claws | cat | 1994-03-17 | | Buffy | dog | 1989-05-13 | | Fang | dog | 1990-08-27 | | Bowser | dog | 1989-08-31 | +--------+---------+------------+ Sorting Rows ............ You may have noticed in the preceding examples that the result rows are displayed in no particular order. However, it's often easier to examine query output when the rows are sorted in some meaningful way. To sort a result, use an `ORDER BY' clause. Here are animal birthdays, sorted by date: mysql> SELECT name, birth FROM pet ORDER BY birth; +----------+------------+ | name | birth | +----------+------------+ | Buffy | 1989-05-13 | | Bowser | 1989-08-31 | | Fang | 1990-08-27 | | Fluffy | 1993-02-04 | | Claws | 1994-03-17 | | Slim | 1996-04-29 | | Whistler | 1997-12-09 | | Chirpy | 1998-09-11 | | Puffball | 1999-03-30 | +----------+------------+ To sort in reverse order, add the `DESC' (descending) keyword to the name of the column you are sorting by: mysql> SELECT name, birth FROM pet ORDER BY birth DESC; +----------+------------+ | name | birth | +----------+------------+ | Puffball | 1999-03-30 | | Chirpy | 1998-09-11 | | Whistler | 1997-12-09 | | Slim | 1996-04-29 | | Claws | 1994-03-17 | | Fluffy | 1993-02-04 | | Fang | 1990-08-27 | | Bowser | 1989-08-31 | | Buffy | 1989-05-13 | +----------+------------+ You can sort on multiple columns. For example, to sort by type of animal, then by birth date within animal type with youngest animals first, use the following query: mysql> SELECT name, species, birth FROM pet ORDER BY species, birth DESC; +----------+---------+------------+ | name | species | birth | +----------+---------+------------+ | Chirpy | bird | 1998-09-11 | | Whistler | bird | 1997-12-09 | | Claws | cat | 1994-03-17 | | Fluffy | cat | 1993-02-04 | | Fang | dog | 1990-08-27 | | Bowser | dog | 1989-08-31 | | Buffy | dog | 1989-05-13 | | Puffball | hamster | 1999-03-30 | | Slim | snake | 1996-04-29 | +----------+---------+------------+ Note that the `DESC' keyword applies only to the column name immediately preceding it (`birth'); `species' values are still sorted in ascending order. Date Calculations ................. MySQL provides several functions that you can use to perform calculations on dates, for example, to calculate ages or extract parts of dates. To determine how many years old each of your pets is, compute the difference in the year part of the current date and the birth date, then subtract one if the current date occurs earlier in the calendar year than the birth date. The following query shows, for each pet, the birth date, the current date, and the age in years. mysql> SELECT name, birth, CURRENT_DATE, -> (YEAR(CURRENT_DATE)-YEAR(birth)) -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5)) -> AS age -> FROM pet; +----------+------------+--------------+------+ | name | birth | CURRENT_DATE | age | +----------+------------+--------------+------+ | Fluffy | 1993-02-04 | 2001-08-29 | 8 | | Claws | 1994-03-17 | 2001-08-29 | 7 | | Buffy | 1989-05-13 | 2001-08-29 | 12 | | Fang | 1990-08-27 | 2001-08-29 | 11 | | Bowser | 1989-08-31 | 2001-08-29 | 11 | | Chirpy | 1998-09-11 | 2001-08-29 | 2 | | Whistler | 1997-12-09 | 2001-08-29 | 3 | | Slim | 1996-04-29 | 2001-08-29 | 5 | | Puffball | 1999-03-30 | 2001-08-29 | 2 | +----------+------------+--------------+------+ Here, `YEAR()' pulls out the year part of a date and `RIGHT()' pulls off the rightmost five characters that represent the `MM-DD' (calendar year) part of the date. The part of the expression that compares the `MM-DD' values evaluates to 1 or 0, which adjusts the year difference down a year if `CURRENT_DATE' occurs earlier in the year than `birth'. The full expression is somewhat ungainly, so an alias (`age') is used to make the output column label more meaningful. The query works, but the result could be scanned more easily if the rows were presented in some order. This can be done by adding an `ORDER BY name' clause to sort the output by name: mysql> SELECT name, birth, CURRENT_DATE, -> (YEAR(CURRENT_DATE)-YEAR(birth)) -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5)) -> AS age -> FROM pet ORDER BY name; +----------+------------+--------------+------+ | name | birth | CURRENT_DATE | age | +----------+------------+--------------+------+ | Bowser | 1989-08-31 | 2001-08-29 | 11 | | Buffy | 1989-05-13 | 2001-08-29 | 12 | | Chirpy | 1998-09-11 | 2001-08-29 | 2 | | Claws | 1994-03-17 | 2001-08-29 | 7 | | Fang | 1990-08-27 | 2001-08-29 | 11 | | Fluffy | 1993-02-04 | 2001-08-29 | 8 | | Puffball | 1999-03-30 | 2001-08-29 | 2 | | Slim | 1996-04-29 | 2001-08-29 | 5 | | Whistler | 1997-12-09 | 2001-08-29 | 3 | +----------+------------+--------------+------+ To sort the output by `age' rather than `name', just use a different `ORDER BY' clause: mysql> SELECT name, birth, CURRENT_DATE, -> (YEAR(CURRENT_DATE)-YEAR(birth)) -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5)) -> AS age -> FROM pet ORDER BY age; +----------+------------+--------------+------+ | name | birth | CURRENT_DATE | age | +----------+------------+--------------+------+ | Chirpy | 1998-09-11 | 2001-08-29 | 2 | | Puffball | 1999-03-30 | 2001-08-29 | 2 | | Whistler | 1997-12-09 | 2001-08-29 | 3 | | Slim | 1996-04-29 | 2001-08-29 | 5 | | Claws | 1994-03-17 | 2001-08-29 | 7 | | Fluffy | 1993-02-04 | 2001-08-29 | 8 | | Fang | 1990-08-27 | 2001-08-29 | 11 | | Bowser | 1989-08-31 | 2001-08-29 | 11 | | Buffy | 1989-05-13 | 2001-08-29 | 12 | +----------+------------+--------------+------+ A similar query can be used to determine age at death for animals that have died. You determine which animals these are by checking whether or not the `death' value is `NULL'. Then, for those with non-`NULL' values, compute the difference between the `death' and `birth' values: mysql> SELECT name, birth, death, -> (YEAR(death)-YEAR(birth)) - (RIGHT(death,5)<RIGHT(birth,5)) -> AS age -> FROM pet WHERE death IS NOT NULL ORDER BY age; +--------+------------+------------+------+ | name | birth | death | age | +--------+------------+------------+------+ | Bowser | 1989-08-31 | 1995-07-29 | 5 | +--------+------------+------------+------+ The query uses `death IS NOT NULL' rather than `death != NULL' because `NULL' is a special value. This is explained later. *Note Working with `NULL': Working with NULL. What if you want to know which animals have birthdays next month? For this type of calculation, year and day are irrelevant; you simply want to extract the month part of the `birth' column. MySQL provides several date-part extraction functions, such as `YEAR()', `MONTH()', and `DAYOFMONTH()'. `MONTH()' is the appropriate function here. To see how it works, run a simple query that displays the value of both `birth' and `MONTH(birth)': mysql> SELECT name, birth, MONTH(birth) FROM pet; +----------+------------+--------------+ | name | birth | MONTH(birth) | +----------+------------+--------------+ | Fluffy | 1993-02-04 | 2 | | Claws | 1994-03-17 | 3 | | Buffy | 1989-05-13 | 5 | | Fang | 1990-08-27 | 8 | | Bowser | 1989-08-31 | 8 | | Chirpy | 1998-09-11 | 9 | | Whistler | 1997-12-09 | 12 | | Slim | 1996-04-29 | 4 | | Puffball | 1999-03-30 | 3 | +----------+------------+--------------+ Finding animals with birthdays in the upcoming month is easy, too. Suppose the current month is April. Then the month value is `4' and you look for animals born in May (month 5) like this: mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5; +-------+------------+ | name | birth | +-------+------------+ | Buffy | 1989-05-13 | +-------+------------+ There is a small complication if the current month is December, of course. You don't just add one to the month number (`12') and look for animals born in month 13, because there is no such month. Instead, you look for animals born in January (month 1). You can even write the query so that it works no matter what the current month is. That way you don't have to use a particular month number in the query. `DATE_ADD()' allows you to add a time interval to a given date. If you add a month to the value of `NOW()', then extract the month part with `MONTH()', the result produces the month in which to look for birthdays: mysql> SELECT name, birth FROM pet -> WHERE MONTH(birth) = MONTH(DATE_ADD(NOW(), INTERVAL 1 MONTH)); A different way to accomplish the same task is to add `1' to get the next month after the current one (after using the modulo function (`MOD') to wrap around the month value to `0' if it is currently `12'): mysql> SELECT name, birth FROM pet -> WHERE MONTH(birth) = MOD(MONTH(NOW()), 12) + 1; Note that `MONTH' returns a number between 1 and 12. And `MOD(something,12)' returns a number between 0 and 11. So the addition has to be after the `MOD()', otherwise we would go from November (11) to January (1). Working with `NULL' Values .......................... The `NULL' value can be surprising until you get used to it. Conceptually, `NULL' means missing value or unknown value and it is treated somewhat differently than other values. To test for `NULL', you cannot use the arithmetic comparison operators such as `=', `<', or `!='. To demonstrate this for yourself, try the following query: mysql> SELECT 1 = NULL, 1 != NULL, 1 < NULL, 1 > NULL; +----------+-----------+----------+----------+ | 1 = NULL | 1 != NULL | 1 < NULL | 1 > NULL | +----------+-----------+----------+----------+ | NULL | NULL | NULL | NULL | +----------+-----------+----------+----------+ Clearly you get no meaningful results from these comparisons. Use the `IS NULL' and `IS NOT NULL' operators instead: mysql> SELECT 1 IS NULL, 1 IS NOT NULL; +-----------+---------------+ | 1 IS NULL | 1 IS NOT NULL | +-----------+---------------+ | 0 | 1 | +-----------+---------------+ In MySQL, 0 or `NULL' means false and anything else means true. The default truth value from a boolean operation is 1. This special treatment of `NULL' is why, in the previous section, it was necessary to determine which animals are no longer alive using `death IS NOT NULL' instead of `death != NULL'. Pattern Matching ................ MySQL provides standard SQL pattern matching as well as a form of pattern matching based on extended regular expressions similar to those used by Unix utilities such as `vi', `grep', and `sed'. SQL pattern matching allows you to use `_' to match any single character and `%' to match an arbitrary number of characters (including zero characters). In MySQL, SQL patterns are case insensitive by default. Some examples are shown below. Note that you do not use `=' or `!=' when you use SQL patterns; use the `LIKE' or `NOT LIKE' comparison operators instead. To find names beginning with `b': mysql> SELECT * FROM pet WHERE name LIKE "b%"; +--------+--------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+------------+ | Buffy | Harold | dog | f | 1989-05-13 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | +--------+--------+---------+------+------------+------------+ To find names ending with `fy': mysql> SELECT * FROM pet WHERE name LIKE "%fy"; +--------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+-------+ | Fluffy | Harold | cat | f | 1993-02-04 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +--------+--------+---------+------+------------+-------+ To find names containing a `w': mysql> SELECT * FROM pet WHERE name LIKE "%w%"; +----------+-------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+------------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | +----------+-------+---------+------+------------+------------+ To find names containing exactly five characters, use the `_' pattern character: mysql> SELECT * FROM pet WHERE name LIKE "_____"; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+ The other type of pattern matching provided by MySQL uses extended regular expressions. When you test for a match for this type of pattern, use the `REGEXP' and `NOT REGEXP' operators (or `RLIKE' and `NOT RLIKE', which are synonyms). Some characteristics of extended regular expressions are: * `.' matches any single character. * A character class `[...]' matches any character within the brackets. For example, `[abc]' matches `a', `b', or `c'. To name a range of characters, use a dash. `[a-z]' matches any lowercase letter, whereas `[0-9]' matches any digit. * `*' matches zero or more instances of the thing preceding it. For example, `x*' matches any number of `x' characters, `[0-9]*' matches any number of digits, and `.*' matches any number of anything. * Regular expressions are case sensitive, but you can use a character class to match both lettercases if you wish. For example, `[aA]' matches lowercase or uppercase `a' and `[a-zA-Z]' matches any letter in either case. * The pattern matches if it occurs anywhere in the value being tested. (SQL patterns match only if they match the entire value.) * To anchor a pattern so that it must match the beginning or end of the value being tested, use `^' at the beginning or `$' at the end of the pattern. To demonstrate how extended regular expressions work, the `LIKE' queries shown above are rewritten below to use `REGEXP'. To find names beginning with `b', use `^' to match the beginning of the name: mysql> SELECT * FROM pet WHERE name REGEXP "^b"; +--------+--------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+------------+ | Buffy | Harold | dog | f | 1989-05-13 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | +--------+--------+---------+------+------------+------------+ Prior to MySQL Version 3.23.4, `REGEXP' is case sensitive, and the previous query will return no rows. To match either lowercase or uppercase `b', use this query instead: mysql> SELECT * FROM pet WHERE name REGEXP "^[bB]"; From MySQL 3.23.4 on, to force a `REGEXP' comparison to be case sensitive, use the `BINARY' keyword to make one of the strings a binary string. This query will match only lowercase `b' at the beginning of a name: mysql> SELECT * FROM pet WHERE name REGEXP BINARY "^b"; To find names ending with `fy', use `$' to match the end of the name: mysql> SELECT * FROM pet WHERE name REGEXP "fy$"; +--------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+-------+ | Fluffy | Harold | cat | f | 1993-02-04 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +--------+--------+---------+------+------------+-------+ To find names containing a lowercase or uppercase `w', use this query: mysql> SELECT * FROM pet WHERE name REGEXP "w"; +----------+-------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+------------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | +----------+-------+---------+------+------------+------------+ Because a regular expression pattern matches if it occurs anywhere in the value, it is not necessary in the previous query to put a wild card on either side of the pattern to get it to match the entire value like it would be if you used a SQL pattern. To find names containing exactly five characters, use `^' and `$' to match the beginning and end of the name, and five instances of `.' in between: mysql> SELECT * FROM pet WHERE name REGEXP "^.....$"; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+ You could also write the previous query using the `{n}' "repeat-`n'-times" operator: mysql> SELECT * FROM pet WHERE name REGEXP "^.{5}$"; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+ Counting Rows ............. Databases are often used to answer the question, "How often does a certain type of data occur in a table?" For example, you might want to know how many pets you have, or how many pets each owner has, or you might want to perform various kinds of censuses on your animals. Counting the total number of animals you have is the same question as "How many rows are in the `pet' table?" because there is one record per pet. The `COUNT()' function counts the number of non-`NULL' results, so the query to count your animals looks like this: mysql> SELECT COUNT(*) FROM pet; +----------+ | COUNT(*) | +----------+ | 9 | +----------+ Earlier, you retrieved the names of the people who owned pets. You can use `COUNT()' if you want to find out how many pets each owner has: mysql> SELECT owner, COUNT(*) FROM pet GROUP BY owner; +--------+----------+ | owner | COUNT(*) | +--------+----------+ | Benny | 2 | | Diane | 2 | | Gwen | 3 | | Harold | 2 | +--------+----------+ Note the use of `GROUP BY' to group together all records for each `owner'. Without it, all you get is an error message: mysql> SELECT owner, COUNT(owner) FROM pet; ERROR 1140 at line 1: Mixing of GROUP columns (MIN(),MAX(),COUNT()...) with no GROUP columns is illegal if there is no GROUP BY clause `COUNT()' and `GROUP BY' are useful for characterizing your data in various ways. The following examples show different ways to perform animal census operations. Number of animals per species: mysql> SELECT species, COUNT(*) FROM pet GROUP BY species; +---------+----------+ | species | COUNT(*) | +---------+----------+ | bird | 2 | | cat | 2 | | dog | 3 | | hamster | 1 | | snake | 1 | +---------+----------+ Number of animals per sex: mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex; +------+----------+ | sex | COUNT(*) | +------+----------+ | NULL | 1 | | f | 4 | | m | 4 | +------+----------+ (In this output, `NULL' indicates sex unknown.) Number of animals per combination of species and sex: mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex; +---------+------+----------+ | species | sex | COUNT(*) | +---------+------+----------+ | bird | NULL | 1 | | bird | f | 1 | | cat | f | 1 | | cat | m | 1 | | dog | f | 1 | | dog | m | 2 | | hamster | f | 1 | | snake | m | 1 | +---------+------+----------+ You need not retrieve an entire table when you use `COUNT()'. For example, the previous query, when performed just on dogs and cats, looks like this: mysql> SELECT species, sex, COUNT(*) FROM pet -> WHERE species = "dog" OR species = "cat" -> GROUP BY species, sex; +---------+------+----------+ | species | sex | COUNT(*) | +---------+------+----------+ | cat | f | 1 | | cat | m | 1 | | dog | f | 1 | | dog | m | 2 | +---------+------+----------+ Or, if you wanted the number of animals per sex only for known-sex animals: mysql> SELECT species, sex, COUNT(*) FROM pet -> WHERE sex IS NOT NULL -> GROUP BY species, sex; +---------+------+----------+ | species | sex | COUNT(*) | +---------+------+----------+ | bird | f | 1 | | cat | f | 1 | | cat | m | 1 | | dog | f | 1 | | dog | m | 2 | | hamster | f | 1 | | snake | m | 1 | +---------+------+----------+ Using More Than one Table ......................... The `pet' table keeps track of which pets you have. If you want to record other information about them, such as events in their lives like visits to the vet or when litters are born, you need another table. What should this table look like? It needs: * To contain the pet name so you know which animal each event pertains to. * A date so you know when the event occurred. * A field to describe the event. * An event type field, if you want to be able to categorize events. Given these considerations, the `CREATE TABLE' statement for the `event' table might look like this: mysql> CREATE TABLE event (name VARCHAR(20), date DATE, -> type VARCHAR(15), remark VARCHAR(255)); As with the `pet' table, it's easiest to load the initial records by creating a tab-delimited text file containing the information: Fluffy 1995-05-15 litter 4 kittens, 3 female, 1 male Buffy 1993-06-23 litter 5 puppies, 2 female, 3 male Buffy 1994-06-19 litter 3 puppies, 3 female Chirpy 1999-03-21 vet needed beak straightened Slim 1997-08-03 vet broken rib Bowser 1991-10-12 kennel Fang 1991-10-12 kennel Fang 1998-08-28 birthday Gave him a new chew toy Claws 1998-03-17 birthday Gave him a new flea collar Whistler 1998-12-09 birthday First birthday Load the records like this: mysql> LOAD DATA LOCAL INFILE "event.txt" INTO TABLE event; Based on what you've learned from the queries you've run on the `pet' table, you should be able to perform retrievals on the records in the `event' table; the principles are the same. But when is the `event' table by itself insufficient to answer questions you might ask? Suppose you want to find out the ages of each pet when they had their litters. The `event' table indicates when this occurred, but to calculate the age of the mother, you need her birth date. Because that is stored in the `pet' table, you need both tables for the query: mysql> SELECT pet.name, (TO_DAYS(date) - TO_DAYS(birth))/365 AS age, remark -> FROM pet, event -> WHERE pet.name = event.name AND type = "litter"; +--------+------+-----------------------------+ | name | age | remark | +--------+------+-----------------------------+ | Fluffy | 2.27 | 4 kittens, 3 female, 1 male | | Buffy | 4.12 | 5 puppies, 2 female, 3 male | | Buffy | 5.10 | 3 puppies, 3 female | +--------+------+-----------------------------+ There are several things to note about this query: * The `FROM' clause lists two tables because the query needs to pull information from both of them. * When combining (joining) information from multiple tables, you need to specify how records in one table can be matched to records in the other. This is easy because they both have a `name' column. The query uses `WHERE' clause to match up records in the two tables based on the `name' values. * Because the `name' column occurs in both tables, you must be specific about which table you mean when referring to the column. This is done by prepending the table name to the column name. You need not have two different tables to perform a join. Sometimes it is useful to join a table to itself, if you want to compare records in a table to other records in that same table. For example, to find breeding pairs among your pets, you can join the `pet' table with itself to pair up males and females of like species: mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species -> FROM pet AS p1, pet AS p2 -> WHERE p1.species = p2.species AND p1.sex = "f" AND p2.sex = "m"; +--------+------+--------+------+---------+ | name | sex | name | sex | species | +--------+------+--------+------+---------+ | Fluffy | f | Claws | m | cat | | Buffy | f | Fang | m | dog | | Buffy | f | Bowser | m | dog | +--------+------+--------+------+---------+ In this query, we specify aliases for the table name in order to refer to the columns and keep straight which instance of the table each column reference is associated with. Getting Information About Databases and Tables ============================================== What if you forget the name of a database or table, or what the structure of a given table is (for example, what its columns are called)? MySQL addresses this problem through several statements that provide information about the databases and tables it supports. You have already seen `SHOW DATABASES', which lists the databases managed by the server. To find out which database is currently selected, use the `DATABASE()' function: mysql> SELECT DATABASE(); +------------+ | DATABASE() | +------------+ | menagerie | +------------+ If you haven't selected any database yet, the result is blank. To find out what tables the current database contains (for example, when you're not sure about the name of a table), use this command: mysql> SHOW TABLES; +---------------------+ | Tables in menagerie | +---------------------+ | event | | pet | +---------------------+ If you want to find out about the structure of a table, the `DESCRIBE' command is useful; it displays information about each of a table's columns: mysql> DESCRIBE pet; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | name | varchar(20) | YES | | NULL | | | owner | varchar(20) | YES | | NULL | | | species | varchar(20) | YES | | NULL | | | sex | char(1) | YES | | NULL | | | birth | date | YES | | NULL | | | death | date | YES | | NULL | | +---------+-------------+------+-----+---------+-------+ `Field' indicates the column name, `Type' is the data type for the column, `Null' indicates whether or not the column can contain `NULL' values, `Key' indicates whether or not the column is indexed, and `Default' specifies the column's default value. If you have indexes on a table, `SHOW INDEX FROM tbl_name' produces information about them. Examples of Common Queries ========================== Here are examples of how to solve some common problems with MySQL. Some of the examples use the table `shop' to hold the price of each article (item number) for certain traders (dealers). Supposing that each trader has a single fixed price per article, then (`article', `dealer') is a primary key for the records. Start the command line tool `mysql' and select a database: mysql your-database-name (In most MySQL installations, you can use the database-name 'test'). You can create the example table as: CREATE TABLE shop ( article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL, dealer CHAR(20) DEFAULT '' NOT NULL, price DOUBLE(16,2) DEFAULT '0.00' NOT NULL, PRIMARY KEY(article, dealer)); INSERT INTO shop VALUES (1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),(3,'C',1.69), (3,'D',1.25),(4,'D',19.95); Okay, so the example data is: mysql> SELECT * FROM shop; +---------+--------+-------+ | article | dealer | price | +---------+--------+-------+ | 0001 | A | 3.45 | | 0001 | B | 3.99 | | 0002 | A | 10.99 | | 0003 | B | 1.45 | | 0003 | C | 1.69 | | 0003 | D | 1.25 | | 0004 | D | 19.95 | +---------+--------+-------+ The Maximum Value for a Column ------------------------------ "What's the highest item number?" SELECT MAX(article) AS article FROM shop +---------+ | article | +---------+ | 4 | +---------+ The Row Holding the Maximum of a Certain Column ----------------------------------------------- "Find number, dealer, and price of the most expensive article." In ANSI SQL this is easily done with a sub-query: SELECT article, dealer, price FROM shop WHERE price=(SELECT MAX(price) FROM shop) In MySQL (which does not yet have sub-selects), just do it in two steps: 1. Get the maximum price value from the table with a `SELECT' statement. 2. Using this value compile the actual query: SELECT article, dealer, price FROM shop WHERE price=19.95 Another solution is to sort all rows descending by price and only get the first row using the MySQL specific `LIMIT' clause: SELECT article, dealer, price FROM shop ORDER BY price DESC LIMIT 1 *NOTE*: If there are several most expensive articles (for example, each 19.95) the `LIMIT' solution shows only one of them! Maximum of Column per Group --------------------------- "What's the highest price per article?" SELECT article, MAX(price) AS price FROM shop GROUP BY article +---------+-------+ | article | price | +---------+-------+ | 0001 | 3.99 | | 0002 | 10.99 | | 0003 | 1.69 | | 0004 | 19.95 | +---------+-------+ The Rows Holding the Group-wise Maximum of a Certain Field ---------------------------------------------------------- "For each article, find the dealer(s) with the most expensive price." In ANSI SQL, I'd do it with a sub-query like this: SELECT article, dealer, price FROM shop s1 WHERE price=(SELECT MAX(s2.price) FROM shop s2 WHERE s1.article = s2.article); In MySQL it's best do it in several steps: 1. Get the list of (article,maxprice). 2. For each article get the corresponding rows that have the stored maximum price. This can easily be done with a temporary table: CREATE TEMPORARY TABLE tmp ( article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL, price DOUBLE(16,2) DEFAULT '0.00' NOT NULL); LOCK TABLES shop read; INSERT INTO tmp SELECT article, MAX(price) FROM shop GROUP BY article; SELECT shop.article, dealer, shop.price FROM shop, tmp WHERE shop.article=tmp.article AND shop.price=tmp.price; UNLOCK TABLES; DROP TABLE tmp; If you don't use a `TEMPORARY' table, you must also lock the 'tmp' table. "Can it be done with a single query?" Yes, but only by using a quite inefficient trick that I call the "MAX-CONCAT trick": SELECT article, SUBSTRING( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 7) AS dealer, 0.00+LEFT( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 6) AS price FROM shop GROUP BY article; +---------+--------+-------+ | article | dealer | price | +---------+--------+-------+ | 0001 | B | 3.99 | | 0002 | A | 10.99 | | 0003 | C | 1.69 | | 0004 | D | 19.95 | +---------+--------+-------+ The last example can, of course, be made a bit more efficient by doing the splitting of the concatenated column in the client. Using user variables -------------------- You can use MySQL user variables to remember results without having to store them in temporary variables in the client. *Note Variables::. For example, to find the articles with the highest and lowest price you can do: select @min_price:=min(price),@max_price:=max(price) from shop; select * from shop where price=@min_price or price=@max_price; +---------+--------+-------+ | article | dealer | price | +---------+--------+-------+ | 0003 | D | 1.25 | | 0004 | D | 19.95 | +---------+--------+-------+ Using Foreign Keys ------------------ You don't need foreign keys to join 2 tables. The only thing MySQL doesn't do is `CHECK' to make sure that the keys you use really exist in the table(s) you're referencing and it doesn't automatically delete rows from table with a foreign key definition. If you use your keys like normal, it'll work just fine: CREATE TABLE persons ( id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT, name CHAR(60) NOT NULL, PRIMARY KEY (id) ); CREATE TABLE shirts ( id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT, style ENUM('t-shirt', 'polo', 'dress') NOT NULL, color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL, owner SMALLINT UNSIGNED NOT NULL REFERENCES persons, PRIMARY KEY (id) ); INSERT INTO persons VALUES (NULL, 'Antonio Paz'); INSERT INTO shirts VALUES (NULL, 'polo', 'blue', LAST_INSERT_ID()), (NULL, 'dress', 'white', LAST_INSERT_ID()), (NULL, 't-shirt', 'blue', LAST_INSERT_ID()); INSERT INTO persons VALUES (NULL, 'Lilliana Angelovska'); INSERT INTO shirts VALUES (NULL, 'dress', 'orange', LAST_INSERT_ID()), (NULL, 'polo', 'red', LAST_INSERT_ID()), (NULL, 'dress', 'blue', LAST_INSERT_ID()), (NULL, 't-shirt', 'white', LAST_INSERT_ID()); SELECT * FROM persons; +----+---------------------+ | id | name | +----+---------------------+ | 1 | Antonio Paz | | 2 | Lilliana Angelovska | +----+---------------------+ SELECT * FROM shirts; +----+---------+--------+-------+ | id | style | color | owner | +----+---------+--------+-------+ | 1 | polo | blue | 1 | | 2 | dress | white | 1 | | 3 | t-shirt | blue | 1 | | 4 | dress | orange | 2 | | 5 | polo | red | 2 | | 6 | dress | blue | 2 | | 7 | t-shirt | white | 2 | +----+---------+--------+-------+ SELECT s.* FROM persons p, shirts s WHERE p.name LIKE 'Lilliana%' AND s.owner = p.id AND s.color <> 'white'; +----+-------+--------+-------+ | id | style | color | owner | +----+-------+--------+-------+ | 4 | dress | orange | 2 | | 5 | polo | red | 2 | | 6 | dress | blue | 2 | +----+-------+--------+-------+ Searching on Two Keys --------------------- MySQL doesn't yet optimize when you search on two different keys combined with `OR' (Searching on one key with different `OR' parts is optimized quite good): SELECT field1_index, field2_index FROM test_table WHERE field1_index = '1' OR field2_index = '1' The reason is that we haven't yet had time to come up with an efficient way to handle this in the general case. (The `AND' handling is, in comparison, now completely general and works very well). For the moment you can solve this very efficiently by using a `TEMPORARY' table. This type of optimization is also very good if you are using very complicated queries where the SQL server does the optimizations in the wrong order. CREATE TEMPORARY TABLE tmp SELECT field1_index, field2_index FROM test_table WHERE field1_index = '1'; INSERT INTO tmp SELECT field1_index, field2_index FROM test_table WHERE field2_index = '1'; SELECT * from tmp; DROP TABLE tmp; The above way to solve this query is in effect an `UNION' of two queries. Calculating visits per day -------------------------- The following shows an idea of how you can use the bit group functions to calculate the number of days per month a user has visited a web page. CREATE TABLE t1 (year YEAR(4), month INT(2) UNSIGNED ZEROFILL, day INT(2) UNSIGNED ZEROFILL); INSERT INTO t1 VALUES(2000,1,1),(2000,1,20),(2000,1,30),(2000,2,2),(2000,2,23),(2000,2,23); SELECT year,month,BIT_COUNT(BIT_OR(1<<day)) AS days FROM t1 GROUP BY year,month; Which returns: +------+-------+------+ | year | month | days | +------+-------+------+ | 2000 | 01 | 3 | | 2000 | 02 | 2 | +------+-------+------+ The above calculates how many different days was used for a given year/month combination, with automatic removal of duplicate entries. Using `mysql' in Batch Mode =========================== In the previous sections, you used `mysql' interactively to enter queries and view the results. You can also run `mysql' in batch mode. To do this, put the commands you want to run in a file, then tell `mysql' to read its input from the file: shell> mysql < batch-file If you need to specify connection parameters on the command line, the command might look like this: shell> mysql -h host -u user -p < batch-file Enter password: ******** When you use `mysql' this way, you are creating a script file, then executing the script. Why use a script? Here are a few reasons: * If you run a query repeatedly (say, every day or every week), making it a script allows you to avoid retyping it each time you execute it. * You can generate new queries from existing ones that are similar by copying and editing script files. * Batch mode can also be useful while you're developing a query, particularly for multiple-line commands or multiple-statement sequences of commands. If you make a mistake, you don't have to retype everything. Just edit your script to correct the error, then tell `mysql' to execute it again. * If you have a query that produces a lot of output, you can run the output through a pager rather than watching it scroll off the top of your screen: shell> mysql < batch-file | more * You can catch the output in a file for further processing: shell> mysql < batch-file > mysql.out * You can distribute your script to other people so they can run the commands, too. * Some situations do not allow for interactive use, for example, when you run a query from a `cron' job. In this case, you must use batch mode. The default output format is different (more concise) when you run `mysql' in batch mode than when you use it interactively. For example, the output of `SELECT DISTINCT species FROM pet' looks like this when run interactively: +---------+ | species | +---------+ | bird | | cat | | dog | | hamster | | snake | +---------+ But like this when run in batch mode: species bird cat dog hamster snake If you want to get the interactive output format in batch mode, use `mysql -t'. To echo to the output the commands that are executed, use `mysql -vvv'. Queries from Twin Project ========================= At Analytikerna and Lentus, we have been doing the systems and field work for a big research project. This project is a collaboration between the Institute of Environmental Medicine at Karolinska Institutet Stockholm and the Section on Clinical Research in Aging and Psychology at the University of Southern California. The project involves a screening part where all twins in Sweden older than 65 years are interviewed by telephone. Twins who meet certain criteria are passed on to the next stage. In this latter stage, twins who want to participate are visited by a doctor/nurse team. Some of the examinations include physical and neuropsychological examination, laboratory testing, neuroimaging, psychological status assessment, and family history collection. In addition, data are collected on medical and environmental risk factors. More information about Twin studies can be found at: <http://www.imm.ki.se/TWIN/TWINUKW.HTM> The latter part of the project is administered with a Web interface written using Perl and MySQL. Each night all data from the interviews are moved into a MySQL database. Find all Non-distributed Twins ------------------------------ The following query is used to determine who goes into the second part of the project: select concat(p1.id, p1.tvab) + 0 as tvid, concat(p1.christian_name, " ", p1.surname) as Name, p1.postal_code as Code, p1.city as City, pg.abrev as Area, if(td.participation = "Aborted", "A", " ") as A, p1.dead as dead1, l.event as event1, td.suspect as tsuspect1, id.suspect as isuspect1, td.severe as tsevere1, id.severe as isevere1, p2.dead as dead2, l2.event as event2, h2.nurse as nurse2, h2.doctor as doctor2, td2.suspect as tsuspect2, id2.suspect as isuspect2, td2.severe as tsevere2, id2.severe as isevere2, l.finish_date from twin_project as tp /* For Twin 1 */ left join twin_data as td on tp.id = td.id and tp.tvab = td.tvab left join informant_data as id on tp.id = id.id and tp.tvab = id.tvab left join harmony as h on tp.id = h.id and tp.tvab = h.tvab left join lentus as l on tp.id = l.id and tp.tvab = l.tvab /* For Twin 2 */ left join twin_data as td2 on p2.id = td2.id and p2.tvab = td2.tvab left join informant_data as id2 on p2.id = id2.id and p2.tvab = id2.tvab left join harmony as h2 on p2.id = h2.id and p2.tvab = h2.tvab left join lentus as l2 on p2.id = l2.id and p2.tvab = l2.tvab, person_data as p1, person_data as p2, postal_groups as pg where /* p1 gets main twin and p2 gets his/her twin. */ /* ptvab is a field inverted from tvab */ p1.id = tp.id and p1.tvab = tp.tvab and p2.id = p1.id and p2.ptvab = p1.tvab and /* Just the sceening survey */ tp.survey_no = 5 and /* Skip if partner died before 65 but allow emigration (dead=9) */ (p2.dead = 0 or p2.dead = 9 or (p2.dead = 1 and (p2.death_date = 0 or (((to_days(p2.death_date) - to_days(p2.birthday)) / 365) >= 65)))) and ( /* Twin is suspect */ (td.future_contact = 'Yes' and td.suspect = 2) or /* Twin is suspect - Informant is Blessed */ (td.future_contact = 'Yes' and td.suspect = 1 and id.suspect = 1) or /* No twin - Informant is Blessed */ (ISNULL(td.suspect) and id.suspect = 1 and id.future_contact = 'Yes') or /* Twin broken off - Informant is Blessed */ (td.participation = 'Aborted' and id.suspect = 1 and id.future_contact = 'Yes') or /* Twin broken off - No inform - Have partner */ (td.participation = 'Aborted' and ISNULL(id.suspect) and p2.dead = 0)) and l.event = 'Finished' /* Get at area code */ and substring(p1.postal_code, 1, 2) = pg.code /* Not already distributed */ and (h.nurse is NULL or h.nurse=00 or h.doctor=00) /* Has not refused or been aborted */ and not (h.status = 'Refused' or h.status = 'Aborted' or h.status = 'Died' or h.status = 'Other') order by tvid; Some explanations: `concat(p1.id, p1.tvab) + 0 as tvid' We want to sort on the concatenated `id' and `tvab' in numerical order. Adding `0' to the result causes MySQL to treat the result as a number. column `id' This identifies a pair of twins. It is a key in all tables. column `tvab' This identifies a twin in a pair. It has a value of `1' or `2'. column `ptvab' This is an inverse of `tvab'. When `tvab' is `1' this is `2', and vice versa. It exists to save typing and to make it easier for MySQL to optimize the query. This query demonstrates, among other things, how to do lookups on a table from the same table with a join (`p1' and `p2'). In the example, this is used to check whether a twin's partner died before the age of 65. If so, the row is not returned. All of the above exist in all tables with twin-related information. We have a key on both `id,tvab' (all tables), and `id,ptvab' (`person_data') to make queries faster. On our production machine (A 200MHz UltraSPARC), this query returns about 150-200 rows and takes less than one second. The current number of records in the tables used above: *Table* *Rows* `person_data' 71074 `lentus' 5291 `twin_project' 5286 `twin_data' 2012 `informant_data' 663 `harmony' 381 `postal_groups' 100 Show a Table on Twin Pair Status -------------------------------- Each interview ends with a status code called `event'. The query shown below is used to display a table over all twin pairs combined by event. This indicates in how many pairs both twins are finished, in how many pairs one twin is finished and the other refused, and so on. select t1.event, t2.event, count(*) from lentus as t1, lentus as t2, twin_project as tp where /* We are looking at one pair at a time */ t1.id = tp.id and t1.tvab=tp.tvab and t1.id = t2.id /* Just the sceening survey */ and tp.survey_no = 5 /* This makes each pair only appear once */ and t1.tvab='1' and t2.tvab='2' group by t1.event, t2.event; Using MySQL with Apache ======================= The Contrib section includes programs that let you authenticate your users from a MySQL database and also let you log your log files into a MySQL table. *Note Contrib::. You can change the Apache logging format to be easily readable by MySQL by putting the following into the Apache configuration file: LogFormat \ "\"%h\",%{%Y%m%d%H%M%S}t,%>s,\"%b\",\"%{Content-Type}o\", \ \"%U\",\"%{Referer}i\",\"%{User-Agent}i\"" In MySQL you can do something like this: LOAD DATA INFILE '/local/access_log' INTO TABLE table_name FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' ESCAPED BY '\\' MySQL Database Administration ***************************** Configuring MySQL ================= mysqld Command-line Options --------------------------- `mysqld' accepts the following command-line options: `--ansi' Use ANSI SQL syntax instead of MySQL syntax. *Note ANSI mode::. `-b, --basedir=path' Path to installation directory. All paths are usually resolved relative to this. `--big-tables' Allow big result sets by saving all temporary sets on file. It solves most 'table full' errors, but also slows down the queries where in-memory tables would suffice. Since Version 3.23.2, MySQL is able to solve it automatically by using memory for small temporary tables and switching to disk tables where necessary. `--bind-address=IP' IP address to bind to. `--character-sets-dir=path' Directory where character sets are. *Note Character sets::. `--chroot=path' Chroot `mysqld' daemon during startup. Recommended security measure. It will somewhat limit `LOAD DATA INFILE' and `SELECT ... INTO OUTFILE' though. `--core-file' Write a core file if `mysqld' dies. For some systems you must also specify `--core-file-size' to `safe_mysqld'. *Note `safe_mysqld': safe_mysqld. `-h, --datadir=path' Path to the database root. `--default-character-set=charset' Set the default character set. *Note Character sets::. `--default-table-type=type' Set the default table type for tables. *Note Table types::. `--debug[...]=' If MySQL is configured with `--with-debug', you can use this option to get a trace file of what `mysqld' is doing. *Note Making trace files::. `--delay-key-write-for-all-tables' Don't flush key buffers between writes for any `MyISAM' table. *Note Server parameters::. `--enable-locking' Enable system locking. Note that if you use this option on a system which a not fully working lockd() (as on Linux) you will easily get mysqld to deadlock. `-T, --exit-info' This is a bit mask of different flags one can use for debugging the mysqld server; One should not use this option if one doesn't know exactly what it does! `--flush' Flush all changes to disk after each SQL command. Normally MySQL only does a write of all changes to disk after each SQL command and lets the operating system handle the syncing to disk. *Note Crashing::. `-?, --help' Display short help and exit. `--init-file=file' Read SQL commands from this file at startup. `-L, --language=...' Client error messages in given language. May be given as a full path. *Note Languages::. `-l, --log[=file]' Log connections and queries to file. *Note Query log::. `--log-isam[=file]' Log all ISAM/MyISAM changes to file (only used when debugging ISAM/MyISAM). `--log-slow-queries[=file]' Log all queries that have taken more than `long_query_time' seconds to execute to file. *Note Slow query log::. `--log-update[=file]' Log updates to `file.#' where `#' is a unique number if not given. *Note Update log::. `--log-long-format' Log some extra information to update log. If you are using `--log-slow-queries' then queries that are not using indexes are logged to the slow query log. `--low-priority-updates' Table-modifying operations (`INSERT'/`DELETE'/`UPDATE') will have lower priority than selects. It can also be done via `{INSERT | REPLACE | UPDATE | DELETE} LOW_PRIORITY ...' to lower the priority of only one query, or by `SET OPTION SQL_LOW_PRIORITY_UPDATES=1' to change the priority in one thread. *Note Table locking::. `--memlock' Lock the `mysqld' process in memory. This works only if your system supports the `mlockall()' system call (like Solaris). This may help if you have a problem where the operating system is causing `mysqld' to swap on disk. `--myisam-recover [=option[,option...]]] where option is any combination' of `DEFAULT', `BACKUP', `FORCE' or `QUICK'. You can also set this explicitely to `""' if you want to disable this option. If this option is used, `mysqld' will on open check if the table is marked as crashed or if if the table wasn't closed properly. (The last option only works if you are running with `--skip-locking'). If this is the case `mysqld' will run check on the table. If the table was corrupted, `mysqld' will attempt to repair it. The following options affects how the repair works. DEFAULT The same as not giving any option to `--myisam-recover'. BACKUP If the data table was changed during recover, save a backup of the `table_name.MYD' data file as `table_name-datetime.BAK'. FORCE Run recover even if we will loose more than one row from the .MYD file. QUICK Don't check the rows in the table if there isn't any delete blocks. Before a table is automatically repaired, MySQL will add a note about this in the error log. If you want to be able to recover from most things without user intervention, you should use the options `BACKUP,FORCE'. This will force a repair of a table even if some rows would be deleted, but it will keep the old data file as a backup so that you can later examine what happened. `--pid-file=path' Path to pid file used by `safe_mysqld'. `-P, --port=...' Port number to listen for TCP/IP connections. `-o, --old-protocol' Use the 3.20 protocol for compatibility with some very old clients. *Note Upgrading-from-3.20::. `--one-thread' Only use one thread (for debugging under Linux). *Note Debugging server::. `-O, --set-variable var=option' Give a variable a value. `--help' lists variables. You can find a full description for all variables in the `SHOW VARIABLES' section in this manual. *Note SHOW VARIABLES::. The tuning server parameters section includes information of how to optimize these. *Note Server parameters::. `--safe-mode' Skip some optimize stages. Implies `--skip-delay-key-write'. `--safe-show-database' Don't show databases for which the user doesn't have any privileges. `--safe-user-create' If this is enabled, a user can't create new users with the GRANT command, if the user doesn't have `INSERT' privilege to the `mysql.user' table or any column in this table. `--skip-concurrent-insert' Turn off the ability to select and insert at the same time on `MyISAM' tables. (This is only to be used if you think you have found a bug in this feature). `--skip-delay-key-write' Ignore the `delay_key_write' option for all tables. *Note Server parameters::. `--skip-grant-tables' This option causes the server not to use the privilege system at all. This gives everyone _full access_ to all databases! (You can tell a running server to start using the grant tables again by executing `mysqladmin flush-privileges' or `mysqladmin reload'.) `--skip-host-cache' Never use host name cache for faster name-ip resolution, but query DNS server on every connect instead. *Note DNS::. `--skip-locking' Don't use system locking. To use `isamchk' or `myisamchk' you must shut down the server. *Note Stability::. Note that in MySQL Version 3.23 you can use `REPAIR' and `CHECK' to repair/check `MyISAM' tables. `--skip-name-resolve' Hostnames are not resolved. All `Host' column values in the grant tables must be IP numbers or `localhost'. *Note DNS::. `--skip-networking' Don't listen for TCP/IP connections at all. All interaction with `mysqld' must be made via Unix sockets. This option is highly recommended for systems where only local requests are allowed. *Note DNS::. `--skip-new' Don't use new, possible wrong routines. Implies `--skip-delay-key-write'. This will also set default table type to `ISAM'. *Note ISAM::. `--skip-symlink' Don't delete or rename files that a symlinked file in the data directory points to. `--skip-safemalloc' If MySQL is configured with `--with-debug=full', all programs will check the memory for overruns for every memory allocation and memory freeing. As this checking is very slow, you can avoid this, when you don't need memory checking, by using this option. `--skip-show-database' Don't allow 'SHOW DATABASE' commands, unless the user has *process* privilege. `--skip-stack-trace' Don't write stack traces. This option is useful when you are running `mysqld' under a debugger. *Note Debugging server::. `--skip-thread-priority' Disable using thread priorities for faster response time. `--socket=path' Socket file to use for local connections instead of default `/tmp/mysql.sock'. `--sql-mode=option[,option[,option...]]' Option can be any combination of: `REAL_AS_FLOAT', `PIPES_AS_CONCAT', `ANSI_QUOTES', `IGNORE_SPACE', `SERIALIZE', `ONLY_FULL_GROUP_BY'. It can also be empty (`""') if you want to reset this. By specifying all of the above options is same as using -ansi. With this option one can turn on only needed SQL modes. *Note ANSI mode::. `transaction-isolation= { READ-UNCOMMITTED | READ-COMMITTED | REPEATABLE-READ | SERIALIZABLE }' Sets the default transaction isolation level. *Note SET TRANSACTION::. `-t, --tmpdir=path' Path for temporary files. It may be useful if your default `/tmp' directory resides on a partition too small to hold temporary tables. `-u, --user=user_name' Run `mysqld' daemon as user `user_name'. This option is _mandatory_ when starting `mysqld' as root. `-V, --version' Output version information and exit. `-W, --warnings' Print out warnings like `Aborted connection...' to the `.err' file. *Note Communication errors::. my.cnf Option Files ------------------- MySQL can, since Version 3.22, read default startup options for the server and for clients from option files. MySQL reads default options from the following files on Unix: *Filename* *Purpose* `/etc/my.cnf' Global options `DATADIR/my.cnf' Server-specific options `defaults-extra-file' The file specified with -defaults-extra-file=# `~/.my.cnf' User-specific options `DATADIR' is the MySQL data directory (typically `/usr/local/mysql/data' for a binary installation or `/usr/local/var' for a source installation). Note that this is the directory that was specified at configuration time, not the one specified with `--datadir' when `mysqld' starts up! (`--datadir' has no effect on where the server looks for option files, because it looks for them before it processes any command-line arguments.) MySQL reads default options from the following files on Windows: *Filename* *Purpose* `windows-system-directory\my.ini'Global options `C:\my.cnf' Global options `C:\mysql\data\my.cnf' Server-specific options Note that on Windows, you should specify all paths with `/' instead of `\'. If you use `\', you need to specify this twice, as `\' is the escape character in MySQL. MySQL tries to read option files in the order listed above. If multiple option files exist, an option specified in a file read later takes precedence over the same option specified in a file read earlier. Options specified on the command line take precedence over options specified in any option file. Some options can be specified using environment variables. Options specified on the command line or in option files take precedence over environment variable values. *Note Environment variables::. The following programs support option files: `mysql', `mysqladmin', `mysqld', `mysqldump', `mysqlimport', `mysql.server', `myisamchk', and `myisampack'. You can use option files to specify any long option that a program supports! Run the program with `--help' to get a list of available options. An option file can contain lines of the following forms: `#comment' Comment lines start with `#' or `;'. Empty lines are ignored. `[group]' `group' is the name of the program or group for which you want to set options. After a group line, any `option' or `set-variable' lines apply to the named group until the end of the option file or another group line is given. `option' This is equivalent to `--option' on the command line. `option=value' This is equivalent to `--option=value' on the command line. `set-variable = variable=value' This is equivalent to `--set-variable variable=value' on the command line. This syntax must be used to set a `mysqld' variable. The `client' group allows you to specify options that apply to all MySQL clients (not `mysqld'). This is the perfect group to use to specify the password you use to connect to the server. (But make sure the option file is readable and writable only by yourself.) Note that for options and values, all leading and trailing blanks are automatically deleted. You may use the escape sequences `\b', `\t', `\n', `\r', `\\', and `\s' in your value string (`\s' == blank). Here is a typical global option file: [client] port=3306 socket=/tmp/mysql.sock [mysqld] port=3306 socket=/tmp/mysql.sock set-variable = key_buffer_size=16M set-variable = max_allowed_packet=1M [mysqldump] quick Here is typical user option file: [client] # The following password will be sent to all standard MySQL clients password=my_password [mysql] no-auto-rehash set-variable = connect_timeout=2 [mysqlhotcopy] interactive-timeout If you have a source distribution, you will find sample configuration files named `my-xxxx.cnf' in the `support-files' directory. If you have a binary distribution, look in the `DIR/support-files' directory, where `DIR' is the pathname to the MySQL installation directory (typically `/usr/local/mysql'). Currently there are sample configuration files for small, medium, large, and very large systems. You can copy `my-xxxx.cnf' to your home directory (rename the copy to `.my.cnf') to experiment with this. All MySQL clients that support option files support the following options: -no-defaults Don't read any option files. -print-defaults Print the program name and all options that it will get. -defaults-file=full-path-to-default-fileOnly use the given configuration file. -defaults-extra-file=full-path-to-default-fileRead this configuration file after the global configuration file but before the user configuration file. Note that the above options must be first on the command line to work! `--print-defaults' may however be used directly after the `--defaults-xxx-file' commands. Note for developers: Option file handling is implemented simply by processing all matching options (that is, options in the appropriate group) before any command-line arguments. This works nicely for programs that use the last instance of an option that is specified multiple times. If you have an old program that handles multiply-specified options this way but doesn't read option files, you need add only two lines to give it that capability. Check the source code of any of the standard MySQL clients to see how to do this. In shell scripts you can use the `my_print_defaults' command to parse the config files: shell> my_print_defaults client mysql --port=3306 --socket=/tmp/mysql.sock --no-auto-rehash The above output contains all options for the groups 'client' and 'mysql'. Installing Many Servers on the Same Machine ------------------------------------------- In some cases you may want to have many different `mysqld' daemons (servers) running on the same machine. You may for example want to run a new version of MySQL for testing together with an old version that is in production. Another case is when you want to give different users access to different `mysqld' servers that they manage themselves. One way to get a new server running is by starting it with a different socket and port as follows: shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock shell> MYSQL_TCP_PORT=3307 shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT shell> scripts/mysql_install_db shell> bin/safe_mysqld & The environment variables appendix includes a list of other environment variables you can use to affect `mysqld'. *Note Environment variables::. The above is the quick and dirty way that one commonly uses for testing. The nice thing with this is that all connections you do in the above shell will automatically be directed to the new running server! If you need to do this more permanently, you should create an option file for each server. *Note Option files::. In your startup script that is executed at boot time (mysql.server?) you should specify for both servers: `safe_mysqld --default-file=path-to-option-file' At least the following options should be different per server: `port=#' `socket=path' `pid-file=path' The following options should be different, if they are used: `log=path' `log-bin=path' `log-update=path' `log-isam=path' `bdb-logdir=path' If you want more performance, you can also specify the following differently: `tmpdir=path' `bdb-tmpdir=path' *Note Command-line options::. If you are installing binary MySQL versions (.tar files) and start them with `./bin/safe_mysqld' then in most cases the only option you need to add/change is the `socket' and `port' argument to `safe_mysqld'. Running Multiple MySQL Servers on the Same Machine -------------------------------------------------- There are circumstances when you might want to run multiple servers on the same machine. For example, you might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you might be an Internet service provider that wants to provide independent MySQL installations for different customers. If you want to run multiple servers, the easiest way is to compile the servers with different TCP/IP ports and socket files so they are not both listening to the same TCP/IP port or socket file. *Note `mysqld_multi': mysqld_multi. Assume an existing server is configured for the default port number and socket file. Then configure the new server with a `configure' command something like this: shell> ./configure --with-tcp-port=port_number \ --with-unix-socket-path=file_name \ --prefix=/usr/local/mysql-3.22.9 Here `port_number' and `file_name' should be different than the default port number and socket file pathname, and the `--prefix' value should specify an installation directory different than the one under which the existing MySQL installation is located. You can check the socket used by any currently executing MySQL server with this command: shell> mysqladmin -h hostname --port=port_number variables Note that if you specify "`localhost'" as a hostname, `mysqladmin' will default to using Unix sockets instead of TCP/IP. If you have a MySQL server running on the port you used, you will get a list of some of the most important configurable variables in MySQL, including the socket name. You don't have to recompile a new MySQL server just to start with a different port and socket. You can change the port and socket to be used by specifying them at run time as options to `safe_mysqld': shell> /path/to/safe_mysqld --socket=file_name --port=port_number `mysqld_multi' can also take `safe_mysqld' (or `mysqld') as an argument and pass the options from a configuration file to `safe_mysqld' and further to `mysqld'. If you run the new server on the same database directory as another server with logging enabled, you should also specify the name of the log files to `safe_mysqld' with `--log', `--log-update', or `--log-slow-queries'. Otherwise, both servers may be trying to write to the same log file. *WARNING*: Normally you should never have two servers that update data in the same database! If your OS doesn't support fault-free system locking, this may lead to unpleasant surprises! If you want to use another database directory for the second server, you can use the `--datadir=path' option to `safe_mysqld'. *NOTE* also that starting several MySQL servers (`mysqlds') in different machines and letting them access one data directory over `NFS' is generally a *BAD IDEA*! The problem is that the `NFS' will become the bottleneck with the speed. It is not meant for such use. And last but not least, you would still have to come up with a solution how to make sure that two or more `mysqlds' are not interfering with each other. At the moment there is no platform that would 100% reliable do the file locking (`lockd' daemon usually) in every situation. Yet there would be one more possible risk with `NFS'; it would make the work even more complicated for `lockd' daemon to handle. So make it easy for your self and forget about the idea. The working solution is to have one computer with an operating system that efficiently handles threads and have several CPUs in it. When you want to connect to a MySQL server that is running with a different port than the port that is compiled into your client, you can use one of the following methods: * Start the client with `--host 'hostname' --port=port_number' to connect with TCP/IP, or `[--host localhost] --socket=file_name' to connect via a Unix socket. * In your C or Perl programs, you can give the port or socket arguments when connecting to the MySQL server. * If your are using the Perl `DBD::mysql' module you can read the options from the MySQL option files. *Note Option files::. $dsn = "DBI:mysql:test;mysql_read_default_group=client;mysql_read_default_file=/usr/local/mysql/data/my.cnf" $dbh = DBI->connect($dsn, $user, $password); * Set the `MYSQL_UNIX_PORT' and `MYSQL_TCP_PORT' environment variables to point to the Unix socket and TCP/IP port before you start your clients. If you normally use a specific socket or port, you should place commands to set these environment variables in your `.login' file. *Note Environment variables::. * Specify the default socket and TCP/IP port in the `.my.cnf' file in your home directory. *Note Option files::. General Security Issues and the MySQL Access Privilege System ============================================================= MySQL has an advanced but non-standard security/privilege system. This section describes how it works. General Security Guidelines --------------------------- Anyone using MySQL on a computer connected to the Internet should read this section to avoid the most common security mistakes. In discussing security, we emphasize the necessity of fully protecting the entire server host (not simply the MySQL server) against all types of applicable attacks: eavesdropping, altering, playback, and denial of service. We do not cover all aspects of availability and fault tolerance here. MySQL uses security based on Access Control Lists (ACLs) for all connections, queries, and other operations that a user may attempt to perform. There is also some support for SSL-encrypted connections between MySQL clients and servers. Many of the concepts discussed here are not specific to MySQL at all; the same general ideas apply to almost all applications. When running MySQL, follow these guidelines whenever possible: * DON'T EVER GIVE ANYONE (EXCEPT THE MySQL ROOT USER) ACCESS TO THE `user' TABLE IN THE `mysql' DATABASE! The encrypted password is the real password in MySQL. If you know the password listed in the `user' table for a given user, you can easily log in as that user if you have access to the host listed for that account. * Learn the MySQL access privilege system. The `GRANT' and `REVOKE' commands are used for controlling access to MySQL. Do not grant any more privileges than necessary. Never grant privileges to all hosts. Checklist: - Try `mysql -u root'. If you are able to connect successfully to the server without being asked for a password, you have problems. Anyone can connect to your MySQL server as the MySQL `root' user with full privileges! Review the MySQL installation instructions, paying particular attention to the item about setting a `root' password. - Use the command `SHOW GRANTS' and check to see who has access to what. Remove those privileges that are not necessary using the `REVOKE' command. * Do not keep any plain-text passwords in your database. When your computer becomes compromised, the intruder can take the full list of passwords and use them. Instead use `MD5()' or another one-way hashing function. * Do not choose passwords from dictionaries. There are special programs to break them. Even passwords like "xfish98" are very bad. Much better is "duag98" which contains the same word "fish" but typed one key to the left on a standard QWERTY keyboard. Another method is to use "Mhall" which is taken from the first characters of each word in the sentence "Mary had a little lamb." This is easy to remember and type, but difficult to guess for someone who does not know it. * Invest in a firewall. This protects you from at least 50% of all types of exploits in any software. Put MySQL behind the firewall or in a demilitarized zone (DMZ). Checklist: - Try to scan your ports from the Internet using a tool such as `nmap'. MySQL uses port 3306 by default. This port should be inaccessible from untrusted hosts. Another simple way to check whether or not your MySQL port is open is to try the following command from some remote machine, where `server_host' is the hostname of your MySQL server: shell> telnet server_host 3306 If you get a connection and some garbage characters, the port is open, and should be closed on your firewall or router, unless you really have a good reason to keep it open. If `telnet' just hangs or the connection is refused, everything is OK; the port is blocked. * Do not trust any data entered by your users. They can try to trick your code by entering special or escaped character sequences in Web forms, URLs, or whatever application you have built. Be sure that your application remains secure if a user enters something like "`; DROP DATABASE mysql;'". This is an extreme example, but large security leaks and data loss may occur as a result of hackers using similar techniques, if you do not prepare for them. Also remember to check numeric data. A common mistake is to protect only strings. Sometimes people think that if a database contains only publicly available data that it need not be protected. This is incorrect. At least denial-of-service type attacks can be performed on such databases. The simplest way to protect from this type of attack is to use apostrophes around the numeric constants: `SELECT * FROM table WHERE ID='234'' rather than `SELECT * FROM table WHERE ID=234'. MySQL automatically converts this string to a number and strips all non-numeric symbols from it. Checklist: - All Web applications: * Try to enter `'' and `"' in all your Web forms. If you get any kind of MySQL error, investigate the problem right away. * Try to modify any dynamic URLs by adding `%22' (`"'), `%23' (`#'), and `%27' (`'') in the URL. * Try to modify datatypes in dynamic URLs from numeric ones to character ones containing characters from previous examples. Your application should be safe against this and similar attacks. * Try to enter characters, spaces, and special symbols instead of numbers in numeric fields. Your application should remove them before passing them to MySQL or your application should generate an error. Passing unchecked values to MySQL is very dangerous! * Check data sizes before passing them to MySQL. * Consider having your application connect to the database using a different user name than the one you use for administrative purposes. Do not give your applications any more access privileges than they need. - Users of PHP: * Check out the `addslashes()' function. As of PHP 4.0.3, a `mysql_escape_string()' function is available that is based on the function of the same name in the MySQL C API. - Users of MySQL C API: * Check out the `mysql_escape_string()' API call. - Users of MySQL++: * Check out the `escape' and `quote' modifiers for query streams. - Users of Perl DBI: * Check out the `quote()' method or use placeholders. - Users of Java JDBC: * Use a `PreparedStatement' object and placeholders. * Do not transmit plain (unencrypted) data over the Internet. These data are accessible to everyone who has the time and ability to intercept it and use it for their own purposes. Instead, use an encrypted protocol such as SSL or SSH. MySQL supports internal SSL connections as of Version 3.23.9. SSH port-forwarding can be used to create an encrypted (and compressed) tunnel for the communication. * Learn to use the `tcpdump' and `strings' utilities. For most cases, you can check whether or not MySQL data streams are unencrypted by issuing a command like the following: shell> tcpdump -l -i eth0 -w - src or dst port 3306 | strings (This works under Linux and should work with small modifications under other systems). Warning: If you do not see data this doesn't always actually mean that it is encrypted. If you need high security, you should consult with a security expert. How to Make MySQL Secure Against Crackers ----------------------------------------- When you connect to a MySQL server, you normally should use a password. The password is not transmitted in clear text over the connection, however the encryption algorithm is not very strong, and with some effort a clever attacker can crack the password if he is able to sniff the traffic between the client and the server. If the connection between the client and the server goes through an untrusted network, you should use an SSH tunnel to encrypt the communication. All other information is transferred as text that can be read by anyone who is able to watch the connection. If you are concerned about this, you can use the compressed protocol (in MySQL Version 3.22 and above) to make things much harder. To make things even more secure you should use `ssh'. You can find an open source `ssh' client at `http://www.openssh.org', and a commercial `ssh' client at `http://www.ssh.com'. With this, you can get an encrypted TCP/IP connection between a MySQL server and a MySQL client. To make a MySQL system secure, you should strongly consider the following suggestions: * Use passwords for all MySQL users. Remember that anyone can log in as any other person as simply as `mysql -u other_user db_name' if `other_user' has no password. It is common behavior with client/server applications that the client may specify any user name. You can change the password of all users by editing the `mysql_install_db' script before you run it, or only the password for the MySQL `root' user like this: shell> mysql -u root mysql mysql> UPDATE user SET Password=PASSWORD('new_password') WHERE user='root'; mysql> FLUSH PRIVILEGES; * Don't run the MySQL daemon as the Unix `root' user. This is very dangerous, because any user with `FILE' privileges will be able to create files as `root' (for example, `~root/.bashrc'). To prevent this, `mysqld' will refuse to run as `root' unless it is specified directly using a `--user=root' option. `mysqld' can be run as an ordinary unprivileged user instead. You can also create a new Unix user `mysql' to make everything even more secure. If you run `mysqld' as another Unix user, you don't need to change the `root' user name in the `user' table, because MySQL user names have nothing to do with Unix user names. To start `mysqld' as another Unix user, add a `user' line that specifies the user name to the `[mysqld]' group of the `/etc/my.cnf' option file or the `my.cnf' option file in the server's data directory. For example: [mysqld] user=mysql This will cause the server to start as the designated user whether you start it manually or by using `safe_mysqld' or `mysql.server'. For more details, see *Note Changing MySQL user: Changing MySQL user. * Don't support symlinks to tables (This can be disabled with the `--skip-symlink' option. This is especially important if you run `mysqld' as root as anyone that has write access to the mysqld data directories could then delete any file in the system! *Note Symbolic links to tables::. * Check that the Unix user that `mysqld' runs as is the only user with read/write privileges in the database directories. * Don't give the *process* privilege to all users. The output of `mysqladmin processlist' shows the text of the currently executing queries, so any user who is allowed to execute that command might be able to see if another user issues an `UPDATE user SET password=PASSWORD('not_secure')' query. `mysqld' reserves an extra connection for users who have the *process* privilege, so that a MySQL `root' user can log in and check things even if all normal connections are in use. * Don't give the *file* privilege to all users. Any user that has this privilege can write a file anywhere in the file system with the privileges of the `mysqld' daemon! To make this a bit safer, all files generated with `SELECT ... INTO OUTFILE' are readable to everyone, and you cannot overwrite existing files. The *file* privilege may also be used to read any file accessible to the Unix user that the server runs as. This could be abused, for example, by using `LOAD DATA' to load `/etc/passwd' into a table, which can then be read with `SELECT'. * If you don't trust your DNS, you should use IP numbers instead of hostnames in the grant tables. In any case, you should be very careful about creating grant table entries using hostname values that contain wild cards! * If you want to restrict the number of connections for a single user, you can do this by setting the `max_user_connections' variable in `mysqld'. Startup Options for `mysqld' Concerning Security ------------------------------------------------ The following `mysqld' options affect security: `--safe-show-database' With this option, `SHOW DATABASES' returns only those databases for which the user has some kind of privilege. `--safe-user-create' If this is enabled, an user can't create new users with the `GRANT' command, if the user doesn't have `INSERT' privilege to the `mysql.user' table. If you want to give a user access to just create new users with those privileges that the user has right to grant, you should give the user the following privilege: GRANT INSERT(user) on mysql.user to 'user'h'ostname'; This will ensure that the user can't change any privilege columns directly, but has to use the `GRANT' command to give privileges to other users. `--skip-grant-tables' This option causes the server not to use the privilege system at all. This gives everyone _full access_ to all databases! (You can tell a running server to start using the grant tables again by executing `mysqladmin flush-privileges' or `mysqladmin reload'.) `--skip-name-resolve' Hostnames are not resolved. All `Host' column values in the grant tables must be IP numbers or `localhost'. `--skip-networking' Don't allow TCP/IP connections over the network. All connections to `mysqld' must be made via Unix sockets. This option is unsuitable for systems that use MIT-pthreads, because the MIT-pthreads package doesn't support Unix sockets. `--skip-show-database' With this option, the `SHOW DATABASES' statement doesn't return anything. What the Privilege System Does ------------------------------ The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with privileges on a database such as *select*, *insert*, *update* and *delete*. Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as `LOAD DATA INFILE' and administrative operations. How the Privilege System Works ------------------------------ The MySQL privilege system ensures that all users may do exactly the things that they are supposed to be allowed to do. When you connect to a MySQL server, your identity is determined by *the host from which you connect* and *the user name you specify*. The system grants privileges according to your identity and *what you want to do*. MySQL considers both your hostname and user name in identifying you because there is little reason to assume that a given user name belongs to the same person everywhere on the Internet. For example, the user `bill' who connects from `whitehouse.gov' need not be the same person as the user `bill' who connects from `microsoft.com'. MySQL handles this by allowing you to distinguish users on different hosts that happen to have the same name: you can grant `bill' one set of privileges for connections from `whitehouse.gov', and a different set of privileges for connections from `microsoft.com'. MySQL access control involves two stages: * Stage 1: The server checks whether or not you are even allowed to connect. * Stage 2: Assuming you can connect, the server checks each request you issue to see whether or not you have sufficient privileges to perform it. For example, if you try to select rows from a table in a database or drop a table from the database, the server makes sure you have the *select* privilege for the table or the *drop* privilege for the database. The server uses the `user', `db', and `host' tables in the `mysql' database at both stages of access control. The fields in these grant tables are shown below: *Table name* `user' `db' `host' *Scope `Host' `Host' `Host' fields* `User' `Db' `Db' `Password' `User' *Privilege `Select_priv' `Select_priv' `Select_priv' fields* `Insert_priv' `Insert_priv' `Insert_priv' `Update_priv' `Update_priv' `Update_priv' `Delete_priv' `Delete_priv' `Delete_priv' `Index_priv' `Index_priv' `Index_priv' `Alter_priv' `Alter_priv' `Alter_priv' `Create_priv' `Create_priv' `Create_priv' `Drop_priv' `Drop_priv' `Drop_priv' `Grant_priv' `Grant_priv' `Grant_priv' `References_priv' `Reload_priv' `Shutdown_priv' `Process_priv' `File_priv' For the second stage of access control (request verification), the server may, if the request involves tables, additionally consult the `tables_priv' and `columns_priv' tables. The fields in these tables are shown below: *Table name* `tables_priv' `columns_priv' *Scope `Host' `Host' fields* `Db' `Db' `User' `User' `Table_name' `Table_name' `Column_name' *Privilege `Table_priv' `Column_priv' fields* `Column_priv' *Other `Timestamp' `Timestamp' fields* `Grantor' Each grant table contains scope fields and privilege fields. Scope fields determine the scope of each entry in the tables, that is, the context in which the entry applies. For example, a `user' table entry with `Host' and `User' values of `'thomas.loc.gov'' and `'bob'' would be used for authenticating connections made to the server by `bob' from the host `thomas.loc.gov'. Similarly, a `db' table entry with `Host', `User', and `Db' fields of `'thomas.loc.gov'', `'bob'' and `'reports'' would be used when `bob' connects from the host `thomas.loc.gov' to access the `reports' database. The `tables_priv' and `columns_priv' tables contain scope fields indicating tables or table/column combinations to which each entry applies. For access-checking purposes, comparisons of `Host' values are case insensitive. `User', `Password', `Db', and `Table_name' values are case sensitive. `Column_name' values are case insensitive in MySQL Version 3.22.12 or later. Privilege fields indicate the privileges granted by a table entry, that is, what operations can be performed. The server combines the information in the various grant tables to form a complete description of a user's privileges. The rules used to do this are described in *Note Request access::. Scope fields are strings, declared as shown below; the default value for each is the empty string: *Field *Type* name* `Host' `CHAR(60)' `User' `CHAR(16)' `Password' `CHAR(16)' `Db' `CHAR(64)' (`CHAR(60)' for the `tables_priv' and `columns_priv' tables) `Table_name'`CHAR(60)' `Column_name'`CHAR(60)' In the `user', `db' and `host' tables, all privilege fields are declared as `ENUM('N','Y')' -- each can have a value of `'N'' or `'Y'', and the default value is `'N''. In the `tables_priv' and `columns_priv' tables, the privilege fields are declared as `SET' fields: *Table name* *Field name* *Possible set elements* `tables_priv' `Table_priv' `'Select', 'Insert', 'Update', 'Delete', 'Create', 'Drop', 'Grant', 'References', 'Index', 'Alter'' `tables_priv' `Column_priv' `'Select', 'Insert', 'Update', 'References'' `columns_priv' `Column_priv' `'Select', 'Insert', 'Update', 'References'' Briefly, the server uses the grant tables like this: * The `user' table scope fields determine whether to allow or reject incoming connections. For allowed connections, any privileges granted in the `user' table indicate the user's global (superuser) privileges. These privileges apply to *all* databases on the server. * The `db' and `host' tables are used together: - The `db' table scope fields determine which users can access which databases from which hosts. The privilege fields determine which operations are allowed. - The `host' table is used as an extension of the `db' table when you want a given `db' table entry to apply to several hosts. For example, if you want a user to be able to use a database from several hosts in your network, leave the `Host' value empty in the user's `db' table entry, then populate the `host' table with an entry for each of those hosts. This mechanism is described more detail in *Note Request access::. * The `tables_priv' and `columns_priv' tables are similar to the `db' table, but are more fine-grained: they apply at the table and column levels rather than at the database level. Note that administrative privileges (*reload*, *shutdown*, etc.) are specified only in the `user' table. This is because administrative operations are operations on the server itself and are not database-specific, so there is no reason to list such privileges in the other grant tables. In fact, only the `user' table need be consulted to determine whether or not you can perform an administrative operation. The *file* privilege is specified only in the `user' table, too. It is not an administrative privilege as such, but your ability to read or write files on the server host is independent of the database you are accessing. The `mysqld' server reads the contents of the grant tables once, when it starts up. Changes to the grant tables take effect as indicated in *Note Privilege changes::. When you modify the contents of the grant tables, it is a good idea to make sure that your changes set up privileges the way you want. For help in diagnosing problems, see *Note Access denied::. For advice on security issues, *note Security::. A useful diagnostic tool is the `mysqlaccess' script, which Yves Carlier has provided for the MySQL distribution. Invoke `mysqlaccess' with the `--help' option to find out how it works. Note that `mysqlaccess' checks access using only the `user', `db' and `host' tables. It does not check table- or column-level privileges. Privileges Provided by MySQL ---------------------------- Information about user privileges is stored in the `user', `db', `host', `tables_priv', and `columns_priv' tables in the `mysql' database (that is, in the database named `mysql'). The MySQL server reads the contents of these tables when it starts up and under the circumstances indicated in *Note Privilege changes::. The names used in this manual to refer to the privileges provided by MySQL are shown below, along with the table column name associated with each privilege in the grant tables and the context in which the privilege applies: *Privilege* *Column* *Context* *select* `Select_priv' tables *insert* `Insert_priv' tables *update* `Update_priv' tables *delete* `Delete_priv' tables *index* `Index_priv' tables *alter* `Alter_priv' tables *create* `Create_priv' databases, tables, or indexes *drop* `Drop_priv' databases or tables *grant* `Grant_priv' databases or tables *references*`References_priv' databases or tables *reload* `Reload_priv' server administration *shutdown* `Shutdown_priv' server administration *process* `Process_priv' server administration *file* `File_priv' file access on server The *select*, *insert*, *update*, and *delete* privileges allow you to perform operations on rows in existing tables in a database. `SELECT' statements require the *select* privilege only if they actually retrieve rows from a table. You can execute certain `SELECT' statements even without permission to access any of the databases on the server. For example, you could use the `mysql' client as a simple calculator: mysql> SELECT 1+1; mysql> SELECT PI()*2; The *index* privilege allows you to create or drop (remove) indexes. The *alter* privilege allows you to use `ALTER TABLE'. The *create* and *drop* privileges allow you to create new databases and tables, or to drop (remove) existing databases and tables. Note that if you grant the *drop* privilege for the `mysql' database to a user, that user can drop the database in which the MySQL access privileges are stored! The *grant* privilege allows you to give to other users those privileges you yourself possess. The *file* privilege gives you permission to read and write files on the server using the `LOAD DATA INFILE' and `SELECT ... INTO OUTFILE' statements. Any user to whom this privilege is granted can read or write any file that the MySQL server can read or write. The remaining privileges are used for administrative operations, which are performed using the `mysqladmin' program. The table below shows which `mysqladmin' commands each administrative privilege allows you to execute: *Privilege* *Commands permitted to privilege holders* *reload* `reload', `refresh', `flush-privileges', `flush-hosts', `flush-logs', and `flush-tables' *shutdown* `shutdown' *process* `processlist', `kill' The `reload' command tells the server to re-read the grant tables. The `refresh' command flushes all tables and opens and closes the log files. `flush-privileges' is a synonym for `reload'. The other `flush-*' commands perform functions similar to `refresh' but are more limited in scope, and may be preferable in some instances. For example, if you want to flush just the log files, `flush-logs' is a better choice than `refresh'. The `shutdown' command shuts down the server. The `processlist' command displays information about the threads executing within the server. The `kill' command kills server threads. You can always display or kill your own threads, but you need the *process* privilege to display or kill threads initiated by other users. *Note KILL::. It is a good idea in general to grant privileges only to those users who need them, but you should exercise particular caution in granting certain privileges: * The *grant* privilege allows users to give away their privileges to other users. Two users with different privileges and with the *grant* privilege are able to combine privileges. * The *alter* privilege may be used to subvert the privilege system by renaming tables. * The *file* privilege can be abused to read any world-readable file on the server into a database table, the contents of which can then be accessed using `SELECT'. This includes the contents of all databases hosted by the server! * The *shutdown* privilege can be abused to deny service to other users entirely, by terminating the server. * The *process* privilege can be used to view the plain text of currently executing queries, including queries that set or change passwords. * Privileges on the `mysql' database can be used to change passwords and other access privilege information. (Passwords are stored encrypted, so a malicious user cannot simply read them to know the plain text password). If they can access the `mysql.user' password column, they can use it to log into the MySQL server for the given user. (With sufficient privileges, the same user can replace a password with a different one.) There are some things that you cannot do with the MySQL privilege system: * You cannot explicitly specify that a given user should be denied access. That is, you cannot explicitly match a user and then refuse the connection. * You cannot specify that a user has privileges to create or drop tables in a database but not to create or drop the database itself. Connecting to the MySQL Server ------------------------------ MySQL client programs generally require that you specify connection parameters when you want to access a MySQL server: the host you want to connect to, your user name, and your password. For example, the `mysql' client can be started like this (optional arguments are enclosed between `[' and `]'): shell> mysql [-h host_name] [-u user_name] [-pyour_pass] Alternate forms of the `-h', `-u', and `-p' options are `--host=host_name', `--user=user_name', and `--password=your_pass'. Note that there is _no space_ between `-p' or `--password=' and the password following it. *NOTE:* Specifying a password on the command line is not secure! Any user on your system may then find out your password by typing a command like: `ps auxww'. *Note Option files::. `mysql' uses default values for connection parameters that are missing from the command line: * The default hostname is `localhost'. * The default user name is your Unix login name. * No password is supplied if `-p' is missing. Thus, for a Unix user `joe', the following commands are equivalent: shell> mysql -h localhost -u joe shell> mysql -h localhost shell> mysql -u joe shell> mysql Other MySQL clients behave similarly. On Unix systems, you can specify different default values to be used when you make a connection, so that you need not enter them on the command line each time you invoke a client program. This can be done in a couple of ways: * You can specify connection parameters in the `[client]' section of the `.my.cnf' configuration file in your home directory. The relevant section of the file might look like this: [client] host=host_name user=user_name password=your_pass *Note Option files::. * You can specify connection parameters using environment variables. The host can be specified for `mysql' using `MYSQL_HOST'. The MySQL user name can be specified using `USER' (this is for Windows only). The password can be specified using `MYSQL_PWD' (but this is insecure; see the next section). *Note Environment variables::. Access Control, Stage 1: Connection Verification ------------------------------------------------ When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether or not you can verify your identity by supplying the correct password. If not, the server denies access to you completely. Otherwise, the server accepts the connection, then enters Stage 2 and waits for requests. Your identity is based on two pieces of information: * The host from which you connect * Your MySQL user name Identity checking is performed using the three `user' table scope fields (`Host', `User', and `Password'). The server accepts the connection only if a `user' table entry matches your hostname and user name, and you supply the correct password. Values in the `user' table scope fields may be specified as follows: * A `Host' value may be a hostname or an IP number, or `'localhost'' to indicate the local host. * You can use the wild-card characters `%' and `_' in the `Host' field. * A `Host' value of `'%'' matches any hostname. * A blank `Host' value means that the privilege should be anded with the entry in the `host' table that matches the given host name. You can find more information about this in the next chapter. * As of MySQL Version 3.23, for `Host' values specified as IP numbers, you can specify a netmask indicating how many address bits to use for the network number. For example: GRANT ALL PRIVILEGES on db.* to david@'192.58.197.0/255.255.255.0'; This will allow everyone to connect from an IP where the following is true: user_ip & netmask = host_ip. In the above example all IP:s in the interval 192.58.197.0 - 192.58.197.255 can connect to the MySQL server. * Wild-card characters are not allowed in the `User' field, but you can specify a blank value, which matches any name. If the `user' table entry that matches an incoming connection has a blank user name, the user is considered to be the anonymous user (the user with no name), rather than the name that the client actually specified. This means that a blank user name is used for all further access checking for the duration of the connection (that is, during Stage 2). * The `Password' field can be blank. This does not mean that any password matches, it means the user must connect without specifying a password. Non-blank `Password' values represent encrypted passwords. MySQL does not store passwords in plaintext form for anyone to see. Rather, the password supplied by a user who is attempting to connect is encrypted (using the `PASSWORD()' function). The encrypted password is then used when the client/server is checking if the password is correct (This is done without the encrypted password ever traveling over the connection.) Note that from MySQL's point of view the encrypted password is the REAL password, so you should not give anyone access to it! In particular, don't give normal users read access to the tables in the `mysql' database! The examples below show how various combinations of `Host' and `User' values in `user' table entries apply to incoming connections: `Host' *value* `User' *Connections matched by entry* *value* `'thomas.loc.gov'' `'fred'' `fred', connecting from `thomas.loc.gov' `'thomas.loc.gov'' `''' Any user, connecting from `thomas.loc.gov' `'%'' `'fred'' `fred', connecting from any host `'%'' `''' Any user, connecting from any host `'%.loc.gov'' `'fred'' `fred', connecting from any host in the `loc.gov' domain `'x.y.%'' `'fred'' `fred', connecting from `x.y.net', `x.y.com',`x.y.edu', etc. (this is probably not useful) `'144.155.166.177''`'fred'' `fred', connecting from the host with IP address `144.155.166.177' `'144.155.166.%'' `'fred'' `fred', connecting from any host in the `144.155.166' class C subnet `'144.155.166.0/255.255.255.0''`'fred'' Same as previous example Because you can use IP wild-card values in the `Host' field (for example, `'144.155.166.%'' to match every host on a subnet), there is the possibility that someone might try to exploit this capability by naming a host `144.155.166.somewhere.com'. To foil such attempts, MySQL disallows matching on hostnames that start with digits and a dot. Thus, if you have a host named something like `1.2.foo.com', its name will never match the `Host' column of the grant tables. Only an IP number can match an IP wild-card value. An incoming connection may be matched by more than one entry in the `user' table. For example, a connection from `thomas.loc.gov' by `fred' would be matched by several of the entries just shown above. How does the server choose which entry to use if more than one matches? The server resolves this question by sorting the `user' table after reading it at startup time, then looking through the entries in sorted order when a user attempts to connect. The first matching entry is the one that is used. `user' table sorting works as follows. Suppose the `user' table looks like this: +-----------+----------+- | Host | User | ... +-----------+----------+- | % | root | ... | % | jeffrey | ... | localhost | root | ... | localhost | | ... +-----------+----------+- When the server reads in the table, it orders the entries with the most-specific `Host' values first (`'%'' in the `Host' column means "any host" and is least specific). Entries with the same `Host' value are ordered with the most-specific `User' values first (a blank `User' value means "any user" and is least specific). The resulting sorted `user' table looks like this: +-----------+----------+- | Host | User | ... +-----------+----------+- | localhost | root | ... | localhost | | ... | % | jeffrey | ... | % | root | ... +-----------+----------+- When a connection is attempted, the server looks through the sorted entries and uses the first match found. For a connection from `localhost' by `jeffrey', the entries with `'localhost'' in the `Host' column match first. Of those, the entry with the blank user name matches both the connecting hostname and user name. (The `'%'/'jeffrey'' entry would have matched, too, but it is not the first match in the table.) Here is another example. Suppose the `user' table looks like this: +----------------+----------+- | Host | User | ... +----------------+----------+- | % | jeffrey | ... | thomas.loc.gov | | ... +----------------+----------+- The sorted table looks like this: +----------------+----------+- | Host | User | ... +----------------+----------+- | thomas.loc.gov | | ... | % | jeffrey | ... +----------------+----------+- A connection from `thomas.loc.gov' by `jeffrey' is matched by the first entry, whereas a connection from `whitehouse.gov' by `jeffrey' is matched by the second. A common misconception is to think that for a given user name, all entries that explicitly name that user will be used first when the server attempts to find a match for the connection. This is simply not true. The previous example illustrates this, where a connection from `thomas.loc.gov' by `jeffrey' is first matched not by the entry containing `'jeffrey'' as the `User' field value, but by the entry with no user name! If you have problems connecting to the server, print out the `user' table and sort it by hand to see where the first match is being made. Access Control, Stage 2: Request Verification --------------------------------------------- Once you establish a connection, the server enters Stage 2. For each request that comes in on the connection, the server checks whether you have sufficient privileges to perform it, based on the type of operation you wish to perform. This is where the privilege fields in the grant tables come into play. These privileges can come from any of the `user', `db', `host', `tables_priv', or `columns_priv' tables. The grant tables are manipulated with `GRANT' and `REVOKE' commands. *Note `GRANT': GRANT. (You may find it helpful to refer to *Note Privileges::, which lists the fields present in each of the grant tables.) The `user' table grants privileges that are assigned to you on a global basis and that apply no matter what the current database is. For example, if the `user' table grants you the *delete* privilege, you can delete rows from any database on the server host! In other words, `user' table privileges are superuser privileges. It is wise to grant privileges in the `user' table only to superusers such as server or database administrators. For other users, you should leave the privileges in the `user' table set to `'N'' and grant privileges on a database-specific basis only, using the `db' and `host' tables. The `db' and `host' tables grant database-specific privileges. Values in the scope fields may be specified as follows: * The wild-card characters `%' and `_' can be used in the `Host' and `Db' fields of either table. * A `'%'' `Host' value in the `db' table means "any host." A blank `Host' value in the `db' table means "consult the `host' table for further information." * A `'%'' or blank `Host' value in the `host' table means "any host." * A `'%'' or blank `Db' value in either table means "any database." * A blank `User' value in either table matches the anonymous user. The `db' and `host' tables are read in and sorted when the server starts up (at the same time that it reads the `user' table). The `db' table is sorted on the `Host', `Db', and `User' scope fields, and the `host' table is sorted on the `Host' and `Db' scope fields. As with the `user' table, sorting puts the most-specific values first and least-specific values last, and when the server looks for matching entries, it uses the first match that it finds. The `tables_priv' and `columns_priv' tables grant table- and column-specific privileges. Values in the scope fields may be specified as follows: * The wild-card characters `%' and `_' can be used in the `Host' field of either table. * A `'%'' or blank `Host' value in either table means "any host." * The `Db', `Table_name' and `Column_name' fields cannot contain wild cards or be blank in either table. The `tables_priv' and `columns_priv' tables are sorted on the `Host', `Db', and `User' fields. This is similar to `db' table sorting, although the sorting is simpler because only the `Host' field may contain wild cards. The request verification process is described below. (If you are familiar with the access-checking source code, you will notice that the description here differs slightly from the algorithm used in the code. The description is equivalent to what the code actually does; it differs only to make the explanation simpler.) For administrative requests (*shutdown*, *reload*, etc.), the server checks only the `user' table entry, because that is the only table that specifies administrative privileges. Access is granted if the entry allows the requested operation and denied otherwise. For example, if you want to execute `mysqladmin shutdown' but your `user' table entry doesn't grant the *shutdown* privilege to you, access is denied without even checking the `db' or `host' tables. (They contain no `Shutdown_priv' column, so there is no need to do so.) For database-related requests (*insert*, *update*, etc.), the server first checks the user's global (superuser) privileges by looking in the `user' table entry. If the entry allows the requested operation, access is granted. If the global privileges in the `user' table are insufficient, the server determines the user's database-specific privileges by checking the `db' and `host' tables: 1. The server looks in the `db' table for a match on the `Host', `Db', and `User' fields. The `Host' and `User' fields are matched to the connecting user's hostname and MySQL user name. The `Db' field is matched to the database the user wants to access. If there is no entry for the `Host' and `User', access is denied. 2. If there is a matching `db' table entry and its `Host' field is not blank, that entry defines the user's database-specific privileges. 3. If the matching `db' table entry's `Host' field is blank, it signifies that the `host' table enumerates which hosts should be allowed access to the database. In this case, a further lookup is done in the `host' table to find a match on the `Host' and `Db' fields. If no `host' table entry matches, access is denied. If there is a match, the user's database-specific privileges are computed as the intersection (_not_ the union!) of the privileges in the `db' and `host' table entries, that is, the privileges that are `'Y'' in both entries. (This way you can grant general privileges in the `db' table entry and then selectively restrict them on a host-by-host basis using the `host' table entries.) After determining the database-specific privileges granted by the `db' and `host' table entries, the server adds them to the global privileges granted by the `user' table. If the result allows the requested operation, access is granted. Otherwise, the server checks the user's table and column privileges in the `tables_priv' and `columns_priv' tables and adds those to the user's privileges. Access is allowed or denied based on the result. Expressed in boolean terms, the preceding description of how a user's privileges are calculated may be summarized like this: global privileges OR (database privileges AND host privileges) OR table privileges OR column privileges It may not be apparent why, if the global `user' entry privileges are initially found to be insufficient for the requested operation, the server adds those privileges to the database-, table-, and column-specific privileges later. The reason is that a request might require more than one type of privilege. For example, if you execute an `INSERT ... SELECT' statement, you need both *insert* and *select* privileges. Your privileges might be such that the `user' table entry grants one privilege and the `db' table entry grants the other. In this case, you have the necessary privileges to perform the request, but the server cannot tell that from either table by itself; the privileges granted by the entries in both tables must be combined. The `host' table can be used to maintain a list of secure servers. At TcX, the `host' table contains a list of all machines on the local network. These are granted all privileges. You can also use the `host' table to indicate hosts that are _not_ secure. Suppose you have a machine `public.your.domain' that is located in a public area that you do not consider secure. You can allow access to all hosts on your network except that machine by using `host' table entries like this: +--------------------+----+- | Host | Db | ... +--------------------+----+- | public.your.domain | % | ... (all privileges set to 'N') | %.your.domain | % | ... (all privileges set to 'Y') +--------------------+----+- Naturally, you should always test your entries in the grant tables (for example, using `mysqlaccess') to make sure your access privileges are actually set up the way you think they are. Causes of `Access denied' Errors -------------------------------- If you encounter `Access denied' errors when you try to connect to the MySQL server, the list below indicates some courses of action you can take to correct the problem: * After installing MySQL, did you run the `mysql_install_db' script to set up the initial grant table contents? If not, do so. *Note Default privileges::. Test the initial privileges by executing this command: shell> mysql -u root test The server should let you connect without error. You should also make sure you have a file `user.MYD' in the MySQL database directory. Ordinarily, this is `PATH/var/mysql/user.MYD', where `PATH' is the pathname to the MySQL installation root. * After a fresh installation, you should connect to the server and set up your users and their access permissions: shell> mysql -u root mysql The server should let you connect because the MySQL `root' user has no password initially. That is also a security risk, so setting the `root' password is something you should do while you're setting up your other MySQL users. If you try to connect as `root' and get this error: Access denied for user: '@unknown' to database mysql this means that you don't have an entry in the `user' table with a `User' column value of `'root'' and that `mysqld' cannot resolve the hostname for your client. In this case, you must restart the server with the `--skip-grant-tables' option and edit your `/etc/hosts' or `\windows\hosts' file to add an entry for your host. * If you get an error like the following: shell> mysqladmin -u root -pxxxx ver Access denied for user: 'root@localhost' (Using password: YES) It means that you are using a wrong password. *Note Passwords::. If you have forgot the root password, you can restart `mysqld' with `--skip-grant-tables' to change the password. You can find more about this option later on in this manual section. If you get the above error even if you haven't specified a password, this means that you a wrong password in some `my.ini' file. *Note Option files::. You can avoid using option files with the `--no-defaults' option, as follows: shell> mysqladmin --no-defaults -u root ver * If you updated an existing MySQL installation from a version earlier than Version 3.22.11 to Version 3.22.11 or later, did you run the `mysql_fix_privilege_tables' script? If not, do so. The structure of the grant tables changed with MySQL Version 3.22.11 when the `GRANT' statement became functional. * If your privileges seem to have changed in the middle of a session, it may be that a superuser has changed them. Reloading the grant tables affects new client connections, but it also affects existing connections as indicated in *Note Privilege changes::. * If you can't get your password to work, remember that you must use the `PASSWORD()' function if you set the password with the `INSERT', `UPDATE', or `SET PASSWORD' statements. The `PASSWORD()' function is unnecessary if you specify the password using the `GRANT ... INDENTIFIED BY' statement or the `mysqladmin password' command. *Note Passwords::. * `localhost' is a synonym for your local hostname, and is also the default host to which clients try to connect if you specify no host explicitly. However, connections to `localhost' do not work if you are running on a system that uses MIT-pthreads (`localhost' connections are made using Unix sockets, which are not supported by MIT-pthreads). To avoid this problem on such systems, you should use the `--host' option to name the server host explicitly. This will make a TCP/IP connection to the `mysqld' server. In this case, you must have your real hostname in `user' table entries on the server host. (This is true even if you are running a client program on the same host as the server.) * If you get an `Access denied' error when trying to connect to the database with `mysql -u user_name db_name', you may have a problem with the `user' table. Check this by executing `mysql -u root mysql' and issuing this SQL statement: mysql> SELECT * FROM user; The result should include an entry with the `Host' and `User' columns matching your computer's hostname and your MySQL user name. * The `Access denied' error message will tell you who you are trying to log in as, the host from which you are trying to connect, and whether or not you were using a password. Normally, you should have one entry in the `user' table that exactly matches the hostname and user name that were given in the error message. For example if you get an error message that contains `Using password: NO', this means that you tried to login without an password. * If you get the following error when you try to connect from a different host than the one on which the MySQL server is running, then there is no row in the `user' table that matches that host: Host ... is not allowed to connect to this MySQL server You can fix this by using the command-line tool `mysql' (on the server host!) to add a row to the `user', `db', or `host' table for the user/hostname combination from which you are trying to connect and then execute `mysqladmin flush-privileges'. If you are not running MySQL Version 3.22 and you don't know the IP number or hostname of the machine from which you are connecting, you should put an entry with `'%'' as the `Host' column value in the `user' table and restart `mysqld' with the `--log' option on the server machine. After trying to connect from the client machine, the information in the MySQL log will indicate how you really did connect. (Then replace the `'%'' in the `user' table entry with the actual hostname that shows up in the log. Otherwise, you'll have a system that is insecure.) Another reason for this error on Linux is that you are using a binary MySQL version that is compiled with a different glibc version than the one you are using. In this case you should either upgrade your OS/glibc or download the source MySQL version and compile this yourself. A source RPM is normally trivial to compile and install, so this isn't a big problem. * If you get an error message where the hostname is not shown or where the hostname is an IP, even if you try to connect with a hostname: shell> mysqladmin -u root -pxxxx -h some-hostname ver Access denied for user: 'root('Using password: YES) This means that MySQL got some error when trying to resolve the IP to a hostname. In this case you can execute `mysqladmin flush-hosts' to reset the internal DNS cache. *Note DNS::. Some permanent solutions are: - Try to find out what is wrong with your DNS server and fix this. - Specify IPs instead of hostnames in the MySQL privilege tables. - Start `mysqld' with `--skip-name-resolve'. - Start `mysqld' with `--skip-host-cache'. - Connect to `localhost' if you are running the server and the client on the same machine. - Put the client machine names in `/etc/hosts'. * If `mysql -u root test' works but `mysql -h your_hostname -u root test' results in `Access denied', then you may not have the correct name for your host in the `user' table. A common problem here is that the `Host' value in the user table entry specifies an unqualified hostname, but your system's name resolution routines return a fully qualified domain name (or vice-versa). For example, if you have an entry with host `'tcx'' in the `user' table, but your DNS tells MySQL that your hostname is `'tcx.subnet.se'', the entry will not work. Try adding an entry to the `user' table that contains the IP number of your host as the `Host' column value. (Alternatively, you could add an entry to the `user' table with a `Host' value that contains a wild card--for example, `'tcx.%''. However, use of hostnames ending with `%' is _insecure_ and is _not_ recommended!) * If `mysql -u user_name test' works but `mysql -u user_name other_db_name' doesn't work, you don't have an entry for `other_db_name' listed in the `db' table. * If `mysql -u user_name db_name' works when executed on the server machine, but `mysql -u host_name -u user_name db_name' doesn't work when executed on another client machine, you don't have the client machine listed in the `user' table or the `db' table. * If you can't figure out why you get `Access denied', remove from the `user' table all entries that have `Host' values containing wild cards (entries that contain `%' or `_'). A very common error is to insert a new entry with `Host'=`'%'' and `User'=`'some user'', thinking that this will allow you to specify `localhost' to connect from the same machine. The reason that this doesn't work is that the default privileges include an entry with `Host'=`'localhost'' and `User'=`'''. Because that entry has a `Host' value `'localhost'' that is more specific than `'%'', it is used in preference to the new entry when connecting from `localhost'! The correct procedure is to insert a second entry with `Host'=`'localhost'' and `User'=`'some_user'', or to remove the entry with `Host'=`'localhost'' and `User'=`'''. * If you get the following error, you may have a problem with the `db' or `host' table: Access to database denied If the entry selected from the `db' table has an empty value in the `Host' column, make sure there are one or more corresponding entries in the `host' table specifying which hosts the `db' table entry applies to. If you get the error when using the SQL commands `SELECT ... INTO OUTFILE' or `LOAD DATA INFILE', your entry in the `user' table probably doesn't have the *file* privilege enabled. * Remember that client programs will use connection parameters specified in configuration files or environment variables. *Note Environment variables::. If a client seems to be sending the wrong default connection parameters when you don't specify them on the command line, check your environment and the `.my.cnf' file in your home directory. You might also check the system-wide MySQL configuration files, though it is far less likely that client connection parameters will be specified there. *Note Option files::. If you get `Access denied' when you run a client without any options, make sure you haven't specified an old password in any of your option files! *Note Option files::. * If you make changes to the grant tables directly (using an `INSERT' or `UPDATE' statement) and your changes seem to be ignored, remember that you must issue a `FLUSH PRIVILEGES' statement or execute a `mysqladmin flush-privileges' command to cause the server to re-read the privilege tables. Otherwise your changes have no effect until the next time the server is restarted. Remember that after you set the `root' password with an `UPDATE' command, you won't need to specify it until after you flush the privileges, because the server won't know you've changed the password yet! * If you have access problems with a Perl, PHP, Python, or ODBC program, try to connect to the server with `mysql -u user_name db_name' or `mysql -u user_name -pyour_pass db_name'. If you are able to connect using the `mysql' client, there is a problem with your program and not with the access privileges. (Note that there is no space between `-p' and the password; you can also use the `--password=your_pass' syntax to specify the password. If you use the `-p' option alone, MySQL will prompt you for the password.) * For testing, start the `mysqld' daemon with the `--skip-grant-tables' option. Then you can change the MySQL grant tables and use the `mysqlaccess' script to check whether or not your modifications have the desired effect. When you are satisfied with your changes, execute `mysqladmin flush-privileges' to tell the `mysqld' server to start using the new grant tables. *Note:* Reloading the grant tables overrides the `--skip-grant-tables' option. This allows you to tell the server to begin using the grant tables again without bringing it down and restarting it. * If everything else fails, start the `mysqld' daemon with a debugging option (for example, `--debug=d,general,query'). This will print host and user information about attempted connections, as well as information about each command issued. *Note Making trace files::. * If you have any other problems with the MySQL grant tables and feel you must post the problem to the mailing list, always provide a dump of the MySQL grant tables. You can dump the tables with the `mysqldump mysql' command. As always, post your problem using the `mysqlbug' script. *Note Bug reports::. In some cases you may need to restart `mysqld' with `--skip-grant-tables' to run `mysqldump'. MySQL User Account Management ============================= `GRANT' and `REVOKE' Syntax --------------------------- GRANT priv_type [(column_list)] [, priv_type [(column_list)] ...] ON {tbl_name | * | *.* | db_name.*} TO user_name [IDENTIFIED BY 'password'] [, user_name [IDENTIFIED BY 'password'] ...] [WITH GRANT OPTION] REVOKE priv_type [(column_list)] [, priv_type [(column_list)] ...] ON {tbl_name | * | *.* | db_name.*} FROM user_name [, user_name ...] `GRANT' is implemented in MySQL Version 3.22.11 or later. For earlier MySQL versions, the `GRANT' statement does nothing. The `GRANT' and `REVOKE' commands allow system administrators to create users and grant and revoke rights to MySQL users at four privilege levels: *Global level* Global privileges apply to all databases on a given server. These privileges are stored in the `mysql.user' table. *Database level* Database privileges apply to all tables in a given database. These privileges are stored in the `mysql.db' and `mysql.host' tables. *Table level* Table privileges apply to all columns in a given table. These privileges are stored in the `mysql.tables_priv' table. *Column level* Column privileges apply to single columns in a given table. These privileges are stored in the `mysql.columns_priv' table. If you give a grant for a users that doesn't exists, that user is created. For examples of how `GRANT' works, see *Note Adding users::. For the `GRANT' and `REVOKE' statements, `priv_type' may be specified as any of the following: ALL PRIVILEGES FILE RELOAD ALTER INDEX SELECT CREATE INSERT SHUTDOWN DELETE PROCESS UPDATE DROP REFERENCES USAGE `ALL' is a synonym for `ALL PRIVILEGES'. `REFERENCES' is not yet implemented. `USAGE' is currently a synonym for "no privileges." It can be used when you want to create a user that has no privileges. To revoke the *grant* privilege from a user, use a `priv_type' value of `GRANT OPTION': REVOKE GRANT OPTION ON ... FROM ...; The only `priv_type' values you can specify for a table are `SELECT', `INSERT', `UPDATE', `DELETE', `CREATE', `DROP', `GRANT', `INDEX', and `ALTER'. The only `priv_type' values you can specify for a column (that is, when you use a `column_list' clause) are `SELECT', `INSERT', and `UPDATE'. You can set global privileges by using `ON *.*' syntax. You can set database privileges by using `ON db_name.*' syntax. If you specify `ON *' and you have a current database, you will set the privileges for that database. (*WARNING:* If you specify `ON *' and you _don't_ have a current database, you will affect the global privileges!) In order to accommodate granting rights to users from arbitrary hosts, MySQL supports specifying the `user_name' value in the form `user@host'. If you want to specify a `user' string containing special characters (such as `-'), or a `host' string containing special characters or wild-card characters (such as `%'), you can quote the user or host name (for example, `'test-user'@'test-hostname''). You can specify wild cards in the hostname. For example, `user@"%.loc.gov"' applies to `user' for any host in the `loc.gov' domain, and `user@"144.155.166.%"' applies to `user' for any host in the `144.155.166' class C subnet. The simple form `user' is a synonym for `user@"%"'. *NOTE:* If you allow anonymous users to connect to the MySQL server (which is the default), you should also add all local users as `user@localhost' because otherwise the anonymous user entry for the local host in the `mysql.user' table will be used when the user tries to log into the MySQL server from the local machine! Anonymous users are defined by inserting entries with `User=''' into the `mysql.user' table. You can verify if this applies to you by executing this query: mysql> SELECT Host,User FROM mysql.user WHERE User=''; For the moment, `GRANT' only supports host, table, database, and column names up to 60 characters long. A user name can be up to 16 characters. The privileges for a table or column are formed from the logical OR of the privileges at each of the four privilege levels. For example, if the `mysql.user' table specifies that a user has a global *select* privilege, this can't be denied by an entry at the database, table, or column level. The privileges for a column can be calculated as follows: global privileges OR (database privileges AND host privileges) OR table privileges OR column privileges In most cases, you grant rights to a user at only one of the privilege levels, so life isn't normally as complicated as above. The details of the privilege-checking procedure are presented in *Note Privilege system::. If you grant privileges for a user/hostname combination that does not exist in the `mysql.user' table, an entry is added and remains there until deleted with a `DELETE' command. In other words, `GRANT' may create `user' table entries, but `REVOKE' will not remove them; you must do that explicitly using `DELETE'. In MySQL Version 3.22.12 or later, if a new user is created or if you have global grant privileges, the user's password will be set to the password specified by the `IDENTIFIED BY' clause, if one is given. If the user already had a password, it is replaced by the new one. *WARNING:* If you create a new user but do not specify an `IDENTIFIED BY' clause, the user has no password. This is insecure. Passwords can also be set with the `SET PASSWORD' command. *Note `SET OPTION': SET OPTION. If you grant privileges for a database, an entry in the `mysql.db' table is created if needed. When all privileges for the database have been removed with `REVOKE', this entry is deleted. If a user doesn't have any privileges on a table, the table is not displayed when the user requests a list of tables (for example, with a `SHOW TABLES' statement). The `WITH GRANT OPTION' clause gives the user the ability to give to other users any privileges the user has at the specified privilege level. You should be careful to whom you give the *grant* privilege, as two users with different privileges may be able to join privileges! You cannot grant another user a privilege you don't have yourself; the *grant* privilege allows you to give away only those privileges you possess. Be aware that when you grant a user the *grant* privilege at a particular privilege level, any privileges the user already possesses (or is given in the future!) at that level are also grantable by that user. Suppose you grant a user the *insert* privilege on a database. If you then grant the *select* privilege on the database and specify `WITH GRANT OPTION', the user can give away not only the *select* privilege, but also *insert*. If you then grant the *update* privilege to the user on the database, the user can give away the *insert*, *select* and *update*. You should not grant *alter* privileges to a normal user. If you do that, the user can try to subvert the privilege system by renaming tables! Note that if you are using table or column privileges for even one user, the server examines table and column privileges for all users and this will slow down MySQL a bit. When `mysqld' starts, all privileges are read into memory. Database, table, and column privileges take effect at once, and user-level privileges take effect the next time the user connects. Modifications to the grant tables that you perform using `GRANT' or `REVOKE' are noticed by the server immediately. If you modify the grant tables manually (using `INSERT', `UPDATE', etc.), you should execute a `FLUSH PRIVILEGES' statement or run `mysqladmin flush-privileges' to tell the server to reload the grant tables. *Note Privilege changes::. The biggest differences between the ANSI SQL and MySQL versions of `GRANT' are: * In MySQL privileges are given for an username + hostname combination and not only for an username. * ANSI SQL doesn't have global or database-level privileges, and ANSI SQL doesn't support all privilege types that MySQL supports. MySQL doesn't support the ANSI SQL `TRIGGER', `EXECUTE' or `UNDER' privileges. * ANSI SQL privileges are structured in a hierarchal manner. If you remove an user, all privileges the user has granted are revoked. In MySQL the granted privileges are not automatically revoked, but you have to revoke these yourself if needed. * If you in MySQL have the `INSERT' grant on only part of the columns in a table, you can execute `INSERT' statements on the table; The columns for which you don't have the `INSERT' privilege will set to their default values. ANSI SQL requires you to have the `INSERT' privilege on all columns. * When you drop a table in ANSI SQL, all privileges for the table are revoked. If you revoke a privilege in ANSI SQL, all privileges that were granted based on this privilege are also revoked. In MySQL, privileges can be dropped only with explicit `REVOKE' commands or by manipulating the MySQL grant tables. MySQL User Names and Passwords ------------------------------ There are several distinctions between the way user names and passwords are used by MySQL and the way they are used by Unix or Windows: * User names, as used by MySQL for authentication purposes, have nothing to do with Unix user names (login names) or Windows user names. Most MySQL clients by default try to log in using the current Unix user name as the MySQL user name, but that is for convenience only. Client programs allow a different name to be specified with the `-u' or `--user' options. This means that you can't make a database secure in any way unless all MySQL user names have passwords. Anyone may attempt to connect to the server using any name, and they will succeed if they specify any name that doesn't have a password. * MySQL user names can be up to 16 characters long; Unix user names typically are limited to 8 characters. * MySQL passwords have nothing to do with Unix passwords. There is no necessary connection between the password you use to log in to a Unix machine and the password you use to access a database on that machine. * MySQL encrypts passwords using a different algorithm than the one used during the Unix login process. See the descriptions of the `PASSWORD()' and `ENCRYPT()' functions in *Note Miscellaneous functions::. Note that even if the password is stored 'scrambled', and knowing your 'scrambled' password is enough to be able to connect to the MySQL server! MySQL users and their privileges are normally created with the `GRANT' command. *Note GRANT::. When you login to a MySQL server with a command line client you should specify the password with `--password=your-password'. *Note Connecting::. mysql --user=monty --password=guess database_name If you want the client to prompt for a password, you should use `--password' without any argument mysql --user=monty --password database_name or the short form: mysql -u monty -p database_name Note that in the last example the password is *NOT* 'database_name'. If you want to use the `-p' option to supply a password you should do so like this: mysql -u monty -pguess database_name On some systems, the library call that MySQL uses to prompt for a password will automatically cut the password to 8 characters. Internally MySQL doesn't have any limit for the length of the password. When Privilege Changes Take Effect ---------------------------------- When `mysqld' starts, all grant table contents are read into memory and become effective at that point. Modifications to the grant tables that you perform using `GRANT', `REVOKE', or `SET PASSWORD' are noticed by the server immediately. If you modify the grant tables manually (using `INSERT', `UPDATE', etc.), you should execute a `FLUSH PRIVILEGES' statement or run `mysqladmin flush-privileges' or `mysqladmin reload' to tell the server to reload the grant tables. Otherwise your changes will have _no effect_ until you restart the server. If you change the grant tables manually but forget to reload the privileges, you will be wondering why your changes don't seem to make any difference! When the server notices that the grant tables have been changed, existing client connections are affected as follows: * Table and column privilege changes take effect with the client's next request. * Database privilege changes take effect at the next `USE db_name' command. Global privilege changes and password changes take effect the next time the client connects. Setting Up the Initial MySQL Privileges --------------------------------------- After installing MySQL, you set up the initial access privileges by running `scripts/mysql_install_db'. *Note Quick install::. The `mysql_install_db' script starts up the `mysqld' server, then initializes the grant tables to contain the following set of privileges: * The MySQL `root' user is created as a superuser who can do anything. Connections must be made from the local host. *NOTE:* The initial `root' password is empty, so anyone can connect as `root' _without a password_ and be granted all privileges. * An anonymous user is created that can do anything with databases that have a name of `'test'' or starting with `'test_''. Connections must be made from the local host. This means any local user can connect without a password and be treated as the anonymous user. * Other privileges are denied. For example, normal users can't use `mysqladmin shutdown' or `mysqladmin processlist'. *NOTE:* The default privileges are different for Windows. *Note Windows running::. Because your installation is initially wide open, one of the first things you should do is specify a password for the MySQL `root' user. You can do this as follows (note that you specify the password using the `PASSWORD()' function): shell> mysql -u root mysql mysql> UPDATE user SET Password=PASSWORD('new_password') WHERE user='root'; mysql> FLUSH PRIVILEGES; You can, in MySQL Version 3.22 and above, use the `SET PASSWORD' statement: shell> mysql -u root mysql mysql> SET PASSWORD FOR root=PASSWORD('new_password'); Another way to set the password is by using the `mysqladmin' command: shell> mysqladmin -u root password new_password Only users with write/update access to the `mysql' database can change the password for others users. All normal users (not anonymous ones) can only change their own password with either of the above commands or with `SET PASSWORD=PASSWORD('new password')'. Note that if you update the password in the `user' table directly using the first method, you must tell the server to re-read the grant tables (with `FLUSH PRIVILEGES'), because the change will go unnoticed otherwise. Once the `root' password has been set, thereafter you must supply that password when you connect to the server as `root'. You may wish to leave the `root' password blank so that you don't need to specify it while you perform additional setup or testing. However, be sure to set it before using your installation for any real production work. See the `scripts/mysql_install_db' script to see how it sets up the default privileges. You can use this as a basis to see how to add other users. If you want the initial privileges to be different than those just described above, you can modify `mysql_install_db' before you run it. To re-create the grant tables completely, remove all the `.frm', `.MYI', and `.MYD' files in the directory containing the `mysql' database. (This is the directory named `mysql' under the database directory, which is listed when you run `mysqld --help'.) Then run the `mysql_install_db' script, possibly after editing it first to have the privileges you want. *NOTE:* For MySQL versions older than Version 3.22.10, you should NOT delete the `.frm' files. If you accidentally do this, you should copy them back from your MySQL distribution before running `mysql_install_db'. Adding New Users to MySQL ------------------------- You can add users two different ways: by using `GRANT' statements or by manipulating the MySQL grant tables directly. The preferred method is to use `GRANT' statements, because they are more concise and less error-prone. *Note GRANT::. There is also a lot of contributed programs like `phpmyadmin' that can be used to create and administrate users. *Note Contrib::. The examples below show how to use the `mysql' client to set up new users. These examples assume that privileges are set up according to the defaults described in the previous section. This means that to make changes, you must be on the same machine where `mysqld' is running, you must connect as the MySQL `root' user, and the `root' user must have the *insert* privilege for the `mysql' database and the *reload* administrative privilege. Also, if you have changed the `root' user password, you must specify it for the `mysql' commands below. You can add new users by issuing `GRANT' statements: shell> mysql --user=root mysql mysql> GRANT ALL PRIVILEGES ON *.* TO monty@localhost IDENTIFIED BY 'some_pass' WITH GRANT OPTION; mysql> GRANT ALL PRIVILEGES ON *.* TO monty@"%" IDENTIFIED BY 'some_pass' WITH GRANT OPTION; mysql> GRANT RELOAD,PROCESS ON *.* TO admin@localhost; mysql> GRANT USAGE ON *.* TO dummy@localhost; These `GRANT' statements set up three new users: `monty' A full superuser who can connect to the server from anywhere, but who must use a password `'some_pass'' to do so. Note that we must issue `GRANT' statements for both `monty@localhost' and `monty@"%"'. If we don't add the entry with `localhost', the anonymous user entry for `localhost' that is created by `mysql_install_db' will take precedence when we connect from the local host, because it has a more specific `Host' field value and thus comes earlier in the `user' table sort order. `admin' A user who can connect from `localhost' without a password and who is granted the *reload* and *process* administrative privileges. This allows the user to execute the `mysqladmin reload', `mysqladmin refresh', and `mysqladmin flush-*' commands, as well as `mysqladmin processlist' . No database-related privileges are granted. (They can be granted later by issuing additional `GRANT' statements.) `dummy' A user who can connect without a password, but only from the local host. The global privileges are all set to `'N'' -- the `USAGE' privilege type allows you to create a user with no privileges. It is assumed that you will grant database-specific privileges later. You can also add the same user access information directly by issuing `INSERT' statements and then telling the server to reload the grant tables: shell> mysql --user=root mysql mysql> INSERT INTO user VALUES('localhost','monty',PASSWORD('some_pass'), 'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y'); mysql> INSERT INTO user VALUES('%','monty',PASSWORD('some_pass'), 'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y'); mysql> INSERT INTO user SET Host='localhost',User='admin', Reload_priv='Y', Process_priv='Y'; mysql> INSERT INTO user (Host,User,Password) VALUES('localhost','dummy',''); mysql> FLUSH PRIVILEGES; Depending on your MySQL version, you may have to use a different number of `'Y'' values above (versions prior to Version 3.22.11 had fewer privilege columns). For the `admin' user, the more readable extended `INSERT' syntax that is available starting with Version 3.22.11 is used. Note that to set up a superuser, you need only create a `user' table entry with the privilege fields set to `'Y''. No `db' or `host' table entries are necessary. The privilege columns in the `user' table were not set explicitly in the last `INSERT' statement (for the `dummy' user), so those columns are assigned the default value of `'N''. This is the same thing that `GRANT USAGE' does. The following example adds a user `custom' who can connect from hosts `localhost', `server.domain', and `whitehouse.gov'. He wants to access the `bankaccount' database only from `localhost', the `expenses' database only from `whitehouse.gov', and the `customer' database from all three hosts. He wants to use the password `stupid' from all three hosts. To set up this user's privileges using `GRANT' statements, run these commands: shell> mysql --user=root mysql mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP ON bankaccount.* TO custom@localhost IDENTIFIED BY 'stupid'; mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP ON expenses.* TO custom@whitehouse.gov IDENTIFIED BY 'stupid'; mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP ON customer.* TO custom@'%' IDENTIFIED BY 'stupid'; The reason that we do to grant statements for the user 'custom' is that we want the give the user access to MySQL both from the local machine with Unix sockets and from the remote machine 'whitehouse.gov' over TCP/IP. To set up the user's privileges by modifying the grant tables directly, run these commands (note the `FLUSH PRIVILEGES' at the end): shell> mysql --user=root mysql mysql> INSERT INTO user (Host,User,Password) VALUES('localhost','custom',PASSWORD('stupid')); mysql> INSERT INTO user (Host,User,Password) VALUES('server.domain','custom',PASSWORD('stupid')); mysql> INSERT INTO user (Host,User,Password) VALUES('whitehouse.gov','custom',PASSWORD('stupid')); mysql> INSERT INTO db (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv, Create_priv,Drop_priv) VALUES ('localhost','bankaccount','custom','Y','Y','Y','Y','Y','Y'); mysql> INSERT INTO db (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv, Create_priv,Drop_priv) VALUES ('whitehouse.gov','expenses','custom','Y','Y','Y','Y','Y','Y'); mysql> INSERT INTO db (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv, Create_priv,Drop_priv) VALUES('%','customer','custom','Y','Y','Y','Y','Y','Y'); mysql> FLUSH PRIVILEGES; The first three `INSERT' statements add `user' table entries that allow user `custom' to connect from the various hosts with the given password, but grant no permissions to him (all privileges are set to the default value of `'N''). The next three `INSERT' statements add `db' table entries that grant privileges to `custom' for the `bankaccount', `expenses', and `customer' databases, but only when accessed from the proper hosts. As usual, when the grant tables are modified directly, the server must be told to reload them (with `FLUSH PRIVILEGES') so that the privilege changes take effect. If you want to give a specific user access from any machine in a given domain, you can issue a `GRANT' statement like the following: mysql> GRANT ... ON *.* TO myusername@"%.mydomainname.com" IDENTIFIED BY 'mypassword'; To do the same thing by modifying the grant tables directly, do this: mysql> INSERT INTO user VALUES ('%.mydomainname.com', 'myusername', PASSWORD('mypassword'),...); mysql> FLUSH PRIVILEGES; You can also use `xmysqladmin', `mysql_webadmin', and even `xmysql' to insert, change, and update values in the grant tables. You can find these utilities in the Contrib directory of the MySQL Website (http://www.mysql.com/Downloads/Contrib/). Setting Up Passwords -------------------- In most cases you should use `GRANT' to set up your users/passwords, so the following only applies for advanced users. *Note `GRANT': GRANT. The examples in the preceding sections illustrate an important principle: when you store a non-empty password using `INSERT' or `UPDATE' statements, you must use the `PASSWORD()' function to encrypt it. This is because the `user' table stores passwords in encrypted form, not as plaintext. If you forget that fact, you are likely to attempt to set passwords like this: shell> mysql -u root mysql mysql> INSERT INTO user (Host,User,Password) VALUES('%','jeffrey','biscuit'); mysql> FLUSH PRIVILEGES; The result is that the plaintext value `'biscuit'' is stored as the password in the `user' table. When the user `jeffrey' attempts to connect to the server using this password, the `mysql' client encrypts it with `PASSWORD()', generates an authentification vector based on *encrypted* password and a random number, obtained from server, and sends the result to the server. The server uses the `password' value in the `user' table (that is *not encrypted* value `'biscuit'') to perform the same calculations, and compares results. The comparison fails and the server rejects the connection: shell> mysql -u jeffrey -pbiscuit test Access denied Passwords must be encrypted when they are inserted in the `user' table, so the `INSERT' statement should have been specified like this instead: mysql> INSERT INTO user (Host,User,Password) VALUES('%','jeffrey',PASSWORD('biscuit')); You must also use the `PASSWORD()' function when you use `SET PASSWORD' statements: mysql> SET PASSWORD FOR jeffrey@"%" = PASSWORD('biscuit'); If you set passwords using the `GRANT ... IDENTIFIED BY' statement or the `mysqladmin password' command, the `PASSWORD()' function is unnecessary. They both take care of encrypting the password for you, so you would specify a password of `'biscuit'' like this: mysql> GRANT USAGE ON *.* TO jeffrey@"%" IDENTIFIED BY 'biscuit'; or shell> mysqladmin -u jeffrey password biscuit *NOTE:* `PASSWORD()' does not perform password encryption in the same way that Unix passwords are encrypted. You should not assume that if your Unix password and your MySQL password are the same, that `PASSWORD()' will result in the same encrypted value as is stored in the Unix password file. *Note User names::. Keeping Your Password Secure ---------------------------- It is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed below, along with an assessment of the risks of each method: * Never give a normal user access to the `mysql.user' table. Knowing the encrypted password for a user makes it possible to login as this user. The passwords are only scrambled so that one shouldn't be able to see the real password you used (if you happen to use a similar password with your other applications). * Use a `-pyour_pass' or `--password=your_pass' option on the command line. This is convenient but insecure, because your password becomes visible to system status programs (such as `ps') that may be invoked by other users to display command lines. (MySQL clients typically overwrite the command-line argument with zeroes during their initialization sequence, but there is still a brief interval during which the value is visible.) * Use a `-p' or `--password' option (with no `your_pass' value specified). In this case, the client program solicits the password from the terminal: shell> mysql -u user_name -p Enter password: ******** The `*' characters represent your password. It is more secure to enter your password this way than to specify it on the command line because it is not visible to other users. However, this method of entering a password is suitable only for programs that you run interactively. If you want to invoke a client from a script that runs non-interactively, there is no opportunity to enter the password from the terminal. On some systems, you may even find that the first line of your script is read and interpreted (incorrectly) as your password! * Store your password in a configuration file. For example, you can list your password in the `[client]' section of the `.my.cnf' file in your home directory: [client] password=your_pass If you store your password in `.my.cnf', the file should not be group or world readable or writable. Make sure the file's access mode is `400' or `600'. *Note Option files::. * You can store your password in the `MYSQL_PWD' environment variable, but this method must be considered extremely insecure and should not be used. Some versions of `ps' include an option to display the environment of running processes; your password will be in plain sight for all to see if you set `MYSQL_PWD'. Even on systems without such a version of `ps', it is unwise to assume there is no other method to observe process environments. *Note Environment variables::. All in all, the safest methods are to have the client program prompt for the password or to specify the password in a properly protected `.my.cnf' file. Disaster Prevention and Recovery ================================ Database Backups ---------------- Because MySQL tables are stored as files, it is easy to do a backup. To get a consistent backup, do a `LOCK TABLES' on the relevant tables followed by `FLUSH TABLES' for the tables. *Note `LOCK TABLES': LOCK TABLES. *Note `FLUSH': FLUSH. You only need a read lock; this allows other threads to continue to query the tables while you are making a copy of the files in the database directory. The `FLUSH TABLE' is needed to ensure that the all active index pages is written to disk before you start the backup. If you want to make a SQL level backup of a table, you can use `SELECT INTO OUTFILE' or `BACKUP TABLE'. *Note SELECT::. *Note BACKUP TABLE::. Another way to back up a database is to use the `mysqldump' program or the `mysqlhotcopy script'. *Note `mysqldump': mysqldump. *Note `mysqlhotcopy': mysqlhotcopy. 1. Do a full backup of your databases: shell> mysqldump --tab=/path/to/some/dir --opt --full or shell> mysqlhotcopy database /path/to/some/dir You can also simply copy all table files (`*.frm', `*.MYD', and `*.MYI' files) as long as the server isn't updating anything. The script `mysqlhotcopy' does use this method. 2. Stop `mysqld' if it's running, then start it with the `--log-update[=file_name]' option. *Note Update log::. The update log file(s) provide you with the information you need to replicate changes to the database that are made subsequent to the point at which you executed `mysqldump'. If you have to restore something, try to recover your tables using `REPAIR TABLE' or `myisamchk -r' first. That should work in 99.9% of all cases. If `myisamchk' fails, try the following procedure: (This will only work if you have started MySQL with `--log-update'. *Note Update log::.): 1. Restore the original `mysqldump' backup. 2. Execute the following command to re-run the updates in the binary log: shell> mysqlbinlog hostname-bin.[0-9]* | mysql If you are using the update log you can use: shell> ls -1 -t -r hostname.[0-9]* | xargs cat | mysql `ls' is used to get all the update log files in the right order. You can also do selective backups with `SELECT * INTO OUTFILE 'file_name' FROM tbl_name' and restore with `LOAD DATA INFILE 'file_name' REPLACE ...' To avoid duplicate records, you need a `PRIMARY KEY' or a `UNIQUE' key in the table. The `REPLACE' keyword causes old records to be replaced with new ones when a new record duplicates an old record on a unique key value. If you get performance problems in making backups on your system, you can solve this by setting up replication and do the backups on the slave instead of on the master. *Note Replication Intro::. If you are using a Veritas file system, you can do: 1. Execute in a client (perl ?) `FLUSH TABLES WITH READ LOCK' 2. Fork a shell or execute in another client `mount vxfs snapshot'. 3. Execute in the first client `UNLOCK TABLES' 4. Copy files from snapshot 5. Unmount snapshot `BACKUP TABLE' Syntax --------------------- BACKUP TABLE tbl_name[,tbl_name...] TO '/path/to/backup/directory' Make a copy of all the table files to the backup directory that are the minimum needed to restore it. Currenlty only works for `MyISAM' tables. For `MyISAM' table, copies `.frm' (definition) and `.MYD' (data) files. The index file can be rebuilt from those two. Before using this command, please see *Note Backup::. During the backup, read lock will be held for each table, one at time, as they are being backed up. If you want to backup several tables as a snapshot, you must first issue `LOCK TABLES' obtaining a read lock for each table in the group. The command returns a table with the following columns: *Column* *Value* Table Table name Op Always "backup" Msg_type One of `status', `error', `info' or `warning'. Msg_text The message. Note that `BACKUP TABLE' is only available in MySQL version 3.23.25 and later. `RESTORE TABLE' Syntax ---------------------- RESTORE TABLE tbl_name[,tbl_name...] FROM '/path/to/backup/directory' Restores the table(s) from the backup that was made with `BACKUP TABLE'. Existing tables will not be overwritten - if you try to restore over an existing table, you will get an error. Restore will take longer than BACKUP due to the need to rebuilt the index. The more keys you have, the longer it is going to take. Just as `BACKUP TABLE', currently only works of `MyISAM' tables. The command returns a table with the following columns: *Column* *Value* Table Table name Op Always "restore" Msg_type One of `status', `error', `info' or `warning'. Msg_text The message. `CHECK TABLE' Syntax -------------------- CHECK TABLE tbl_name[,tbl_name...] [option [option...]] option = QUICK | FAST | MEDIUM | EXTENDED | CHANGED `CHECK TABLE' only works on `MyISAM' tables. On `MyISAM' tables it's the same thing as running `myisamchk -m table_name' on the table. If you don't specify any option `MEDIUM' is used. Checks the table(s) for errors. For `MyISAM' tables the key statistics is updated. The command returns a table with the following columns: *Column* *Value* Table Table name. Op Always "check". Msg_type One of `status', `error', `info', or `warning'. Msg_text The message. Note that you can get many rows of information for each checked table. The last row will be of `Msg_type status' and should normally be `OK'. If you don't get `OK', or `Not checked' you should normally run a repair of the table. *Note Table maintenance::. `Not checked' means that the table the given `TYPE' told MySQL that there wasn't any need to check the table. The different check types stand for the following: *Type* *Meaning* `QUICK' Don't scan the rows to check for wrong links. `FAST' Only check tables which haven't been closed properly. `CHANGED' Only check tables which have been changed since last check or haven't been closed properly. `MEDIUM' Scan rows to verify that deleted links are ok. This also calculates a key checksum for the rows and verifies this with a calcualted checksum for the keys. `EXTENDED' Do a full key lookup for all keys for each row. This ensures that the table is 100 % consistent, but will take a long time! For dynamic sized `MyISAM' tables a started check will always do a `MEDIUM' check. For static size rows we skip the row scan for `QUICK' and `FAST' as the rows are very seldom corrupted. You can combine check options as in: CHECK TABLE test_table FAST QUICK; Which only would do a quick check on the table if it wasn't closed properly. *NOTE:* that in some case `CHECK TABLE' will change the table! This happens if the table is marked as 'corrupted' or 'not closed properly' but `CHECK TABLE' didn't find any problems in the table. In this case `CHECK TABLE' will mark the table as ok. If a table is corrupted, then it's most likely that the problem is in the indexes and not in the data part. All of the above check types checks the indexes throughly and should thus find most errors. If you just want to check a table that you assume is ok, you should use no check options or the `QUICK' option. The later should be used when you are in a hurry and can take the very small risk that `QUICK' didn't find an error in the data file (In most cases MySQL should find, under normal usage, any error in the data file. If this happens then the table will be marked as 'corrupted', in which case the table can't be used until it's repaired). `FAST' and `CHANGED' are mostly intended to be used from a script (for example to be executed from cron) if you want to check your table from time to time. In most cases you `FAST' is to be prefered over `CHANGED'. (The only case when it isn't is when you suspect a bug you have found a bug in the `MyISAM' code.). `EXTENDED' is only to be used after you have run a normal check but still get strange errors from a table when MySQL tries to update a row or find a row by key (this is VERY unlikely to happen if a normal check has succeeded!). Some things reported by check table, can't be corrected automatically: * `Found row where the auto_increment column has the value 0'. This means that you have in the table a row where the `auto_increment' index column contains the value 0. (It's possible to create a row where the auto_increment column is 0 by explicitely setting the column to 0 with an `UPDATE' statement) This isn't an error in itself, but could cause trouble if you decide to dump the table and restore it or do an `ALTER TABLE' on the table. In this case the auto_increment column will change value, according to the rules of auto_increment columns, which could cause problems like a duplicate key error. To get rid of the warning, just execute an `UPDATE' statement to set the column to some other value than 0. `REPAIR TABLE' Syntax --------------------- REPAIR TABLE tbl_name[,tbl_name...] [QUICK] [EXTENDED] `REPAIR TABLE' only works on `MyISAM' tables and is the same as running `myisamchk -r table_name' on the table. Normally you should never have to run this command, but if disaster strikes you are very likely to get back all your data from a MyISAM table with `REPAIR TABLE'. If your tables get corrupted a lot you should try to find the reason for this! *Note Crashing::. *Note MyISAM table problems::. `REPAIR TABLE' repairs a possible corrupted table. The command returns a table with the following columns: *Column* *Value* Table Table name Op Always "repair" Msg_type One of `status', `error', `info' or `warning'. Msg_text The message. Note that you can get many rows of information for each repaired table. The last one row will be of `Msg_type status' and should normally be `OK'. If you don't get `OK', you should try repairing the table with `myisamchk -o', as `REPAIR TABLE' does not yet implement all the options of `myisamchk'. In the near future, we will make it more flexible. If `QUICK' is given then MySQL will try to do a `REPAIR' of only the index tree. If you use `EXTENDED' then MySQL will create the index row by row instead of creating one index at a time with sorting; This may be better than sorting on fixed-length keys if you have long `char()' keys that compress very good. Using `myisamchk' for Table Maintenance and Crash Recovery ---------------------------------------------------------- Starting with MySQL Version 3.23.13, you can check MyISAM tables with the `CHECK TABLE' command. *Note CHECK TABLE::. You can repair tables with the `REPAIR TABLE' command. *Note REPAIR TABLE::. To check/repair MyISAM tables (`.MYI' and `.MYD') you should use the `myisamchk' utility. To check/repair ISAM tables (`.ISM' and `.ISD') you should use the `isamchk' utility. *Note Table types::. In the following text we will talk about `myisamchk', but everything also applies to the old `isamchk'. You can use the `myisamchk' utility to get information about your database tables, check and repair them, or optimize them. The following sections describe how to invoke `myisamchk' (including a description of its options), how to set up a table maintenance schedule, and how to use `myisamchk' to perform its various functions. You can, in most cases, also use the command `OPTIMIZE TABLES' to optimize and repair tables, but this is not as fast or reliable (in case of real fatal errors) as `myisamchk'. On the other hand, `OPTIMIZE TABLE' is easier to use and you don't have to worry about flushing tables. *Note `OPTIMIZE TABLE': OPTIMIZE TABLE. Even that the repair in `myisamchk' is quite secure, it's always a good idea to make a backup BEFORE doing a repair (or anything that could make a lot of changes to a table) `myisamchk' Invocation Syntax ............................. `myisamchk' is invoked like this: shell> myisamchk [options] tbl_name The `options' specify what you want `myisamchk' to do. They are described below. (You can also get a list of options by invoking `myisamchk --help'.) With no options, `myisamchk' simply checks your table. To get more information or to tell `myisamchk' to take corrective action, specify options as described below and in the following sections. `tbl_name' is the database table you want to check/repair. If you run `myisamchk' somewhere other than in the database directory, you must specify the path to the file, because `myisamchk' has no idea where your database is located. Actually, `myisamchk' doesn't care whether or not the files you are working on are located in a database directory; you can copy the files that correspond to a database table into another location and perform recovery operations on them there. You can name several tables on the `myisamchk' command line if you wish. You can also specify a name as an index file name (with the `.MYI' suffix), which allows you to specify all tables in a directory by using the pattern `*.MYI'. For example, if you are in a database directory, you can check all the tables in the directory like this: shell> myisamchk *.MYI If you are not in the database directory, you can check all the tables there by specifying the path to the directory: shell> myisamchk /path/to/database_dir/*.MYI You can even check all tables in all databases by specifying a wild card with the path to the MySQL data directory: shell> myisamchk /path/to/datadir/*/*.MYI The recommended way to quickly check all tables is: myisamchk --silent --fast /path/to/datadir/*/*.MYI isamchk --silent /path/to/datadir/*/*.ISM If you want to check all tables and repair all tables that are corrupted, you can use the following line: myisamchk --silent --force --fast --update-state -O key_buffer=64M -O sort_buffer=64M -O read_buffer=1M -O write_buffer=1M /path/to/datadir/*/*.MYI isamchk --silent --force -O key_buffer=64M -O sort_buffer=64M -O read_buffer=1M -O write_buffer=1M /path/to/datadir/*/*.ISM The above assumes that you have more than 64 M free. Note that if you get an error like: myisamchk: warning: 1 clients is using or hasn't closed the table properly This means that you are trying to check a table that has been updated by the another program (like the `mysqld' server) that hasn't yet closed the file or that has died without closing the file properly. If you `mysqld' is running, you must force a sync/close of all tables with `FLUSH TABLES' and ensure that no one is using the tables while you are running `myisamchk'. In MySQL Version 3.23 the easiest way to avoid this problem is to use `CHECK TABLE' instead of `myisamchk' to check tables. General Options for `myisamchk' ............................... `myisamchk' supports the following options. `-# or --debug=debug_options' Output debug log. The `debug_options' string often is `'d:t:o,filename''. `-? or --help' Display a help message and exit. `-O var=option, --set-variable var=option' Set the value of a variable. The possible variables and their default values for myisamchk can be examined with `myisamchk --help': key_buffer_size 523264 read_buffer_size 262136 write_buffer_size 262136 sort_buffer_size 2097144 sort_key_blocks 16 decode_bits 9 `sort_buffer_size' is used when the keys are repaired by sorting keys, which is the normal case when you use `--recover'. `key_buffer_size' is used when you are checking the table with `--extended-check' or when the keys are repaired by inserting key row by row in to the table (like when doing normal inserts). Repairing through the key buffer is used in the following cases: * If you use `--safe-recover'. * If you are using a `FULLTEXT' index. * If the temporary files needed to sort the keys would be more than twice as big as when creating the key file directly. This is often the case when you have big `CHAR', `VARCHAR' or `TEXT' keys as the sort needs to store the whole keys during sorting. If you have lots of temporary space and you can force `myisamchk' to repair by sorting you can use the `--sort-recover' option. Reparing through the key buffer takes much less disk space than using sorting, but is also much slower. If you want a faster repair, set the above variables to about 1/4 of your available memory. You can set both variables to big values, as only one of the above buffers will be used at a time. `-s or --silent' Silent mode. Write output only when errors occur. You can use `-s' twice (`-ss') to make `myisamchk' very silent. `-v or --verbose' Verbose mode. Print more information. This can be used with `-d' and `-e'. Use `-v' multiple times (`-vv', `-vvv') for more verbosity! `-V or --version' Print the `myisamchk' version and exit. `-w or, --wait' Instead of giving an error if the table is locked, wait until the table is unlocked before continuing. Note that if you are running `mysqld' on the table with `--skip-locking', the table can only be locked by another `myisamchk' command. Check Options for `myisamchk' ............................. `-c or --check' Check table for errors. This is the default operation if you are not giving `myisamchk' any options that override this. `-e or --extend-check' Check the table VERY thoroughly (which is quite slow if you have many indexes). This option should only be used in extreme cases. Normally, `myisamchk' or `myisamchk --medium-check' should, in most cases, be able to find out if there are any errors in the table. If you are using `--extended-check' and have much memory, you should increase the value of `key_buffer_size' a lot! `-F or --fast' Check only tables that haven't been closed properly. `-C or --check-only-changed' Check only tables that have changed since the last check. `-f or --force' Restart `myisamchk' with `-r' (repair) on the table, if `myisamchk' finds any errors in the table. `-i or --information' Print informational statistics about the table that is checked. `-m or --medium-check' Faster than extended-check, but only finds 99.99% of all errors. Should, however, be good enough for most cases. `-U or --update-state' Store in the `.MYI' file when the table was checked and if the table crashed. This should be used to get full benefit of the `--check-only-changed' option, but you shouldn't use this option if the `mysqld' server is using the table and you are running `mysqld' with `--skip-locking'. `-T or --read-only' Don't mark table as checked. This is useful if you use `myisamchk' to check a table that is in use by some other application that doesn't use locking (like `mysqld --skip-locking'). Repair Options for myisamchk ............................ The following options are used if you start `myisamchk' with `-r' or `-o': `-D # or --data-file-length=#' Max length of data file (when re-creating data file when it's 'full'). `-e or --extend-check' Try to recover every possible row from the data file. Normally this will also find a lot of garbage rows. Don't use this option if you are not totally desperate. `-f or --force' Overwrite old temporary files (`table_name.TMD') instead of aborting. `-k # or keys-used=#' If you are using ISAM, tells the ISAM table handler to update only the first `#' indexes. If you are using `MyISAM', tells which keys to use, where each binary bit stands for one key (first key is bit 0). This can be used to get faster inserts! Deactivated indexes can be reactivated by using `myisamchk -r'. keys. `-l or --no-symlinks' Do not follow symbolic links. Normally `myisamchk' repairs the table a symlink points at. This option doesn't exist in MySQL 4.0, as MySQL 4.0 will not remove symlinks during repair. `-r or --recover' Can fix almost anything except unique keys that aren't unique (which is an extremely unlikely error with ISAM/MyISAM tables). If you want to recover a table, this is the option to try first. Only if myisamchk reports that the table can't be recovered by `-r', you should then try `-o'. (Note that in the unlikely case that `-r' fails, the data file is still intact.) If you have lots of memory, you should increase the size of `sort_buffer_size'! `-o or --safe-recover' Uses an old recovery method (reads through all rows in order and updates all index trees based on the found rows); this is a magnitude slower than `-r', but can handle a couple of very unlikely cases that `-r' cannot handle. This recovery method also uses much less disk space than `-r'. Normally one should always first repair with `-r', and only if this fails use `-o'. If you have lots of memory, you should increase the size of `key_buffer_size'! `-n or --sort-recover' Force `myisamchk' to use sorting to resolve the keys even if the temporary files should be very big. This will not have any effect if you have fulltext keys in the table. `--character-sets-dir=...' Directory where character sets are stored. `--set-character-set=name' Change the character set used by the index `.t or --tmpdir=path' Path for storing temporary files. If this is not set, `myisamchk' will use the environment variable `TMPDIR' for this. `-q or --quick' Faster repair by not modifying the data file. One can give a second `-q' to force `myisamchk' to modify the original datafile in case of duplicate keys `-u or --unpack' Unpack file packed with myisampack. Other Options for `myisamchk' ............................. Other actions that `myisamchk' can do, besides repair and check tables: `-a or --analyze' Analyze the distribution of keys. This improves join performance by enabling the join optimizer to better choose in which order it should join the tables and which keys it should use: `myisamchk --describe --verbose table_name'' or using `SHOW KEYS' in MySQL. `-d or --description' Prints some information about table. `-A or --set-auto-increment[=value]' Force auto_increment to start at this or higher value. If no value is given, then sets the next auto_increment value to the highest used value for the auto key + 1. `-S or --sort-index' Sort the index tree blocks in high-low order. This will optimize seeks and will make table scanning by key faster. `-R or --sort-records=#' Sorts records according to an index. This makes your data much more localized and may speed up ranged `SELECT' and `ORDER BY' operations on this index. (It may be VERY slow to do a sort the first time!) To find out a table's index numbers, use `SHOW INDEX', which shows a table's indexes in the same order that `myisamchk' sees them. Indexes are numbered beginning with 1. `myisamchk' Memory Usage ........................ Memory allocation is important when you run `myisamchk'. `myisamchk' uses no more memory than you specify with the `-O' options. If you are going to use `myisamchk' on very large files, you should first decide how much memory you want it to use. The default is to use only about 3M to fix things. By using larger values, you can get `myisamchk' to operate faster. For example, if you have more than 32M RAM, you could use options such as these (in addition to any other options you might specify): shell> myisamchk -O sort=16M -O key=16M -O read=1M -O write=1M ... Using `-O sort=16M' should probably be enough for most cases. Be aware that `myisamchk' uses temporary files in `TMPDIR'. If `TMPDIR' points to a memory file system, you may easily get out of memory errors. If this happens, set `TMPDIR' to point at some directory with more space and restart `myisamchk'. When repairing, `myisamchk' will also need a lot of disk space: * Double the size of the record file (the original one and a copy). This space is not needed if one does a repair with `--quick', as in this case only the index file will be re-created. This space is needed on the same disk as the original record file! * Space for the new index file that replaces the old one. The old index file is truncated at start, so one usually ignore this space. This space is needed on the same disk as the original index file! * When using `--recover' or `--sort-recover' (but not when using `--safe-recover', you will need space for a sort buffer for: `(largest_key + row_pointer_length)*number_of_rows * 2'. You can check the length of the keys and the row_pointer_length with `myisamchk -dv table'. This space is allocated on the temporary disk (specified by `TMPDIR' or `--tmpdir=#'). If you have a problem with disk space during repair, you can try to use `--safe-recover' instead of `--recover'. Using `myisamchk' for Crash Recovery .................................... If you run `mysqld' with `--skip-locking' (which is the default on some systems, like Linux), you can't reliably use `myisamchk' to check a table when `mysqld' is using the same table. If you can be sure that no one is accessing the tables through `mysqld' while you run `myisamchk', you only have to do `mysqladmin flush-tables' before you start checking the tables. If you can't guarantee the above, then you must take down `mysqld' while you check the tables. If you run `myisamchk' while `mysqld' is updating the tables, you may get a warning that a table is corrupt even if it isn't. If you are not using `--skip-locking', you can use `myisamchk' to check tables at any time. While you do this, all clients that try to update the table will wait until `myisamchk' is ready before continuing. If you use `myisamchk' to repair or optimize tables, you *MUST* always ensure that the `mysqld' server is not using the table (this also applies if you are using `--skip-locking'). If you don't take down `mysqld' you should at least do a `mysqladmin flush-tables' before you run `myisamchk'. This chapter describes how to check for and deal with data corruption in MySQL databases. If your tables get corrupted a lot you should try to find the reason for this! *Note Crashing::. The `MyISAM' table section contains reason for why a table could be corrupted. *Note MyISAM table problems::. When performing crash recovery, it is important to understand that each table `tbl_name' in a database corresponds to three files in the database directory: *File* *Purpose* `tbl_name.frm' Table definition (form) file `tbl_name.MYD' Data file `tbl_name.MYI' Index file Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files. `myisamchk' works by creating a copy of the `.MYD' (data) file row by row. It ends the repair stage by removing the old `.MYD' file and renaming the new file to the original file name. If you use `--quick', `myisamchk' does not create a temporary `.MYD' file, but instead assumes that the `.MYD' file is correct and only generates a new index file without touching the `.MYD' file. This is safe, because `myisamchk' automatically detects if the `.MYD' file is corrupt and aborts the repair in this case. You can also give two `--quick' options to `myisamchk'. In this case, `myisamchk' does not abort on some errors (like duplicate key) but instead tries to resolve them by modifying the `.MYD' file. Normally the use of two `--quick' options is useful only if you have too little free disk space to perform a normal repair. In this case you should at least make a backup before running `myisamchk'. How to Check Tables for Errors .............................. To check a MyISAM table, use the following commands: `myisamchk tbl_name' This finds 99.99% of all errors. What it can't find is corruption that involves *ONLY* the data file (which is very unusual). If you want to check a table, you should normally run `myisamchk' without options or with either the `-s' or `--silent' option. `myisamchk -m tbl_name' This finds 99.999% of all errors. It checks first all index entries for errors and then it reads through all rows. It calculates a checksum for all keys in the rows and verifies that they checksum matches the checksum for the keys in the index tree. `myisamchk -e tbl_name' This does a complete and thorough check of all data (`-e' means "extended check"). It does a check-read of every key for each row to verify that they indeed point to the correct row. This may take a LONG time on a big table with many keys. `myisamchk' will normally stop after the first error it finds. If you want to obtain more information, you can add the `--verbose' (`-v') option. This causes `myisamchk' to keep going, up through a maximum of 20 errors. In normal usage, a simple `myisamchk' (with no arguments other than the table name) is sufficient. `myisamchk -e -i tbl_name' Like the previous command, but the `-i' option tells `myisamchk' to print some informational statistics, too. How to Repair Tables .................... In the following section we only talk about using `myisamchk' on `MyISAM' tables (extensions `.MYI' and `.MYD'). If you are using `ISAM' tables (extensions `.ISM' and `.ISD'), you should use `isamchk' instead. Starting with MySQL Version 3.23.14, you can repair MyISAM tables with the `REPAIR TABLE' command. *Note REPAIR TABLE::. The symptoms of a corrupted table include queries that abort unexpectedly and observable errors such as these: * `tbl_name.frm' is locked against change * Can't find file `tbl_name.MYI' (Errcode: ###) * Unexpected end of file * Record file is crashed * Got error ### from table handler To get more information about the error you can run `perror ###'. Here is the most common errors that indicates a problem with the table: shell> perror 126 127 132 134 135 136 141 144 145 126 = Index file is crashed / Wrong file format 127 = Record-file is crashed 132 = Old database file 134 = Record was already deleted (or record file crashed) 135 = No more room in record file 136 = No more room in index file 141 = Duplicate unique key or constraint on write or update 144 = Table is crashed and last repair failed 145 = Table was marked as crashed and should be repaired Note that error 135, no more room in record file, is not an error that can be fixed by a simple repair. In this case you have to do: ALTER TABLE table MAX_ROWS=xxx AVG_ROW_LENGTH=yyy; In the other cases, you must repair your tables. `myisamchk' can usually detect and fix most things that go wrong. The repair process involves up to four stages, described below. Before you begin, you should `cd' to the database directory and check the permissions of the table files. Make sure they are readable by the Unix user that `mysqld' runs as (and to you, because you need to access the files you are checking). If it turns out you need to modify files, they must also be writable by you. If you are using MySQL Version 3.23.16 and above, you can (and should) use the `CHECK' and `REPAIR' commands to check and repair `MyISAM' tables. *Note CHECK TABLE::. *Note REPAIR TABLE::. The manual section about table maintenance includes the options to `isamchk'/`myisamchk'. *Note Table maintenance::. The following section is for the cases where the above command fails or if you want to use the extended features that `isamchk'/`myisamchk' provides. If you are going to repair a table from the command line, you must first take down the `mysqld' server. Note that when you do `mysqladmin shutdown' on a remote server, the `mysqld' server will still be alive for a while after `mysqladmin' returns, until all queries are stopped and all keys have been flushed to disk. *Stage 1: Checking your tables* Run `myisamchk *.MYI' or `myisamchk -e *.MYI' if you have more time. Use the `-s' (silent) option to suppress unnecessary information. If the `mysqld' server is done you should use the -update option to tell `myisamchk' to mark the table as 'checked'. You have to repair only those tables for which `myisamchk' announces an error. For such tables, proceed to Stage 2. If you get weird errors when checking (such as `out of memory' errors), or if `myisamchk' crashes, go to Stage 3. *Stage 2: Easy safe repair* NOTE: If you want repairing to go much faster, you should add: `-O sort_buffer=# -O key_buffer=#' (where # is about 1/4 of the available memory) to all `isamchk/myisamchk' commands. First, try `myisamchk -r -q tbl_name' (`-r -q' means "quick recovery mode"). This will attempt to repair the index file without touching the data file. If the data file contains everything that it should and the delete links point at the correct locations within the data file, this should work, and the table is fixed. Start repairing the next table. Otherwise, use the following procedure: 1. Make a backup of the data file before continuing. 2. Use `myisamchk -r tbl_name' (`-r' means "recovery mode"). This will remove incorrect records and deleted records from the data file and reconstruct the index file. 3. If the preceding step fails, use `myisamchk --safe-recover tbl_name'. Safe recovery mode uses an old recovery method that handles a few cases that regular recovery mode doesn't (but is slower). If you get weird errors when repairing (such as `out of memory' errors), or if `myisamchk' crashes, go to Stage 3. *Stage 3: Difficult repair* You should only reach this stage if the first 16K block in the index file is destroyed or contains incorrect information, or if the index file is missing. In this case, it's necessary to create a new index file. Do so as follows: 1. Move the data file to some safe place. 2. Use the table description file to create new (empty) data and index files: shell> mysql db_name mysql> SET AUTOCOMMIT=1; mysql> TRUNCATE TABLE table_name; mysql> quit If your SQL version doesn't have `TRUNCATE TABLE', use `DELETE FROM table_name' instead. 3. Copy the old data file back onto the newly created data file. (Don't just move the old file back onto the new file; you want to retain a copy in case something goes wrong.) Go back to Stage 2. `myisamchk -r -q' should work now. (This shouldn't be an endless loop.) *Stage 4: Very difficult repair* You should reach this stage only if the description file has also crashed. That should never happen, because the description file isn't changed after the table is created: 1. Restore the description file from a backup and go back to Stage 3. You can also restore the index file and go back to Stage 2. In the latter case, you should start with `myisamchk -r'. 2. If you don't have a backup but know exactly how the table was created, create a copy of the table in another database. Remove the new data file, then move the description and index files from the other database to your crashed database. This gives you new description and index files, but leaves the data file alone. Go back to Stage 2 and attempt to reconstruct the index file. Table Optimization .................. To coalesce fragmented records and eliminate wasted space resulting from deleting or updating records, run `myisamchk' in recovery mode: shell> myisamchk -r tbl_name You can optimize a table in the same way using the SQL `OPTIMIZE TABLE' statement. `OPTIMIZE TABLE' does a repair of the table, a key analyzes and also sorts the index tree to give faster key lookups. There is also no possibility of unwanted interaction between a utility and the server, because the server does all the work when you use `OPTIMIZE TABLE'. *Note OPTIMIZE TABLE::. `myisamchk' also has a number of other options you can use to improve the performance of a table: `-S, --sort-index' `-R index_num, --sort-records=index_num' `-a, --analyze' For a full description of the option. *Note myisamchk syntax::. Setting Up a Table Maintenance Regimen -------------------------------------- Starting with MySQL Version 3.23.13, you can check MyISAM tables with the `CHECK TABLE' command. *Note CHECK TABLE::. You can repair tables with the `REPAIR TABLE' command. *Note REPAIR TABLE::. It is a good idea to perform table checks on a regular basis rather than waiting for problems to occur. For maintenance purposes, you can use `myisamchk -s' to check tables. The `-s' option (short for `--silent') causes `myisamchk' to run in silent mode, printing messages only when errors occur. It's also a good idea to check tables when the server starts up. For example, whenever the machine has done a reboot in the middle of an update, you usually need to check all the tables that could have been affected. (This is an "expected crashed table".) You could add a test to `safe_mysqld' that runs `myisamchk' to check all tables that have been modified during the last 24 hours if there is an old `.pid' (process ID) file left after a reboot. (The `.pid' file is created by `mysqld' when it starts up and removed when it terminates normally. The presence of a `.pid' file at system startup time indicates that `mysqld' terminated abnormally.) An even better test would be to check any table whose last-modified time is more recent than that of the `.pid' file. You should also check your tables regularly during normal system operation. At MySQL AB, we run a `cron' job to check all our important tables once a week, using a line like this in a `crontab' file: 35 0 * * 0 /path/to/myisamchk --fast --silent /path/to/datadir/*/*.MYI This prints out information about crashed tables so we can examine and repair them when needed. As we haven't had any unexpectedly crashed tables (tables that become corrupted for reasons other than hardware trouble) for a couple of years now (this is really true), once a week is more than enough for us. We recommend that to start with, you execute `myisamchk -s' each night on all tables that have been updated during the last 24 hours, until you come to trust MySQL as much as we do. Normally you don't need to maintain MySQL tables that much. If you are changing tables with dynamic size rows (tables with `VARCHAR', `BLOB' or `TEXT' columns) or have tables with many deleted rows you may want to from time to time (once a month?) defragment/reclaim space from the tables. You can do this by using `OPTIMIZE TABLE' on the tables in question or if you can take the `mysqld' server down for a while do: isamchk -r --silent --sort-index -O sort_buffer_size=16M */*.ISM myisamchk -r --silent --sort-index -O sort_buffer_size=16M */*.MYI Getting Information About a Table --------------------------------- To get a description of a table or statistics about it, use the commands shown below. We explain some of the information in more detail later: `myisamchk -d tbl_name' Runs `myisamchk' in "describe mode" to produce a description of your table. If you start the MySQL server using the `--skip-locking' option, `myisamchk' may report an error for a table that is updated while it runs. However, because `myisamchk' doesn't change the table in describe mode, there isn't any risk of destroying data. `myisamchk -d -v tbl_name' To produce more information about what `myisamchk' is doing, add `-v' to tell it to run in verbose mode. `myisamchk -eis tbl_name' Shows only the most important information from a table. It is slow because it must read the whole table. `myisamchk -eiv tbl_name' This is like `-eis', but tells you what is being done. Example of `myisamchk -d' output: MyISAM file: company.MYI Record format: Fixed length Data records: 1403698 Deleted blocks: 0 Recordlength: 226 table description: Key Start Len Index Type 1 2 8 unique double 2 15 10 multip. text packed stripped 3 219 8 multip. double 4 63 10 multip. text packed stripped 5 167 2 multip. unsigned short 6 177 4 multip. unsigned long 7 155 4 multip. text 8 138 4 multip. unsigned long 9 177 4 multip. unsigned long 193 1 text Example of `myisamchk -d -v' output: MyISAM file: company Record format: Fixed length File-version: 1 Creation time: 1999-10-30 12:12:51 Recover time: 1999-10-31 19:13:01 Status: checked Data records: 1403698 Deleted blocks: 0 Datafile parts: 1403698 Deleted data: 0 Datafilepointer (bytes): 3 Keyfile pointer (bytes): 3 Max datafile length: 3791650815 Max keyfile length: 4294967294 Recordlength: 226 table description: Key Start Len Index Type Rec/key Root Blocksize 1 2 8 unique double 1 15845376 1024 2 15 10 multip. text packed stripped 2 25062400 1024 3 219 8 multip. double 73 40907776 1024 4 63 10 multip. text packed stripped 5 48097280 1024 5 167 2 multip. unsigned short 4840 55200768 1024 6 177 4 multip. unsigned long 1346 65145856 1024 7 155 4 multip. text 4995 75090944 1024 8 138 4 multip. unsigned long 87 85036032 1024 9 177 4 multip. unsigned long 178 96481280 1024 193 1 text Example of `myisamchk -eis' output: Checking MyISAM file: company Key: 1: Keyblocks used: 97% Packed: 0% Max levels: 4 Key: 2: Keyblocks used: 98% Packed: 50% Max levels: 4 Key: 3: Keyblocks used: 97% Packed: 0% Max levels: 4 Key: 4: Keyblocks used: 99% Packed: 60% Max levels: 3 Key: 5: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 6: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 7: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 8: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 9: Keyblocks used: 98% Packed: 0% Max levels: 4 Total: Keyblocks used: 98% Packed: 17% Records: 1403698 M.recordlength: 226 Packed: 0% Recordspace used: 100% Empty space: 0% Blocks/Record: 1.00 Record blocks: 1403698 Delete blocks: 0 Recorddata: 317235748 Deleted data: 0 Lost space: 0 Linkdata: 0 User time 1626.51, System time 232.36 Maximum resident set size 0, Integral resident set size 0 Non physical pagefaults 0, Physical pagefaults 627, Swaps 0 Blocks in 0 out 0, Messages in 0 out 0, Signals 0 Voluntary context switches 639, Involuntary context switches 28966 Example of `myisamchk -eiv' output: Checking MyISAM file: company Data records: 1403698 Deleted blocks: 0 - check file-size - check delete-chain block_size 1024: index 1: index 2: index 3: index 4: index 5: index 6: index 7: index 8: index 9: No recordlinks - check index reference - check data record references index: 1 Key: 1: Keyblocks used: 97% Packed: 0% Max levels: 4 - check data record references index: 2 Key: 2: Keyblocks used: 98% Packed: 50% Max levels: 4 - check data record references index: 3 Key: 3: Keyblocks used: 97% Packed: 0% Max levels: 4 - check data record references index: 4 Key: 4: Keyblocks used: 99% Packed: 60% Max levels: 3 - check data record references index: 5 Key: 5: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 6 Key: 6: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 7 Key: 7: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 8 Key: 8: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 9 Key: 9: Keyblocks used: 98% Packed: 0% Max levels: 4 Total: Keyblocks used: 9% Packed: 17% - check records and index references [LOTS OF ROW NUMBERS DELETED] Records: 1403698 M.recordlength: 226 Packed: 0% Recordspace used: 100% Empty space: 0% Blocks/Record: 1.00 Record blocks: 1403698 Delete blocks: 0 Recorddata: 317235748 Deleted data: 0 Lost space: 0 Linkdata: 0 User time 1639.63, System time 251.61 Maximum resident set size 0, Integral resident set size 0 Non physical pagefaults 0, Physical pagefaults 10580, Swaps 0 Blocks in 4 out 0, Messages in 0 out 0, Signals 0 Voluntary context switches 10604, Involuntary context switches 122798 Here are the sizes of the data and index files for the table used in the preceding examples: -rw-rw-r-- 1 monty tcx 317235748 Jan 12 17:30 company.MYD -rw-rw-r-- 1 davida tcx 96482304 Jan 12 18:35 company.MYM Explanations for the types of information `myisamchk' produces are given below. The "keyfile" is the index file. "Record" and "row" are synonymous: `ISAM file' Name of the ISAM (index) file. `Isam-version' Version of ISAM format. Currently always 2. `Creation time' When the data file was created. `Recover time' When the index/data file was last reconstructed. `Data records' How many records are in the table. `Deleted blocks' How many deleted blocks still have reserved space. You can optimize your table to minimize this space. *Note Optimization::. `Datafile: Parts' For dynamic record format, this indicates how many data blocks there are. For an optimized table without fragmented records, this is the same as `Data records'. `Deleted data' How many bytes of non-reclaimed deleted data there are. You can optimize your table to minimize this space. *Note Optimization::. `Datafile pointer' The size of the data file pointer, in bytes. It is usually 2, 3, 4, or 5 bytes. Most tables manage with 2 bytes, but this cannot be controlled from MySQL yet. For fixed tables, this is a record address. For dynamic tables, this is a byte address. `Keyfile pointer' The size of the index file pointer, in bytes. It is usually 1, 2, or 3 bytes. Most tables manage with 2 bytes, but this is calculated automatically by MySQL. It is always a block address. `Max datafile length' How long the table's data file (`.MYD' file) can become, in bytes. `Max keyfile length' How long the table's key file (`.MYI' file) can become, in bytes. `Recordlength' How much space each record takes, in bytes. `Record format' The format used to store table rows. The examples shown above use `Fixed length'. Other possible values are `Compressed' and `Packed'. `table description' A list of all keys in the table. For each key, some low-level information is presented: `Key' This key's number. `Start' Where in the record this index part starts. `Len' How long this index part is. For packed numbers, this should always be the full length of the column. For strings, it may be shorter than the full length of the indexed column, because you can index a prefix of a string column. `Index' `unique' or `multip.' (multiple). Indicates whether or not one value can exist multiple times in this index. `Type' What data-type this index part has. This is an ISAM data-type with the options `packed', `stripped' or `empty'. `Root' Address of the root index block. `Blocksize' The size of each index block. By default this is 1024, but the value may be changed at compile time. `Rec/key' This is a statistical value used by the optimizer. It tells how many records there are per value for this key. A unique key always has a value of 1. This may be updated after a table is loaded (or greatly changed) with `myisamchk -a'. If this is not updated at all, a default value of 30 is given. `' In the first example above, the 9th key is a multi-part key with two parts. `Keyblocks used' What percentage of the keyblocks are used. Because the table used in the examples had just been reorganized with `myisamchk', the values are very high (very near the theoretical maximum). `Packed' MySQL tries to pack keys with a common suffix. This can only be used for `CHAR'/`VARCHAR'/`DECIMAL' keys. For long strings like names, this can significantly reduce the space used. In the third example above, the 4th key is 10 characters long and a 60% reduction in space is achieved. `Max levels' How deep the B-tree for this key is. Large tables with long keys get high values. `Records' How many rows are in the table. `M.recordlength' The average record length. For tables with fixed-length records, this is the exact record length. `Packed' MySQL strips spaces from the end of strings. The `Packed' value indicates the percentage of savings achieved by doing this. `Recordspace used' What percentage of the data file is used. `Empty space' What percentage of the data file is unused. `Blocks/Record' Average number of blocks per record (that is, how many links a fragmented record is composed of). This is always 1 for fixed-format tables. This value should stay as close to 1.0 as possible. If it gets too big, you can reorganize the table with `myisamchk'. *Note Optimization::. `Recordblocks' How many blocks (links) are used. For fixed format, this is the same as the number of records. `Deleteblocks' How many blocks (links) are deleted. `Recorddata' How many bytes in the data file are used. `Deleted data' How many bytes in the data file are deleted (unused). `Lost space' If a record is updated to a shorter length, some space is lost. This is the sum of all such losses, in bytes. `Linkdata' When the dynamic table format is used, record fragments are linked with pointers (4 to 7 bytes each). `Linkdata' is the sum of the amount of storage used by all such pointers. If a table has been compressed with `myisampack', `myisamchk -d' prints additional information about each table column. See *Note `myisampack': myisampack, for an example of this information and a description of what it means. Database Administration Language Reference ========================================== `OPTIMIZE TABLE' Syntax ----------------------- OPTIMIZE TABLE tbl_name[,tbl_name]... `OPTIMIZE TABLE' should be used if you have deleted a large part of a table or if you have made many changes to a table with variable-length rows (tables that have `VARCHAR', `BLOB', or `TEXT' columns). Deleted records are maintained in a linked list and subsequent `INSERT' operations reuse old record positions. You can use `OPTIMIZE TABLE' to reclaim the unused space and to defragment the data file. For the moment `OPTIMIZE TABLE' only works on *MyISAM* and `BDB' tables. For `BDB' tables, `OPTIMIZE TABLE' is currently mapped to `ANALYZE TABLE'. *Note ANALYZE TABLE::. You can get optimize table to work on other table types by starting `mysqld' with `--skip-new' or `--safe-mode', but in this case `OPTIMIZE TABLE' is just mapped to `ALTER TABLE'. `OPTIMIZE TABLE' works the following way: * If the table has deleted or split rows, repair the table. * If the index pages are not sorted, sort them. * If the statistics are not up to date (and the repair couldn't be done by sorting the index), update them. `OPTIMIZE TABLE' for `MyISAM' tables is equvialent of running `myisamchk --quick --check-changed-tables --sort-index --analyze' on the table. Note that the table is locked during the time `OPTIMIZE TABLE' is running! `ANALYZE TABLE' Syntax ---------------------- ANALYZE TABLE tbl_name[,tbl_name...] Analyze and store the key distribution for the table. During the analyze the table is locked with a read lock. This works on `MyISAM' and `BDB' tables. This is equivalent to running `myisamchk -a' on the table. MySQL uses the stored key distribution to decide in which order tables should be joined when one does a join on something else than a constant. The command returns a table with the following columns: *Column* *Value* Table Table name Op Always "analyze" Msg_type One of `status', `error', `info' or `warning'. Msg_text The message. You can check the stored key distribution with the `SHOW INDEX' command. *Note SHOW DATABASE INFO::. If the table hasn't changed since the last `ANALYZE TABLE' command, the table will not be analyzed again. `FLUSH' Syntax -------------- FLUSH flush_option [,flush_option] You should use the `FLUSH' command if you want to clear some of the internal caches MySQL uses. To execute `FLUSH', you must have the *RELOAD* privilege. `flush_option' can be any of the following: `HOSTS' Empties the host cache tables. You should flush the host tables if some of your hosts change IP number or if you get the error message `Host ... is blocked'. When more than `max_connect_errors' errors occur in a row for a given host while connection to the MySQL server, MySQL assumes something is wrong and blocks the host from further connection requests. Flushing the host tables allows the host to attempt to connect again. *Note Blocked host::.) You can start `mysqld' with `-O max_connection_errors=999999999' to avoid this error message. `LOGS' Closes and reopens all log files. If you have specified the update log file or a binary log file without an extension, the extension number of the log file will be incremented by one relative to the previous file. If you have used an extension in the file name, MySQL will close and reopen the update log file. *Note Update log::. This is the same thing as sending the `SIGHUP' signal to the `mysqld' server. `PRIVILEGES'Reloads the privileges from the grant tables in the `mysql' database. `TABLES' Closes all open tables and force all tables in use to be closed. `[TABLE | Flushes only the given tables. TABLES] table_name [,table_name...]' `TABLES Closes all open tables and locks all tables for all WITH READ databases with a read until one executes `UNLOCK TABLES'. LOCK' This is very convenient way to get backups if you have a file system, like Veritas,that can take snapshots in time. `STATUS' Resets most status variables to zero. This is something one should only use when debugging a query. You can also access each of the commands shown above with the `mysqladmin' utility, using the `flush-hosts', `flush-logs', `reload', or `flush-tables' commands. Take also a look at the `RESET' command used with replication. *Note Replication SQL::. `KILL' Syntax ------------- KILL thread_id Each connection to `mysqld' runs in a separate thread. You can see which threads are running with the `SHOW PROCESSLIST' command and kill a thread with the `KILL thread_id' command. If you have the *process* privilege, you can see and kill all threads. Otherwise, you can see and kill only your own threads. You can also use the `mysqladmin processlist' and `mysqladmin kill' commands to examine and kill threads. When you do a `KILL', a thread specific `kill flag' is set for the thread. In most cases it may take some time for the thread to die as the kill flag is only checked at specific intervals. * In `SELECT', `ORDER BY' and `GROUP BY' loops, the flag is checked after reading a block of rows. If the kill flag is set the statement is aborted * When doing an `ALTER TABLE' the kill flag is checked before each block of rows are read from the original table. If the kill flag was set the command is aborted and the temporary table is deleted. * When doing an `UPDATE TABLE' and `DELETE TABLE', the kill flag is checked after each block read and after each updated or delete row. If the kill flag is set the statement is aborted. Note that if you are not using transactions, the changes will not be rolled back! * `GET_LOCK()' will abort with `NULL'. * An `INSERT DELAYED' thread will quickly flush all rows it has in memory and die. * If the thread is in the table lock handler (state: `Locked'), the table lock will be quickly aborted. * If the thread is waiting for free disk space in a `write' call, the write is aborted with an disk full error message. `SHOW' Syntax ------------- SHOW DATABASES [LIKE wild] or SHOW [OPEN] TABLES [FROM db_name] [LIKE wild] or SHOW [FULL] COLUMNS FROM tbl_name [FROM db_name] [LIKE wild] or SHOW INDEX FROM tbl_name [FROM db_name] or SHOW TABLE STATUS [FROM db_name] [LIKE wild] or SHOW STATUS [LIKE wild] or SHOW VARIABLES [LIKE wild] or SHOW LOGS or SHOW [FULL] PROCESSLIST or SHOW GRANTS FOR user or SHOW CREATE TABLE table_name or SHOW MASTER STATUS or SHOW MASTER LOGS or SHOW SLAVE STATUS `SHOW' provides information about databases, tables, columns, or status information about the server. If the `LIKE wild' part is used, the `wild' string can be a string that uses the SQL `%' and `_' wild-card characters. Retrieving information about Database, Tables, Columns, and Indexes ................................................................... You can use `db_name.tbl_name' as an alternative to the `tbl_name FROM db_name' syntax. These two statements are equivalent: mysql> SHOW INDEX FROM mytable FROM mydb; mysql> SHOW INDEX FROM mydb.mytable; `SHOW DATABASES' lists the databases on the MySQL server host. You can also get this list using the `mysqlshow' command. `SHOW TABLES' lists the tables in a given database. You can also get this list using the `mysqlshow db_name' command. *NOTE:* If a user doesn't have any privileges for a table, the table will not show up in the output from `SHOW TABLES' or `mysqlshow db_name'. `SHOW OPEN TABLES' lists the tables that are currently open in the table cache. *Note Table cache::. The `Comment' field tells how many times the table is `cached' and `in_use'. `SHOW COLUMNS' lists the columns in a given table. If you specify the `FULL' option, you will also get the privileges you have for each column. If the column types are different than you expect them to be based on a `CREATE TABLE' statement, note that MySQL sometimes changes column types. *Note Silent column changes::. The `DESCRIBE' statement provides information similar to `SHOW COLUMNS'. *Note `DESCRIBE': DESCRIBE. `SHOW FIELDS' is a synonym for `SHOW COLUMNS', and `SHOW KEYS' is a synonym for `SHOW INDEX'. You can also list a table's columns or indexes with `mysqlshow db_name tbl_name' or `mysqlshow -k db_name tbl_name'. `SHOW INDEX' returns the index information in a format that closely resembles the `SQLStatistics' call in ODBC. The following columns are returned: *Column* *Meaning* `Table' Name of the table. `Non_unique' 0 if the index can't contain duplicates. `Key_name' Name of the index. `Seq_in_index' Column sequence number in index, starting with 1. `Column_name' Column name. `Collation' How the column is sorted in the index. In MySQL, this can have values `A' (Ascending) or `NULL' (Not sorted). `Cardinality' Number of unique values in the index. This is updated by running `isamchk -a'. `Sub_part' Number of indexed characters if the column is only partly indexed. `NULL' if the entire key is indexed. `Comment' Various remarks. For now, it tells whether index is FULLTEXT or not. Note that as the `Cardinality' is counted based on statistics stored as integers, it's not necessarily accurate for small tables. `SHOW TABLE STATUS' ................... SHOW TABLE STATUS [FROM db_name] [LIKE wild] `SHOW TABLE STATUS' (new in Version 3.23) works likes `SHOW STATUS', but provides a lot of information about each table. You can also get this list using the `mysqlshow --status db_name' command. The following columns are returned: *Column* *Meaning* `Name' Name of the table. `Type' Type of table. *Note Table types::. `Row_format' The row storage format (Fixed, Dynamic, or Compressed). `Rows' Number of rows. `Avg_row_length' Average row length. `Data_length' Length of the data file. `Max_data_length' Max length of the data file. `Index_length' Length of the index file. `Data_free' Number of allocated but not used bytes. `Auto_increment' Next autoincrement value. `Create_time' When the table was created. `Update_time' When the data file was last updated. `Check_time' When the table was last checked. `Create_options' Extra options used with `CREATE TABLE'. `Comment' The comment used when creating the table (or some information why MySQL couldn't access the table information). `InnoDB' tables will report the free space in the tablespace in the table comment. `SHOW STATUS' ............. `SHOW STATUS' provides server status information (like `mysqladmin extended-status'). The output resembles that shown below, though the format and numbers probably differ: +--------------------------+------------+ | Variable_name | Value | +--------------------------+------------+ | Aborted_clients | 0 | | Aborted_connects | 0 | | Bytes_received | 155372598 | | Bytes_sent | 1176560426 | | Connections | 30023 | | Created_tmp_disk_tables | 0 | | Created_tmp_tables | 8340 | | Created_tmp_files | 60 | | Delayed_insert_threads | 0 | | Delayed_writes | 0 | | Delayed_errors | 0 | | Flush_commands | 1 | | Handler_delete | 462604 | | Handler_read_first | 105881 | | Handler_read_key | 27820558 | | Handler_read_next | 390681754 | | Handler_read_prev | 6022500 | | Handler_read_rnd | 30546748 | | Handler_read_rnd_next | 246216530 | | Handler_update | 16945404 | | Handler_write | 60356676 | | Key_blocks_used | 14955 | | Key_read_requests | 96854827 | | Key_reads | 162040 | | Key_write_requests | 7589728 | | Key_writes | 3813196 | | Max_used_connections | 0 | | Not_flushed_key_blocks | 0 | | Not_flushed_delayed_rows | 0 | | Open_tables | 1 | | Open_files | 2 | | Open_streams | 0 | | Opened_tables | 44600 | | Questions | 2026873 | | Select_full_join | 0 | | Select_full_range_join | 0 | | Select_range | 99646 | | Select_range_check | 0 | | Select_scan | 30802 | | Slave_running | OFF | | Slave_open_temp_tables | 0 | | Slow_launch_threads | 0 | | Slow_queries | 0 | | Sort_merge_passes | 30 | | Sort_range | 500 | | Sort_rows | 30296250 | | Sort_scan | 4650 | | Table_locks_immediate | 1920382 | | Table_locks_waited | 0 | | Threads_cached | 0 | | Threads_created | 30022 | | Threads_connected | 1 | | Threads_running | 1 | | Uptime | 80380 | +--------------------------+------------+ The status variables listed above have the following meaning: *Variable* *Meaning* `Aborted_clients' Number of connections aborted because the client died without closing the connection properly. *Note Communication errors::. `Aborted_connects' Number of tries to connect to the MySQL server that failed. *Note Communication errors::. `Bytes_received' Number of bytes received from all clients. `Bytes_sent' Number of bytes sent to all clients. `Connections' Number of connection attempts to the MySQL server. `Created_tmp_disk_tables' Number of implicit temporary tables on disk created while executing statements. `Created_tmp_tables' Number of implicit temporary tables in memory created while executing statements. `Created_tmp_files' How many temporary files `mysqld' have created. `Delayed_insert_threads' Number of delayed insert handler threads in use. `Delayed_writes' Number of rows written with `INSERT DELAYED'. `Delayed_errors' Number of rows written with `INSERT DELAYED' for which some error occurred (probably `duplicate key'). `Flush_commands' Number of executed `FLUSH' commands. `Handler_delete' Number of times a row was deleted from a table. `Handler_read_first' Number of times the first entry was read from an index. If this is high, it suggests that the server is doing a lot of full index scans, for example, `SELECT col1 FROM foo', assuming that col1 is indexed. `Handler_read_key' Number of requests to read a row based on a key. If this is high, it is a good indication that your queries and tables are properly indexed. `Handler_read_next' Number of requests to read next row in key order. This will be incremented if you are querying an index column with a range constraint. This also will be incremented if you are doing an index scan. `Handler_read_rnd' Number of requests to read a row based on a fixed position. This will be high if you are doing a lot of queries that require sorting of the result. `Handler_read_rnd_next' Number of requests to read the next row in the datafile. This will be high if you are doing a lot of table scans. Generally this suggests that your tables are not properly indexed or that your queries are not written to take advantage of the indexes you have. `Handler_update' Number of requests to update a row in a table. `Handler_write' Number of requests to insert a row in a table. `Key_blocks_used' The number of used blocks in the key cache. `Key_read_requests' The number of requests to read a key block from the cache. `Key_reads' The number of physical reads of a key block from disk. `Key_write_requests' The number of requests to write a key block to the cache. `Key_writes' The number of physical writes of a key block to disk. `Max_used_connections' The maximum number of connections in use simultaneously. `Not_flushed_key_blocks' Keys blocks in the key cache that has changed but hasn't yet been flushed to disk. `Not_flushed_delayed_rows'Number of rows waiting to be written in `INSERT DELAY' queues. `Open_tables' Number of tables that are open. `Open_files' Number of files that are open. `Open_streams' Number of streams that are open (used mainly for logging). `Opened_tables' Number of tables that have been opened. `Select_full_join' Number of joins without keys (Should be 0). `Select_full_range_join' Number of joins where we used a range search on reference table. `Select_range' Number of joins where we used ranges on the first table. (It's normally not critical even if this is big.) `Select_scan' Number of joins where we scanned the first table. `Select_range_check' Number of joins without keys where we check for key usage after each row (Should be 0). `Questions' Number of queries sent to the server. `Slave_open_temp_tables' Number of temporary tables currently open by the slave thread `Slow_launch_threads' Number of threads that have taken more than `slow_launch_time' to connect. `Slow_queries' Number of queries that have taken more than `long_query_time'. *Note Slow query log::. `Sort_merge_passes' Number of merges the sort has to do. If this value is large you should consider increasing `sort_buffer'. `Sort_range' Number of sorts that where done with ranges. `Sort_rows' Number of sorted rows. `Sort_scan' Number of sorts that where done by scanning the table. `Table_locks_immediate' Number of times a table lock was acquired immediately. Available after 3.23.33. `Table_locks_waited' Number of times a table lock could not be acquired immediately and a wait was needed. If this is high, and you have performance problems, you should first optimize your queries, and then either split your table(s) or use replication. Available after 3.23.33. `Threads_cached' Number of threads in the thread cache. `Threads_connected' Number of currently open connections. `Threads_created' Number of threads created to handle connections. `Threads_running' Number of threads that are not sleeping. `Uptime' How many seconds the server has been up. Some comments about the above: * If `Opened_tables' is big, then your `table_cache' variable is probably too small. * If `key_reads' is big, then your `key_cache' is probably too small. The cache hit rate can be calculated with `key_reads'/`key_read_requests'. * If `Handler_read_rnd' is big, then you probably have a lot of queries that require MySQL to scan whole tables or you have joins that don't use keys properly. * If `Threads_created' is big, you may want to increase the `thread_cache_size' variable. `SHOW VARIABLES' ................ SHOW VARIABLES [LIKE wild] `SHOW VARIABLES' shows the values of some MySQL system variables. You can also get this information using the `mysqladmin variables' command. If the default values are unsuitable, you can set most of these variables using command-line options when `mysqld' starts up. *Note Command-line options::. The output resembles that shown below, though the format and numbers may differ somewhat: +-------------------------+---------------------------+ | Variable_name | Value | +-------------------------+---------------------------+ | ansi_mode | OFF | | back_log | 50 | | basedir | /my/monty/ | | bdb_cache_size | 16777216 | | bdb_log_buffer_size | 32768 | | bdb_home | /my/monty/data/ | | bdb_max_lock | 10000 | | bdb_logdir | | | bdb_shared_data | OFF | | bdb_tmpdir | /tmp/ | | binlog_cache_size | 32768 | | concurrent_insert | ON | | connect_timeout | 5 | | datadir | /my/monty/data/ | | delay_key_write | ON | | delayed_insert_limit | 100 | | delayed_insert_timeout | 300 | | delayed_queue_size | 1000 | | flush | OFF | | flush_time | 0 | | have_bdb | YES | | have_innodb | YES | | have_raid | YES | | have_ssl | NO | | init_file | | | interactive_timeout | 28800 | | join_buffer_size | 131072 | | key_buffer_size | 16776192 | | language | /my/monty/share/english/ | | large_files_support | ON | | log | OFF | | log_update | OFF | | log_bin | OFF | | log_slave_updates | OFF | | long_query_time | 10 | | low_priority_updates | OFF | | lower_case_table_names | 0 | | max_allowed_packet | 1048576 | | max_binlog_cache_size | 4294967295 | | max_connections | 100 | | max_connect_errors | 10 | | max_delayed_threads | 20 | | max_heap_table_size | 16777216 | | max_join_size | 4294967295 | | max_sort_length | 1024 | | max_tmp_tables | 32 | | max_write_lock_count | 4294967295 | | myisam_recover_options | DEFAULT | | myisam_sort_buffer_size | 8388608 | | net_buffer_length | 16384 | | net_read_timeout | 30 | | net_retry_count | 10 | | net_write_timeout | 60 | | open_files_limit | 0 | | pid_file | /my/monty/data/donna.pid | | port | 3306 | | protocol_version | 10 | | record_buffer | 131072 | | query_buffer_size | 0 | | safe_show_database | OFF | | server_id | 0 | | skip_locking | ON | | skip_networking | OFF | | skip_show_database | OFF | | slow_launch_time | 2 | | socket | /tmp/mysql.sock | | sort_buffer | 2097116 | | table_cache | 64 | | table_type | MYISAM | | thread_cache_size | 4 | | thread_stack | 65536 | | tmp_table_size | 1048576 | | tmpdir | /tmp/ | | version | 3.23.29a-gamma-debug | | wait_timeout | 28800 | +-------------------------+---------------------------+ Each option is described below. Values for buffer sizes, lengths, and stack sizes are given in bytes. You can specify values with a suffix of `K' or `M' to indicate kilobytes or megabytes. For example, `16M' indicates 16 megabytes. The case of suffix letters does not matter; `16M' and `16m' are equivalent: ``ansi_mode'.' Is `ON' if `mysqld' was started with `--ansi'. *Note ANSI mode::. ``back_log'' The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets *VERY* many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The `back_log' value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time. In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix `listen(2)' system call should have more details. Check your OS documentation for the maximum value for this variable. Attempting to set `back_log' higher than your operating system limit will be ineffective. ``basedir'' The value of the `--basedir' option. ``bdb_cache_size'' The buffer that is allocated to cache index and rows for `BDB' tables. If you don't use `BDB' tables, you should start `mysqld' with `--skip-bdb' to not waste memory for this cache. ``bdb_log_buffer_size'' The buffer that is allocated to cache index and rows for `BDB' tables. If you don't use `BDB' tables, you should set this to 0 or start `mysqld' with `--skip-bdb' to not waste memory for this cache. ``bdb_home'' The value of the `--bdb-home' option. ``bdb_max_lock'' The maximum number of locks (1000 by default) you can have active on a BDB table. You should increase this if you get errors of type `bdb: Lock table is out of available locks' or `Got error 12 from ...' when you have do long transactions or when `mysqld' has to examine a lot of rows to calculate the query. ``bdb_logdir'' The value of the `--bdb-logdir' option. ``bdb_shared_data'' Is `ON' if you are using `--bdb-shared-data'. ``bdb_tmpdir'' The value of the `--bdb-tmpdir' option. ``binlog_cache_size'. The size of the cache to hold the SQL' statements for the binary log during a transaction. If you often use big, multi-statement transactions you can increase this to get more performance. *Note COMMIT::. ``character_set'' The default character set. ``character_sets'' The supported character sets. ``concurrent_inserts'' If `ON' (the default), MySQL will allow you to use `INSERT' on `MyISAM' tables at the same time as you run `SELECT' queries on them. You can turn this option off by starting `mysqld' with `--safe' or `--skip-new'. ``connect_timeout'' The number of seconds the `mysqld' server is waiting for a connect packet before responding with `Bad handshake'. ``datadir'' The value of the `--datadir' option. ``delay_key_write'' If enabled (is on by default), MySQL will honor the `delay_key_write' option `CREATE TABLE'. This means that the key buffer for tables with this option will not get flushed on every index update, but only when a table is closed. This will speed up writes on keys a lot, but you should add automatic checking of all tables with `myisamchk --fast --force' if you use this. Note that if you start `mysqld' with the `--delay-key-write-for-all-tables' option this means that all tables will be treated as if they were created with the `delay_key_write' option. You can clear this flag by starting `mysqld' with `--skip-new' or `--safe-mode'. ``delayed_insert_limit'' After inserting `delayed_insert_limit' rows, the `INSERT DELAYED' handler will check if there are any `SELECT' statements pending. If so, it allows these to execute before continuing. ``delayed_insert_timeout'' How long a `INSERT DELAYED' thread should wait for `INSERT' statements before terminating. ``delayed_queue_size'' What size queue (in rows) should be allocated for handling `INSERT DELAYED'. If the queue becomes full, any client that does `INSERT DELAYED' will wait until there is room in the queue again. ``flush'' This is `ON' if you have started MySQL with the `--flush' option. ``flush_time'' If this is set to a non-zero value, then every `flush_time' seconds all tables will be closed (to free up resources and sync things to disk). We only recommend this option on Win95, Win98, or on systems where you have very little resources. ``have_bdb'' `YES' if `mysqld' supports Berkeley DB tables. `DISABLED' if `--skip-bdb' is used. ``have_innodb'' `YES' if `mysqld' supports InnoDB tables. `DISABLED' if `--skip-innodb' is used. ``have_raid'' `YES' if `mysqld' supports the `RAID' option. ``have_ssl'' `YES' if `mysqld' supports SSL (encryption) on the client/server protocol. ``init_file'' The name of the file specified with the `--init-file' option when you start the server. This is a file of SQL statements you want the server to execute when it starts. ``interactive_timeout'' The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the `CLIENT_INTERACTIVE' option to `mysql_real_connect()'. See also `wait_timeout'. ``join_buffer_size'' The size of the buffer that is used for full joins (joins that do not use indexes). The buffer is allocated one time for each full join between two tables. Increase this value to get a faster full join when adding indexes is not possible. (Normally the best way to get fast joins is to add indexes.) ``key_buffer_size'' Index blocks are buffered and are shared by all threads. `key_buffer_size' is the size of the buffer used for index blocks. Increase this to get better index handling (for all reads and multiple writes) to as much as you can afford; 64M on a 256M machine that mainly runs MySQL is quite common. If you, however, make this too big (more than 50% of your total memory?) your system may start to page and become REALLY slow. Remember that because MySQL does not cache data read, that you will have to leave some room for the OS filesystem cache. You can check the performance of the key buffer by doing `show status' and examine the variables `Key_read_requests', `Key_reads', `Key_write_requests', and `Key_writes'. The `Key_reads/Key_read_request' ratio should normally be < 0.01. The `Key_write/Key_write_requests' is usually near 1 if you are using mostly updates/deletes but may be much smaller if you tend to do updates that affect many at the same time or if you are using `delay_key_write'. *Note SHOW::. To get even more speed when writing many rows at the same time, use `LOCK TABLES'. *Note `LOCK TABLES': LOCK TABLES. ``language'' The language used for error messages. ``large_file_support'' If `mysqld' was compiled with options for big file support. ``locked_in_memory'' If `mysqld' was locked in memory with `--memlock' ``log'' If logging of all queries is enabled. ``log_update'' If the update log is enabled. ``log_bin'' If the binary log is enabled. ``log_slave_updates'' If the updates from the slave should be logged. ``long_query_time'' If a query takes longer than this (in seconds), the `Slow_queries' counter will be incremented. If you are using `--log-slow-queries', the query will be logged to the slow query logfile. *Note Slow query log::. ``lower_case_table_names'' If set to 1 table names are stored in lowercase on disk and table names will be case-insensitive. *Note Name case sensitivity::. ``max_allowed_packet'' The maximum size of one packet. The message buffer is initialized to `net_buffer_length' bytes, but can grow up to `max_allowed_packet' bytes when needed. This value by default is small, to catch big (possibly wrong) packets. You must increase this value if you are using big `BLOB' columns. It should be as big as the biggest `BLOB' you want to use. The current protocol limits `max_allowed_packet' to 16M. ``max_binlog_cache_size'' If a multi-statement transaction requires more than this amount of memory, one will get the error "Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage". ``max_binlog_size'' Available after 3.23.33. If a write to the binary (replication) log exceeds the given value, rotate the logs. You cannot set it to less than 1024 bytes, or more than 1 GB. Default is 1 GB. ``max_connections'' The number of simultaneous clients allowed. Increasing this value increases the number of file descriptors that `mysqld' requires. See below for comments on file descriptor limits. *Note Too many connections::. ``max_connect_errors'' If there is more than this number of interrupted connections from a host this host will be blocked from further connections. You can unblock a host with the command `FLUSH HOSTS'. ``max_delayed_threads'' Don't start more than this number of threads to handle `INSERT DELAYED' statements. If you try to insert data into a new table after all `INSERT DELAYED' threads are in use, the row will be inserted as if the `DELAYED' attribute wasn't specified. ``max_heap_table_size'' Don't allow creation of heap tables bigger than this. ``max_join_size'' Joins that are probably going to read more than `max_join_size' records return an error. Set this value if your users tend to perform joins that lack a `WHERE' clause, that take a long time, and that return millions of rows. ``max_sort_length'' The number of bytes to use when sorting `BLOB' or `TEXT' values (only the first `max_sort_length' bytes of each value are used; the rest are ignored). ``max_user_connections'' The maximum number of active connections for a single user (0 = no limit). ``max_tmp_tables'' (This option doesn't yet do anything.) Maximum number of temporary tables a client can keep open at the same time. ``max_write_lock_count'' After this many write locks, allow some read locks to run in between. ``myisam_recover_options'' The value of the `--myisam-recover' option. ``myisam_sort_buffer_size'' The buffer that is allocated when sorting the index when doing a `REPAIR' or when creating indexes with `CREATE INDEX' or `ALTER TABLE'. ``myisam_max_extra_sort_file_size'.' If the creating of the temporary file for fast index creation would be this much bigger than using the key cache, then prefer the key cache method. This is mainly used to force long character keys in large tables to use the slower key cache method to create the index. *NOTE* that this parameter is given in megabytes! ``myisam_max_sort_file_size'' The maximum size of the temporary file MySQL is allowed to use while recreating the index (during `REPAIR', `ALTER TABLE' or `LOAD DATA INFILE'. If the file size would be bigger than this, the index will be created through the key cache (which is slower). *NOTE* that this parameter is given in megabytes! ``net_buffer_length'' The communication buffer is reset to this size between queries. This should not normally be changed, but if you have very little memory, you can set it to the expected size of a query. (That is, the expected length of SQL statements sent by clients. If statements exceed this length, the buffer is automatically enlarged, up to `max_allowed_packet' bytes.) ``net_read_timeout'' Number of seconds to wait for more data from a connection before aborting the read. Note that when we don't expect data from a connection, the timeout is defined by `write_timeout'. See also `slave_read_timeout'. ``net_retry_count'' If a read on a communication port is interrupted, retry this many times before giving up. This value should be quite high on `FreeBSD' as internal interrupts are sent to all threads. ``net_write_timeout'' Number of seconds to wait for a block to be written to a connection before aborting the write. ``open_files_limit'' If this is not 0, then `mysqld' will use this value to reserve file descriptors to use with `setrlimit()'. If this value is 0 then `mysqld' will reserve `max_connections*5' or `max_connections + table_cache*2' (whichever is larger) number of files. You should try increasing this if `mysqld' gives you the error 'Too many open files'. ``pid_file'' The value of the `--pid-file' option. ``port'' The value of the `--port' option. ``protocol_version'' The protocol version used by the MySQL server. ``record_buffer'' Each thread that does a sequential scan allocates a buffer of this size for each table it scans. If you do many sequential scans, you may want to increase this value. ``record_rnd_buffer'' When reading rows in sorted order after a sort, the rows are read through this buffer to avoid a disk seeks. If not set, then it's set to the value of `record_buffer'. ``query_buffer_size'' The initial allocation of the query buffer. If most of your queries are long (like when inserting blobs), you should increase this! ``safe_show_databases'' Don't show databases for which the user doesn't have any database or table privileges. This can improve security if you're concerned about people being able to see what databases other users have. See also `skip_show_databases'. ``server_id'' The value of the `--server-id' option. ``skip_locking'' Is OFF if `mysqld' uses external locking. ``skip_networking'' Is ON if we only allow local (socket) connections. ``skip_show_databases'' This prevents people from doing `SHOW DATABASES' if they don't have the `PROCESS_PRIV' privilege. This can improve security if you're concerned about people being able to see what databases other users have. See also `safe_show_databases'. ``slave_read_timeout'' Number of seconds to wait for more data from a master/slave connection before aborting the read. ``slow_launch_time'' If creating the thread takes longer than this value (in seconds), the `Slow_launch_threads' counter will be incremented. ``socket'' The Unix socket used by the server. ``sort_buffer'' Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster `ORDER BY' or `GROUP BY' operations. *Note Temporary files::. ``table_cache'' The number of open tables for all threads. Increasing this value increases the number of file descriptors that `mysqld' requires. MySQL needs two file descriptors for each unique open table. See below for comments on file descriptor limits. You can check if you need to increase the table cache by checking the `Opened_tables' variable. *Note SHOW::. If this variable is big and you don't do `FLUSH TABLES' a lot (which just forces all tables to be closed and reopenend), then you should increase the value of this variable. Make sure that your operating system can handle the number of open file descriptors implied by the `table_cache' setting. If `table_cache' is set too high, MySQL may run out of file descriptors and refuse connections, fail to perform queries, and be very unreliable. For information about how the table cache works, see *Note Table cache::. ``table_type'' The default table type ``thread_cache_size'' How many threads we should keep in a cache for reuse. When a client disconnects, the client's threads are put in the cache if there aren't more than `thread_cache_size' threads from before. All new threads are first taken from the cache, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. (Normally this doesn't give a notable performance improvement if you have a good thread implementation.) By examing the difference between the `Connections' and `Threads_created' you can see how efficient the current thread cache is for you. ``thread_concurrency'' On Solaris, `mysqld' will call `thr_setconcurrency()' with this value. `thr_setconcurrency()' permits the application to give the threads system a hint for the desired number of threads that should be run at the same time. ``thread_stack'' The stack size for each thread. Many of the limits detected by the `crash-me' test are dependent on this value. The default is large enough for normal operation. *Note MySQL Benchmarks::. ``timezone'' The timezone for the server. ``tmp_table_size'' If an in-memory temporary table exceeds this size, MySQL will automatically convert it to an on-disk `MyISAM' table. Increase the value of `tmp_table_size' if you do many advanced `GROUP BY' queries and you have lots of memory. ``tmpdir'' The directory used for temporary files and temporary tables. ``version'' The version number for the server. ``wait_timeout'' The number of seconds the server waits for activity on a connection before closing it. See also `interactive_timeout'. The manual section that describes tuning MySQL contains some information of how to tune the above variables. *Note Server parameters::. `SHOW LOGS' ........... `SHOW LOGS' shows you status information about existing log files. It currently only displays information about Berkeley DB log files. * `File' shows the full path to the log file * `Type' shows the type of the log file (`BDB' for Berkeley DB log files) * `Status' shows the status of the log file (`FREE' if the file can be removed, or `IN USE' if the file is needed by the transaction subsystem) `SHOW PROCESSLIST' .................. `SHOW PROCESSLIST' shows you which threads are running. You can also get this information using the `mysqladmin processlist' command. If you have the *process* privilege, you can see all threads. Otherwise, you can see only your own threads. *Note `KILL': KILL. If you don't use the `FULL' option, then only the first 100 characters of each query will be shown. This command is very useful if you get the 'too many connections' error message and want to find out what's going on. MySQL reserves one extra connection for a client with the `Process_priv' privilege to ensure that you should always be able to login and check the system (assuming you are not giving this privilege to all your users). `SHOW GRANTS' ............. `SHOW GRANTS FOR user' lists the grant commands that must be issued to duplicate the grants for a user. mysql> SHOW GRANTS FOR root@localhost; +---------------------------------------------------------------------+ | Grants for root@localhost | +---------------------------------------------------------------------+ | GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' WITH GRANT OPTION | +---------------------------------------------------------------------+ `SHOW CREATE TABLE' ................... Shows a `CREATE TABLE' statement that will create the given table: mysql> show create table t\G *************************** 1. row *************************** Table: t Create Table: CREATE TABLE t ( id int(11) default NULL auto_increment, s char(60) default NULL, PRIMARY KEY (id) ) TYPE=MyISAM `SHOW CREATE TABLE' will quote table and column names according to `SQL_QUOTE_SHOW_CREATE' option. *Note `SET OPTION SQL_QUOTE_SHOW_CREATE': SET OPTION. MySQL Localization and International Usage ========================================== The Character Set Used for Data and Sorting ------------------------------------------- By default, MySQL uses the ISO-8859-1 (Latin1) character set with sorting according to Swedish/Finnish. This is the character set suitable in the USA and western Europe. All standard MySQL binaries are compiled with `--with-extra-charsets=complex'. This will add code to all standard programs to be able to handle `latin1' and all multi-byte character sets within the binary. Other character sets will be loaded from a character-set definition file when needed. The character set determines what characters are allowed in names and how things are sorted by the `ORDER BY' and `GROUP BY' clauses of the `SELECT' statement. You can change the character set with the `--default-character-set' option when you start the server. The character sets available depend on the `--with-charset=charset' and `--with-extra-charset= list-of-charset | complex | all' options to `configure', and the character set configuration files listed in `SHAREDIR/charsets/Index'. *Note configure options::. If you change the character set when running MySQL (which may also change the sort order), you must run myisamchk -r -q on all tables. Otherwise your indexes may not be ordered correctly. When a client connects to a MySQL server, the server sends the default character set in use to the client. The client will switch to use this character set for this connection. One should use `mysql_real_escape_string()' when escaping strings for a SQL query. `mysql_real_escape_string()' is identical to the old `mysql_escape_string()' function, except that it takes the MYSQL connection handle as the first parameter. If the client is compiled with different paths than where the server is installed and the user who configured MySQL didn't included all character sets in the MySQL binary, one must specify for the client where it can find the additional character sets it will need if the server runs with a different character set than the client. One can specify this by putting in a MySQL option file: [client] character-sets-dir=/usr/local/mysql/share/mysql/charsets where the path points to where the dynamic MySQL character sets are stored. One can force the client to use specific character set by specifying: [client] default-character-set=character-set-name but normally this is never needed. Non-English Error Messages -------------------------- `mysqld' can issue error messages in the following languages: Czech, Danish, Dutch, English (the default), Estonian, French, German, Greek, Hungarian, Italian, Japanese, Korean, Norwegian, Norwegian-ny, Polish, Portuguese, Romanian, Russian, Slovak, Spanish, and Swedish. To start `mysqld' with a particular language, use either the `--language=lang' or `-L lang' options. For example: shell> mysqld --language=swedish or: shell> mysqld --language=/usr/local/share/swedish Note that all language names are specified in lowercase. The language files are located (by default) in `MYSQL_BASE_DIR/share/LANGUAGE/'. To update the error message file, you should edit the `errmsg.txt' file and execute the following command to generate the `errmsg.sys' file: shell> comp_err errmsg.txt errmsg.sys If you upgrade to a newer version of MySQL, remember to repeat your changes with the new `errmsg.txt' file. Adding a New Character Set -------------------------- To add another character set to MySQL, use the following procedure. Decide if the set is simple or complex. If the character set does not need to use special string collating routines for sorting and does not need multi-byte character support, it is simple. If it needs either of those features, it is complex. For example, `latin1' and `danish' are simple charactersets while `big5' or `czech' are complex character sets. In the following section, we have assumed that you name your character set `MYSET'. For a simple character set do the following: 1. Add MYSET to the end of the `sql/share/charsets/Index' file Assign an unique number to it. 2. Create the file `sql/share/charsets/MYSET.conf'. (You can use `sql/share/charsets/latin1.conf' as a base for this). The syntax for the file very simple: * Comments start with a '#' character and proceed to the end of the line. * Words are separated by arbitrary amounts of whitespace. * When defining the character set, every word must be a number in hexadecimal format * The `ctype' array takes up the first 257 words. The `to_lower', `to_upper' and `sort_order' arrays take up 256 words each after that. *Note Character arrays::. 3. Add the character set name to the `CHARSETS_AVAILABLE' and `COMPILED_CHARSETS' lists in `configure.in'. 4. Reconfigure, recompile, and test. For a complex character set do the following: 1. Create the file `strings/ctype-MYSET.c' in the MySQL source distribution. 2. Add MYSET to the end of the `sql/share/charsets/Index' file. Assign an unique number to it. 3. Look at one of the existing `ctype-*.c' files to see what needs to be defined, for example `strings/ctype-big5.c'. Note that the arrays in your file must have names like `ctype_MYSET', `to_lower_MYSET', and so on. This corresponds to the arrays in the simple character set. *Note Character arrays::. For a complex character set 4. Near the top of the file, place a special comment like this: /* * This comment is parsed by configure to create ctype.c, * so don't change it unless you know what you are doing. * * .configure. number_MYSET=MYNUMBER * .configure. strxfrm_multiply_MYSET=N * .configure. mbmaxlen_MYSET=N */ The `configure' program uses this comment to include the character set into the MySQL library automatically. The strxfrm_multiply and mbmaxlen lines will be explained in the following sections. Only include them if you the string collating functions or the multi-byte character set functions, respectively. 5. You should then create some of the following functions: * `my_strncoll_MYSET()' * `my_strcoll_MYSET()' * `my_strxfrm_MYSET()' * `my_like_range_MYSET()' *Note String collating::. 6. Add the character set name to the `CHARSETS_AVAILABLE' and `COMPILED_CHARSETS' lists in `configure.in'. 7. Reconfigure, recompile, and test. The file `sql/share/charsets/README' includes some more instructions. If you want to have the character set included in the MySQL distribution, mail a patch to <internals@lists.mysql.com>. The character definition arrays ------------------------------- `to_lower[]' and `to_upper[]' are simple arrays that hold the lowercase and uppercase characters corresponding to each member of the character set. For example: to_lower['A'] should contain 'a' to_upper['a'] should contain 'A' `sort_order[]' is a map indicating how characters should be ordered for comparison and sorting purposes. For many character sets, this is the same as `to_upper[]' (which means sorting will be case insensitive). MySQL will sort characters based on the value of `sort_order[character]'. For more complicated sorting rules, see the discussion of string collating below. *Note String collating::. `ctype[]' is an array of bit values, with one element for one character. (Note that `to_lower[]', `to_upper[]', and `sort_order[]' are indexed by character value, but `ctype[]' is indexed by character value + 1. This is an old legacy to be able to handle EOF.) You can find the following bitmask definitions in `m_ctype.h': #define _U 01 /* Uppercase */ #define _L 02 /* Lowercase */ #define _N 04 /* Numeral (digit) */ #define _S 010 /* Spacing character */ #define _P 020 /* Punctuation */ #define _C 040 /* Control character */ #define _B 0100 /* Blank */ #define _X 0200 /* heXadecimal digit */ The `ctype[]' entry for each character should be the union of the applicable bitmask values that describe the character. For example, `'A'' is an uppercase character (`_U') as well as a hexadecimal digit (`_X'), so `ctype['A'+1]' should contain the value: _U + _X = 01 + 0200 = 0201 String Collating Support ------------------------ If the sorting rules for your language are too complex to be handled with the simple `sort_order[]' table, you need to use the string collating functions. Right now the best documentation on this is the character sets that are already implemented. Look at the big5, czech, gbk, sjis, and tis160 character sets for examples. You must specify the `strxfrm_multiply_MYSET=N' value in the special comment at the top of the file. `N' should be set to the maximum ratio the strings may grow during `my_strxfrm_MYSET' (it must be a positive integer). Multi-byte Character Support ---------------------------- If your want to add support for a new character set that includes multi-byte characters, you need to use the multi-byte character functions. Right now the best documentation on this is the character sets that are already implemented. Look at the euc_kr, gb2312, gbk, sjis and ujis character sets for examples. These are implemented in the `ctype-'charset'.c' files in the `strings' directory. You must specify the `mbmaxlen_MYSET=N' value in the special comment at the top of the source file. `N' should be set to the size in bytes of the largest character in the set. MySQL Server-Side Scripts and Utilities ======================================= Overview of the Server-Side Scripts and Utilities ------------------------------------------------- All MySQL clients that communicate with the server using the `mysqlclient' library use the following environment variables: *Name* *Description* `MYSQL_UNIX_PORT' The default socket; used for connections to `localhost' `MYSQL_TCP_PORT' The default TCP/IP port `MYSQL_PWD' The default password `MYSQL_DEBUG' Debug-trace options when debugging `TMPDIR' The directory where temporary tables/files are created Use of `MYSQL_PWD' is insecure. *Note Connecting::. The `mysql' client uses the file named in the `MYSQL_HISTFILE' environment variable to save the command-line history. The default value for the history file is `$HOME/.mysql_history', where `$HOME' is the value of the `HOME' environment variable. *Note Environment variables::. All MySQL programs take many different options. However, every MySQL program provides a `--help' option that you can use to get a full description of the program's different options. For example, try `mysql --help'. You can override default options for all standard client programs with an option file. *Note Option files::. The list below briefly describes the MySQL programs: `myisamchk' Utility to describe, check, optimize, and repair MySQL tables. Because `myisamchk' has many functions, it is described in its own chapter. *Note MySQL Database Administration::. `make_binary_distribution' Makes a binary release of a compiled MySQL. This could be sent by FTP to `/pub/mysql/Incoming' on `support.mysql.com' for the convenience of other MySQL users. `msql2mysql' A shell script that converts `mSQL' programs to MySQL. It doesn't handle all cases, but it gives a good start when converting. `mysqlaccess' A script that checks the access privileges for a host, user, and database combination. `mysqladmin' Utility for performing administrative operations, such as creating or dropping databases, reloading the grant tables, flushing tables to disk, and reopening log files. `mysqladmin' can also be used to retrieve version, process, and status information from the server. *Note `mysqladmin': mysqladmin. `mysqlbug' The MySQL bug report script. This script should always be used when filing a bug report to the MySQL list. `mysqld' The SQL daemon. This should always be running. `mysqldump' Dumps a MySQL database into a file as SQL statements or as tab-separated text files. Enhanced freeware originally by Igor Romanenko. *Note `mysqldump': mysqldump. `mysqlimport' Imports text files into their respective tables using `LOAD DATA INFILE'. *Note `mysqlimport': mysqlimport. `mysqlshow' Displays information about databases, tables, columns, and indexes. `mysql_install_db' Creates the MySQL grant tables with default privileges. This is usually executed only once, when first installing MySQL on a system. `replace' A utility program that is used by `msql2mysql', but that has more general applicability as well. `replace' changes strings in place in files or on the standard input. Uses a finite state machine to match longer strings first. Can be used to swap strings. For example, this command swaps `a' and `b' in the given files: shell> replace a b b a -- file1 file2 ... safe_mysqld, the wrapper around mysqld -------------------------------------- `safe_mysqld' is the recommended way to start a `mysqld' daemon on Unix. `safe_mysqld' adds some safety features such as restarting the server when an error occurs and logging run-time information to a log file. If you don't use `--mysqld=#' or `--mysqld-version=#' `safe_mysqld' will use an executable named `mysqld-max' if it exists. If not, `safe_mysqld' will start `mysqld'. This makes it very easy to test to use `mysqld-max' instead of `mysqld'; Just copy `mysqld-max' to where you have `mysqld' and it will be used. Normally one should never edit the `safe_mysqld' script, but instead put the options to `safe_mysqld' in the `[safe_mysqld]' section in the `my.cnf' file. `safe_mysqld' will read all options from the `[mysqld]', `[server]' and `[safe_mysqld]' sections from the option files. *Note Option files::. Note that all options on the command line to `safe_mysqld' are passed to `mysqld'. If you wants to use any options in `safe_mysqld' that `mysqld' doesn't support, you must specify these in the option file. Most of the options to `safe_mysqld' are the same as the options to `mysqld'. *Note Command-line options::. `safe_mysqld' supports the following options: `--basedir=path' `--core-file-size=#' Size of the core file `mysqld' should be able to create. Passed to `ulimit -c'. `--datadir=path' `--defaults-extra-file=path' `--defaults-file=path' `--err-log=path' `--ledir=path' Path to `mysqld' `--log=path' `--mysqld=mysqld-version' Name of the `mysqld' version in the `ledir' directory you want to start. `--mysqld-version=version' Similar to `--mysqld=' but here you only give the suffix for `mysqld'. For example if you use `--mysqld-version=max', `safe_mysqld' will start the `ledir/mysqld-max' version. If the argument to `--mysqld-version' is empty, `ledir/mysqld' will be used. `--no-defaults' `--open-files-limit=#' Number of files `mysqld' should be able to open. Passed to `ulimit -n'. Note that you need to start `safe_mysqld' as root for this to work properly! `--pid-file=path' `--port=#' `--socket=path' `--timezone=#' Set the timezone (the `TZ') variable to the value of this parameter. `--user=#' The `safe_mysqld' script is written so that it normally is able to start a server that was installed from either a source or a binary version of MySQL, even if these install the server in slightly different locations. `safe_mysqld' expects one of these conditions to be true: * The server and databases can be found relative to the directory from which `safe_mysqld' is invoked. `safe_mysqld' looks under its working directory for `bin' and `data' directories (for binary distributions) or for `libexec' and `var' directories (for source distributions). This condition should be met if you execute `safe_mysqld' from your MySQL installation directory (for example, `/usr/local/mysql' for a binary distribution). * If the server and databases cannot be found relative to the working directory, `safe_mysqld' attempts to locate them by absolute pathnames. Typical locations are `/usr/local/libexec' and `/usr/local/var'. The actual locations are determined when the distribution was built from which `safe_mysqld' comes. They should be correct if MySQL was installed in a standard location. Because `safe_mysqld' will try to find the server and databases relative to its own working directory, you can install a binary distribution of MySQL anywhere, as long as you start `safe_mysqld' from the MySQL installation directory: shell> cd mysql_installation_directory shell> bin/safe_mysqld & If `safe_mysqld' fails, even when invoked from the MySQL installation directory, you can modify it to use the path to `mysqld' and the pathname options that are correct for your system. Note that if you upgrade MySQL in the future, your modified version of `safe_mysqld' will be overwritten, so you should make a copy of your edited version that you can reinstall. mysqld_multi, program for managing multiple MySQL servers --------------------------------------------------------- `mysqld_multi' is meant for managing several `mysqld' processes running in different UNIX sockets and TCP/IP ports. The program will search for group(s) named [mysqld#] from my.cnf (or the given -config-file=...), where # can be any positive number starting from 1. These groups should be the same as the usual `[mysqld]' group (e.g. options to mysqld, see MySQL manual for detailed information about this group), but with those port, socket etc. options that are wanted for each separate `mysqld' processes. The number in the group name has another function; it can be used for starting, stopping, or reporting some specific `mysqld' servers with this program. See the usage and options below for more information. Usage: mysqld_multi [OPTIONS] {start|stop|report} [GNR,GNR,GNR...] or mysqld_multi [OPTIONS] {start|stop|report} [GNR-GNR,GNR,GNR-GNR,...] The GNR above means the group number. You can start, stop or report any GNR, or several of them at the same time. (See -example) The GNRs list can be comma separated, or a dash combined, of which the latter means that all the GNRs between GNR1-GNR2 will be affected. Without GNR argument all the found groups will be either started, stopped, or reported. Note that you must not have any white spaces in the GNR list. Anything after a white space is ignored. `mysqld_multi' supports the following options: `--config-file=...' Alternative config file. NOTE: This will not affect this program's own options (group `[mysqld_multi]'), but only groups [mysqld#]. Without this option everything will be searched from the ordinary my.cnf file. `--example' Give an example of a config file. `--help' Print this help and exit. `--log=...' Log file. Full path to and the name for the log file. NOTE: If the file exists, everything will be appended. `--mysqladmin=...' `mysqladmin' binary to be used for a server shutdown. `--mysqld=...' `mysqld' binary to be used. Note that you can give `safe_mysqld' to this option also. The options are passed to `mysqld'. Just make sure you have `mysqld' in your environment variable `PATH' or fix `safe_mysqld'. `--no-log' Print to stdout instead of the log file. By default the log file is turned on. `--password=...' Password for user for `mysqladmin'. `--tcp-ip' Connect to the MySQL server(s) via the TCP/IP port instead of the UNIX socket. This affects stopping and reporting. If a socket file is missing, the server may still be running, but can be accessed only via the TCP/IP port. By default connecting is done via the UNIX socket. `--user=...' MySQL user for `mysqladmin'. `--version' Print the version number and exit. Some notes about `mysqld_multi': * Make sure that the MySQL user, who is stopping the `mysqld' services (e.g using the `mysqladmin') have the same password and username for all the data directories accessed (to the 'mysql' database) And make sure that the user has the 'Shutdown_priv' privilege! If you have many data- directories and many different 'mysql' databases with different passwords for the MySQL 'root' user, you may want to create a common 'multi_admin' user for each using the same password (see below). Example how to do it: shell> mysql -u root -S /tmp/mysql.sock -proot_password -e "GRANT SHUTDOWN ON *.* TO multi_admin@localhost IDENTIFIED BY 'multipass'" *Note Privileges::. You will have to do the above for each `mysqld' running in each data directory, that you have (just change the socket, -S=...) * `pid-file' is very important, if you are using `safe_mysqld' to start `mysqld' (e.g. -mysqld=safe_mysqld) Every `mysqld' should have its own `pid-file'. The advantage using `safe_mysqld' instead of `mysqld' directly here is, that `safe_mysqld' 'guards' every `mysqld' process and will restart it, if a `mysqld' process fails due to signal kill -9, or similar. (Like segmentation fault, which MySQL should never do, of course ;) Please note that `safe_mysqld' script may require that you start it from a certain place. This means that you may have to CD to a certain directory, before you start the `mysqld_multi'. If you have problems starting, please see the `safe_mysqld' script. Check especially the lines: -------------------------------------------------------------------------- MY_PWD=`pwd` Check if we are starting this relative (for the binary release) if test -d /data/mysql -a -f ./share/mysql/english/errmsg.sys -a -x ./bin/mysqld -------------------------------------------------------------------------- *Note `safe_mysqld': safe_mysqld. The above test should be successful, or you may encounter problems. * Beware of the dangers starting multiple `mysqlds' in the same data directory. Use separate data directories, unless you *KNOW* what you are doing! * The socket file and the TCP/IP port must be different for every `mysqld'. * The first and fifth `mysqld' group were intentionally left out from the example. You may have 'gaps' in the config file. This gives you more flexibility. The order in which the `mysqlds' are started or stopped depends on the order in which they appear in the config file. * When you want to refer to a certain group using GNR with this program, just use the number in the end of the group name ( [mysqld# <== ). * You may want to use option '-user' for `mysqld', but in order to do this you need to be root when you start the `mysqld_multi' script. Having the option in the config file doesn't matter; you will just get a warning, if you are not the superuser and the `mysqlds' are started under *YOUR* UNIX account. *IMPORTANT*: Make sure that the `pid-file' and the data directory are read+write(+execute for the latter one) accessible for *THAT* UNIX user, who the specific `mysqld' process is started as. *DON'T* use the UNIX root account for this, unless you *KNOW* what you are doing! * *MOST IMPORTANT*: Make sure that you understand the meanings of the options that are passed to the `mysqlds' and why *WOULD YOU WANT* to have separate `mysqld' processes. Starting multiple `mysqlds' in one data directory *WILL NOT* give you extra performance in a threaded system! *Note Multiple servers::. This is an example of the config file on behalf of `mysqld_multi'. # This file should probably be in your home dir (~/.my.cnf) or /etc/my.cnf # Version 2.1 by Jani Tolonen [mysqld_multi] mysqld = /usr/local/bin/safe_mysqld mysqladmin = /usr/local/bin/mysqladmin user = multi_admin password = multipass [mysqld2] socket = /tmp/mysql.sock2 port = 3307 pid-file = /usr/local/mysql/var2/hostname.pid2 datadir = /usr/local/mysql/var2 language = /usr/local/share/mysql/english user = john [mysqld3] socket = /tmp/mysql.sock3 port = 3308 pid-file = /usr/local/mysql/var3/hostname.pid3 datadir = /usr/local/mysql/var3 language = /usr/local/share/mysql/swedish user = monty [mysqld4] socket = /tmp/mysql.sock4 port = 3309 pid-file = /usr/local/mysql/var4/hostname.pid4 datadir = /usr/local/mysql/var4 language = /usr/local/share/mysql/estonia user = tonu [mysqld6] socket = /tmp/mysql.sock6 port = 3311 pid-file = /usr/local/mysql/var6/hostname.pid6 datadir = /usr/local/mysql/var6 language = /usr/local/share/mysql/japanese user = jani *Note Option files::. myisampack, The MySQL Compressed Read-only Table Generator ---------------------------------------------------------- `myisampack' is used to compress MyISAM tables, and `pack_isam' is used to compress ISAM tables. Because ISAM tables are deprecated, we will only discuss `myisampack' here, but everything said about `myisampack' should also be true for `pack_isam'. `myisampack' works by compressing each column in the table separately. The information needed to decompress columns is read into memory when the table is opened. This results in much better performance when accessing individual records, because you only have to uncompress exactly one record, not a much larger disk block as when using Stacker on MS-DOS. Usually, `myisampack' packs the data file 40%-70%. MySQL uses memory mapping (`mmap()') on compressed tables and falls back to normal read/write file usage if `mmap()' doesn't work. There are currently two limitations with `myisampack': * After packing, the table is read-only. * `myisampack' can also pack `BLOB' or `TEXT' columns. The older `pack_isam' could not do this. Fixing these limitations is on our TODO list but with low priority. `myisampack' is invoked like this: shell> myisampack [options] filename ... Each filename should be the name of an index (`.MYI') file. If you are not in the database directory, you should specify the pathname to the file. It is permissible to omit the `.MYI' extension. `myisampack' supports the following options: `-b, --backup' Make a backup of the table as `tbl_name.OLD'. `-#, --debug=debug_options' Output debug log. The `debug_options' string often is `'d:t:o,filename''. `-f, --force' Force packing of the table even if it becomes bigger or if the temporary file exists. `myisampack' creates a temporary file named `tbl_name.TMD' while it compresses the table. If you kill `myisampack', the `.TMD' file may not be deleted. Normally, `myisampack' exits with an error if it finds that `tbl_name.TMD' exists. With `--force', `myisampack' packs the table anyway. `-?, --help' Display a help message and exit. `-j big_tbl_name, --join=big_tbl_name' Join all tables named on the command line into a single table `big_tbl_name'. All tables that are to be combined MUST be identical (same column names and types, same indexes, etc.). `-p #, --packlength=#' Specify the record length storage size, in bytes. The value should be 1, 2, or 3. (`myisampack' stores all rows with length pointers of 1, 2, or 3 bytes. In most normal cases, `myisampack' can determine the right length value before it begins packing the file, but it may notice during the packing process that it could have used a shorter length. In this case, `myisampack' will print a note that the next time you pack the same file, you could use a shorter record length.) `-s, --silent' Silent mode. Write output only when errors occur. `-t, --test' Don't actually pack table, just test packing it. `-T dir_name, --tmp_dir=dir_name' Use the named directory as the location in which to write the temporary table. `-v, --verbose' Verbose mode. Write information about progress and packing result. `-V, --version' Display version information and exit. `-w, --wait' Wait and retry if table is in use. If the `mysqld' server was invoked with the `--skip-locking' option, it is not a good idea to invoke `myisampack' if the table might be updated during the packing process. The sequence of commands shown below illustrates a typical table compression session: shell> ls -l station.* -rw-rw-r-- 1 monty my 994128 Apr 17 19:00 station.MYD -rw-rw-r-- 1 monty my 53248 Apr 17 19:00 station.MYI -rw-rw-r-- 1 monty my 5767 Apr 17 19:00 station.frm shell> myisamchk -dvv station MyISAM file: station Isam-version: 2 Creation time: 1996-03-13 10:08:58 Recover time: 1997-02-02 3:06:43 Data records: 1192 Deleted blocks: 0 Datafile: Parts: 1192 Deleted data: 0 Datafile pointer (bytes): 2 Keyfile pointer (bytes): 2 Max datafile length: 54657023 Max keyfile length: 33554431 Recordlength: 834 Record format: Fixed length table description: Key Start Len Index Type Root Blocksize Rec/key 1 2 4 unique unsigned long 1024 1024 1 2 32 30 multip. text 10240 1024 1 Field Start Length Type 1 1 1 2 2 4 3 6 4 4 10 1 5 11 20 6 31 1 7 32 30 8 62 35 9 97 35 10 132 35 11 167 4 12 171 16 13 187 35 14 222 4 15 226 16 16 242 20 17 262 20 18 282 20 19 302 30 20 332 4 21 336 4 22 340 1 23 341 8 24 349 8 25 357 8 26 365 2 27 367 2 28 369 4 29 373 4 30 377 1 31 378 2 32 380 8 33 388 4 34 392 4 35 396 4 36 400 4 37 404 1 38 405 4 39 409 4 40 413 4 41 417 4 42 421 4 43 425 4 44 429 20 45 449 30 46 479 1 47 480 1 48 481 79 49 560 79 50 639 79 51 718 79 52 797 8 53 805 1 54 806 1 55 807 20 56 827 4 57 831 4 shell> myisampack station.MYI Compressing station.MYI: (1192 records) - Calculating statistics normal: 20 empty-space: 16 empty-zero: 12 empty-fill: 11 pre-space: 0 end-space: 12 table-lookups: 5 zero: 7 Original trees: 57 After join: 17 - Compressing file 87.14% shell> ls -l station.* -rw-rw-r-- 1 monty my 127874 Apr 17 19:00 station.MYD -rw-rw-r-- 1 monty my 55296 Apr 17 19:04 station.MYI -rw-rw-r-- 1 monty my 5767 Apr 17 19:00 station.frm shell> myisamchk -dvv station MyISAM file: station Isam-version: 2 Creation time: 1996-03-13 10:08:58 Recover time: 1997-04-17 19:04:26 Data records: 1192 Deleted blocks: 0 Datafile: Parts: 1192 Deleted data: 0 Datafilepointer (bytes): 3 Keyfile pointer (bytes): 1 Max datafile length: 16777215 Max keyfile length: 131071 Recordlength: 834 Record format: Compressed table description: Key Start Len Index Type Root Blocksize Rec/key 1 2 4 unique unsigned long 10240 1024 1 2 32 30 multip. text 54272 1024 1 Field Start Length Type Huff tree Bits 1 1 1 constant 1 0 2 2 4 zerofill(1) 2 9 3 6 4 no zeros, zerofill(1) 2 9 4 10 1 3 9 5 11 20 table-lookup 4 0 6 31 1 3 9 7 32 30 no endspace, not_always 5 9 8 62 35 no endspace, not_always, no empty 6 9 9 97 35 no empty 7 9 10 132 35 no endspace, not_always, no empty 6 9 11 167 4 zerofill(1) 2 9 12 171 16 no endspace, not_always, no empty 5 9 13 187 35 no endspace, not_always, no empty 6 9 14 222 4 zerofill(1) 2 9 15 226 16 no endspace, not_always, no empty 5 9 16 242 20 no endspace, not_always 8 9 17 262 20 no endspace, no empty 8 9 18 282 20 no endspace, no empty 5 9 19 302 30 no endspace, no empty 6 9 20 332 4 always zero 2 9 21 336 4 always zero 2 9 22 340 1 3 9 23 341 8 table-lookup 9 0 24 349 8 table-lookup 10 0 25 357 8 always zero 2 9 26 365 2 2 9 27 367 2 no zeros, zerofill(1) 2 9 28 369 4 no zeros, zerofill(1) 2 9 29 373 4 table-lookup 11 0 30 377 1 3 9 31 378 2 no zeros, zerofill(1) 2 9 32 380 8 no zeros 2 9 33 388 4 always zero 2 9 34 392 4 table-lookup 12 0 35 396 4 no zeros, zerofill(1) 13 9 36 400 4 no zeros, zerofill(1) 2 9 37 404 1 2 9 38 405 4 no zeros 2 9 39 409 4 always zero 2 9 40 413 4 no zeros 2 9 41 417 4 always zero 2 9 42 421 4 no zeros 2 9 43 425 4 always zero 2 9 44 429 20 no empty 3 9 45 449 30 no empty 3 9 46 479 1 14 4 47 480 1 14 4 48 481 79 no endspace, no empty 15 9 49 560 79 no empty 2 9 50 639 79 no empty 2 9 51 718 79 no endspace 16 9 52 797 8 no empty 2 9 53 805 1 17 1 54 806 1 3 9 55 807 20 no empty 3 9 56 827 4 no zeros, zerofill(2) 2 9 57 831 4 no zeros, zerofill(1) 2 9 The information printed by `myisampack' is described below: `normal' The number of columns for which no extra packing is used. `empty-space' The number of columns containing values that are only spaces; these will occupy 1 bit. `empty-zero' The number of columns containing values that are only binary 0's; these will occupy 1 bit. `empty-fill' The number of integer columns that don't occupy the full byte range of their type; these are changed to a smaller type (for example, an `INTEGER' column may be changed to `MEDIUMINT'). `pre-space' The number of decimal columns that are stored with leading spaces. In this case, each value will contain a count for the number of leading spaces. `end-space' The number of columns that have a lot of trailing spaces. In this case, each value will contain a count for the number of trailing spaces. `table-lookup' The column had only a small number of different values, which were converted to an `ENUM' before Huffman compression. `zero' The number of columns for which all values are zero. `Original trees' The initial number of Huffman trees. `After join' The number of distinct Huffman trees left after joining trees to save some header space. After a table has been compressed, `myisamchk -dvv' prints additional information about each field: `Type' The field type may contain the following descriptors: `constant' All rows have the same value. `no endspace' Don't store endspace. `no endspace, not_always' Don't store endspace and don't do end space compression for all values. `no endspace, no empty' Don't store endspace. Don't store empty values. `table-lookup' The column was converted to an `ENUM'. `zerofill(n)' The most significant `n' bytes in the value are always 0 and are not stored. `no zeros' Don't store zeros. `always zero' 0 values are stored in 1 bit. `Huff tree' The Huffman tree associated with the field. `Bits' The number of bits used in the Huffman tree. After you have run `pack_isam'/`myisampack' you must run `isamchk'/`myisamchk' to re-create the index. At this time you can also sort the index blocks and create statistics needed for the MySQL optimizer to work more efficiently: myisamchk -rq --analyze --sort-index table_name.MYI isamchk -rq --analyze --sort-index table_name.ISM After you have installed the packed table into the MySQL database directory you should do `mysqladmin flush-tables' to force `mysqld' to start using the new table. If you want to unpack a packed table, you can do this with the `--unpack' option to `isamchk' or `myisamchk'. mysqld-max, An extended mysqld server ------------------------------------- `mysqld-max' is the MySQL server (`mysqld') configured with the following configure options: *Option* *Comment* -with-server-suffix=-maxAdd a suffix to the `mysqld' version string. -with-bdb Support for Berkeley DB (BDB) tables -with-innodb Support for InnoDB tables. CFLAGS=-DUSE_SYMDIR Symbolic links support for Windows. You can find the MySQL-max binaries at `http://www.mysql.com/downloads/mysql-max-3.23.html'. The Windows MySQL 3.23 binary distribution includes both the standard `mysqld.exe' binary and the `mysqld-max.exe' binary. `http://www.mysql.com/downloads/mysql-3.23.html'. *Note Windows installation::. Note that as Berkeley DB and InnoDB are not available for all platforms, some of the `Max' binaries may not have support for both of these. You can check which table types are supported by doing the following query: mysql> show variables like "have_%"; +---------------+-------+ | Variable_name | Value | +---------------+-------+ | have_bdb | YES | | have_innodb | NO | | have_isam | YES | | have_raid | NO | | have_ssl | NO | +---------------+-------+ The meaning of the values are: *Value* *Meaning*. YES The option is activated and usable. NO MySQL is not compiled with support for this option. DISABLED The xxxx option is disabled because one started `mysqld' with `--skip-xxxx' or because one didn't start `mysqld' with all needed options to enable the option. In this case the `hostname.err' file should contain a reason for why the option is disabled. *NOTE*: To be able to create InnoDB tables you *MUST* edit your startup options to include at least the `innodb_data_file_path' option. *Note InnoDB start::. To get better performance for BDB tables, you should add some configuration options for these too. *Note BDB start::. `safe_mysqld' will automatically try to start any `mysqld' binary with the `-max' prefix. This makes it very easy to test out a another `mysqld' binary in an existing installation. Just run `configure' with the options you want and then install the new `mysqld' binary as `mysqld-max' in the same directory where your old `mysqld' binary is. *Note `safe_mysqld': safe_mysqld. The `mysqld-max' RPM uses the above mentioned `safe_mysqld' feature. It just installs the `mysqld-max' executable and `safe_mysqld' will automatically use this executable when `safe_mysqld' is restarted. The following table shows which table types our standard *MySQL-Max* binaries includes: *System* *BDB* *InnoDB* AIX 4.3 N Y HP-UX 11.0 N Y Linux-Alpha N Y Linux-Intel Y Y Linux-Ia64 N Y Solaris-intel N Y Solaris-sparc Y Y SCO OSR5 Y Y UnixWare Y Y Windows/NT Y Y MySQL Client-Side Scripts and Utilities ======================================= Overview of the Client-Side Scripts and Utilities ------------------------------------------------- All MySQL clients that communicate with the server using the `mysqlclient' library use the following environment variables: *Name* *Description* `MYSQL_UNIX_PORT' The default socket; used for connections to `localhost' `MYSQL_TCP_PORT' The default TCP/IP port `MYSQL_PWD' The default password `MYSQL_DEBUG' Debug-trace options when debugging `TMPDIR' The directory where temporary tables/files are created Use of `MYSQL_PWD' is insecure. *Note Connecting::. The `mysql' client uses the file named in the `MYSQL_HISTFILE' environment variable to save the command-line history. The default value for the history file is `$HOME/.mysql_history', where `$HOME' is the value of the `HOME' environment variable. *Note Environment variables::. All MySQL programs take many different options. However, every MySQL program provides a `--help' option that you can use to get a full description of the program's different options. For example, try `mysql --help'. You can override default options for all standard client programs with an option file. *Note Option files::. The list below briefly describes the MySQL programs: `myisamchk' Utility to describe, check, optimize, and repair MySQL tables. Because `myisamchk' has many functions, it is described in its own chapter. *Note MySQL Database Administration::. `make_binary_distribution' Makes a binary release of a compiled MySQL. This could be sent by FTP to `/pub/mysql/Incoming' on `support.mysql.com' for the convenience of other MySQL users. `msql2mysql' A shell script that converts `mSQL' programs to MySQL. It doesn't handle all cases, but it gives a good start when converting. `mysqlaccess' A script that checks the access privileges for a host, user, and database combination. `mysqladmin' Utility for performing administrative operations, such as creating or dropping databases, reloading the grant tables, flushing tables to disk, and reopening log files. `mysqladmin' can also be used to retrieve version, process, and status information from the server. *Note `mysqladmin': mysqladmin. `mysqlbug' The MySQL bug report script. This script should always be used when filing a bug report to the MySQL list. `mysqld' The SQL daemon. This should always be running. `mysqldump' Dumps a MySQL database into a file as SQL statements or as tab-separated text files. Enhanced freeware originally by Igor Romanenko. *Note `mysqldump': mysqldump. `mysqlimport' Imports text files into their respective tables using `LOAD DATA INFILE'. *Note `mysqlimport': mysqlimport. `mysqlshow' Displays information about databases, tables, columns, and indexes. `mysql_install_db' Creates the MySQL grant tables with default privileges. This is usually executed only once, when first installing MySQL on a system. `replace' A utility program that is used by `msql2mysql', but that has more general applicability as well. `replace' changes strings in place in files or on the standard input. Uses a finite state machine to match longer strings first. Can be used to swap strings. For example, this command swaps `a' and `b' in the given files: shell> replace a b b a -- file1 file2 ... The Command-line Tool --------------------- `mysql' is a simple SQL shell (with GNU `readline' capabilities). It supports interactive and non-interactive use. When used interactively, query results are presented in an ASCII-table format. When used non-interactively (for example, as a filter), the result is presented in tab-separated format. (The output format can be changed using command-line options.) You can run scripts simply like this: shell> mysql database < script.sql > output.tab If you have problems due to insufficient memory in the client, use the `--quick' option! This forces `mysql' to use `mysql_use_result()' rather than `mysql_store_result()' to retrieve the result set. Using `mysql' is very easy. Just start it as follows: `mysql database' or `mysql --user=user_name --password=your_password database'. Type a SQL statement, end it with `;', `\g', or `\G' and press RETURN/ENTER. `mysql' supports the following options: `-?, --help' Display this help and exit. `-A, --no-auto-rehash' No automatic rehashing. One has to use 'rehash' to get table and field completion. This gives a quicker start of mysql. `-B, --batch' Print results with a tab as separator, each row on a new line. Doesn't use history file. `--character-sets-dir=...' Directory where character sets are located. `-C, --compress' Use compression in server/client protocol. `-#, --debug[=...]' Debug log. Default is 'd:t:o,/tmp/mysql.trace'. `-D, --database=...' Database to use. This is mainly useful in the `my.cnf' file. `--default-character-set=...' Set the default character set. `-e, --execute=...' Execute command and quit. (Output like with -batch) `-E, --vertical' Print the output of a query (rows) vertically. Without this option you can also force this output by ending your statements with `\G'. `-f, --force' Continue even if we get a SQL error. `-g, --no-named-commands' Named commands are disabled. Use \* form only, or use named commands only in the beginning of a line ending with a semicolon (;). Since Version 10.9, the client now starts with this option ENABLED by default! With the -g option, long format commands will still work from the first line, however. `-G, --enable-named-commands' Named commands are *enabled*. Long format commands are allowed as well as shortened \* commands. `-i, --ignore-space' Ignore space after function names. `-h, --host=...' Connect to the given host. `-H, --html' Produce HTML output. `-L, --skip-line-numbers' Don't write line number for errors. Useful when one wants to compare result files that includes error messages `--no-pager' Disable pager and print to stdout. See interactive help (\h) also. `--no-tee' Disable outfile. See interactive help (\h) also. `-n, --unbuffered' Flush buffer after each query. `-N, --skip-column-names' Don't write column names in results. `-O, --set-variable var=option' Give a variable a value. `--help' lists variables. `-o, --one-database' Only update the default database. This is useful for skipping updates to other database in the update log. ``--pager[=...]'' Output type. Default is your `ENV' variable `PAGER'. Valid pagers are less, more, cat [> filename], etc. See interactive help (\h) also. This option does not work in batch mode. Pager works only in UNIX. `-p[password], --password[=...]' Password to use when connecting to server. If a password is not given on the command line, you will be prompted for it. Note that if you use the short form `-p' you can't have a space between the option and the password. `-P --port=...' TCP/IP port number to use for connection. `-q, --quick' Don't cache result, print it row-by-row. This may slow down the server if the output is suspended. Doesn't use history file. `-r, --raw' Write column values without escape conversion. Used with `--batch' `-s, --silent' Be more silent. `-S --socket=...' Socket file to use for connection. `-t --table' Output in table format. This is default in non-batch mode. `-T, --debug-info' Print some debug information at exit. `--tee=...' Append everything into outfile. See interactive help (\h) also. Does not work in batch mode. `-u, --user=#' User for login if not current user. `-U, --safe-updates[=#], --i-am-a-dummy[=#]' Only allow `UPDATE' and `DELETE' that uses keys. See below for more information about this option. You can reset this option if you have it in your `my.cnf' file by using `--safe-updates=0'. `-v, --verbose' More verbose output (-v -v -v gives the table output format). `-V, --version' Output version information and exit. `-w, --wait' Wait and retry if connection is down instead of aborting. You can also set the following variables with `-O' or `--set-variable': Variable Name Default Description connect_timeout 0 Number of seconds before timeout connection. max_allowed_packet 16777216 Max packetlength to send/receive from to server net_buffer_length 16384 Buffer for TCP/IP and socket communication select_limit 1000 Automatic limit for SELECT when using -i-am-a-dummy max_join_size 1000000 Automatic limit for rows in a join when using -i-am-a-dummy. If you type 'help' on the command line, `mysql' will print out the commands that it supports: mysql> help MySQL commands: help (\h) Display this text. ? (\h) Synonym for `help'. clear (\c) Clear command. connect (\r) Reconnect to the server. Optional arguments are db and host. edit (\e) Edit command with $EDITOR. ego (\G) Send command to mysql server, display result vertically. exit (\q) Exit mysql. Same as quit. go (\g) Send command to mysql server. nopager (\n) Disable pager, print to stdout. notee (\t) Don't write into outfile. pager (\P) Set PAGER [to_pager]. Print the query results via PAGER. print (\p) Print current command. quit (\q) Quit mysql. rehash (\#) Rebuild completion hash. source (\.) Execute a SQL script file. Takes a file name as an argument. status (\s) Get status information from the server. tee (\T) Set outfile [to_outfile]. Append everything into given outfile. use (\u) Use another database. Takes database name as argument. From the above, pager only works in UNIX. The `status' command gives you some information about the connection and the server you are using. If you are running in the `--safe-updates' mode, `status' will also print the values for the `mysql' variables that affect your queries. A useful startup option for beginners (introduced in MySQL Version 3.23.11) is `--safe-updates' (or `--i-am-a-dummy' for users that has at some time done a `DELETE FROM table_name' but forgot the `WHERE' clause). When using this option, `mysql' sends the following command to the MySQL server when opening the connection: SET SQL_SAFE_UPDATES=1,SQL_SELECT_LIMIT=#select_limit#, SQL_MAX_JOIN_SIZE=#max_join_size#" where `#select_limit#' and `#max_join_size#' are variables that can be set from the `mysql' command line. *Note `SET': SET OPTION. The effect of the above is: * You are not allowed to do an `UPDATE' or `DELETE' statement if you don't have a key constraint in the `WHERE' part. One can, however, force an `UPDATE/DELETE' by using `LIMIT': UPDATE table_name SET not_key_column=# WHERE not_key_column=# LIMIT 1; * All big results are automatically limited to `#select_limit#' rows. * `SELECT''s that will probably need to examine more than `#max_join_size' row combinations will be aborted. Some useful hints about the `mysql' client: Some data is much more readable when displayed vertically, instead of the usual horizontal box type output. For example longer text, which includes new lines, is often much easier to be read with vertical output. mysql> select * from mails where length(txt) < 300 limit 300,1\G *************************** 1. row *************************** msg_nro: 3068 date: 2000-03-01 23:29:50 time_zone: +0200 mail_from: Monty reply: monty@no.spam.com mail_to: "Thimble Smith" <tim@no.spam.com> sbj: UTF-8 txt: >>>>> "Thimble" == Thimble Smith writes: Thimble> Hi. I think this is a good idea. Is anyone familiar with UTF-8 Thimble> or Unicode? Otherwise I'll put this on my TODO list and see what Thimble> happens. Yes, please do that. Regards, Monty file: inbox-jani-1 hash: 190402944 1 row in set (0.09 sec) * For logging, you can use the `tee' option. The `tee' can be started with option `--tee=...', or from the command line interactively with command `tee'. All the data displayed on the screen will also be appended into a given file. This can be very useful for debugging purposes also. The `tee' can be disabled from the command line with command `notee'. Executing `tee' again starts logging again. Without a parameter the previous file will be used. Note that `tee' will flush the results into the file after each command, just before the command line appears again waiting for the next command. * Browsing, or searching the results in the interactive mode in UNIX less, more, or any other similar program, is now possible with option `--pager[=...]'. Without argument, `mysql' client will look for environment variable PAGER and set `pager' to that. `pager' can be started from the interactive command line with command `pager' and disabled with command `nopager'. The command takes an argument optionally and the `pager' will be set to that. Command `pager' can be called without an argument, but this requires that the option `--pager' was used, or the `pager' will default to stdout. `pager' works only in UNIX, since it uses the popen() function, which doesn't exist in Windows. In Windows, the `tee' option can be used instead, although it may not be as handy as `pager' can be in some situations. * A few tips about `pager': You can use it to write to a file: mysql> pager cat > /tmp/log.txt and the results will only go to a file. You can also pass any options for the programs that you want to use with the `pager': mysql> pager less -n -i -S From the above do note the option '-S'. You may find it very useful when browsing the results; try the option with horizontal output (end commands with '\g', or ';') and with vertical output (end commands with '\G'). Sometimes a very wide result set is hard to be read from the screen, with option -S to less you can browse the results within the interactive less from left to right, preventing lines longer than your screen from being continued to the next line. This can make the result set much more readable. You can swith the mode between on and off within the interactive less with '-S'. See the 'h' for more help about less. * Last (unless you already understood this from the above examples ;) you can combine very complex ways to handle the results, for example the following would send the results to two files in two different directories, on two different hard-disks mounted on /dr1 and /dr2, yet let the results still be seen on the screen via less: mysql> pager cat | tee /dr1/tmp/res.txt | tee /dr2/tmp/res2.txt | less -n -i -S * You can also combine the two functions above; have the `tee' enabled, `pager' set to 'less' and you will be able to browse the results in unix 'less' and still have everything appended into a file the same time. The difference between `UNIX tee' used with the `pager' and the `mysql' client in-built `tee', is that the in-built `tee' works even if you don't have the `UNIX tee' available. The in-built `tee' also logs everything that is printed on the screen, where the `UNIX tee' used with `pager' doesn't log quite that much. Last, but not least, the interactive `tee' is more handy to switch on and off, when you want to log something into a file, but want to be able to turn the feature off sometimes. mysqladmin, Administrating a MySQL Server ----------------------------------------- A utility for performing administrative operations. The syntax is: shell> mysqladmin [OPTIONS] command [command-option] command ... You can get a list of the options your version of `mysqladmin' supports by executing `mysqladmin --help'. The current `mysqladmin' supports the following commands: `create databasename' Create a new database. `drop databasename' Delete a database and all its tables. `extended-status' Gives an extended status message from the server. `flush-hosts' Flush all cached hosts. `flush-logs' Flush all logs. `flush-tables' Flush all tables. `flush-privileges' Reload grant tables (same as reload). `kill id,id,...' Kill mysql threads. `password' Set a new password. Change old password to new-password. `ping' Check if mysqld is alive. `processlist' Show list of active threads in server. `reload' Reload grant tables. `refresh' Flush all tables and close and open logfiles. `shutdown' Take server down. `slave-start' Start slave replication thread. `slave-stop' Stop slave replication thread. `status' Gives a short status message from the server. `variables' Prints variables available. `version' Get version info from server. All commands can be shortened to their unique prefix. For example: shell> mysqladmin proc stat +----+-------+-----------+----+-------------+------+-------+------+ | Id | User | Host | db | Command | Time | State | Info | +----+-------+-----------+----+-------------+------+-------+------+ | 6 | monty | localhost | | Processlist | 0 | | | +----+-------+-----------+----+-------------+------+-------+------+ Uptime: 10077 Threads: 1 Questions: 9 Slow queries: 0 Opens: 6 Flush tables: 1 Open tables: 2 Memory in use: 1092K Max memory used: 1116K The `mysqladmin status' command result has the following columns: Uptime Number of seconds the MySQL server has been up. Threads Number of active threads (clients). Questions Number of questions from clients since `mysqld' was started. Slow queries Queries that have taken more than `long_query_time' seconds. *Note Slow query log::. Opens How many tables `mysqld' has opened. Flush tables Number of `flush ...', `refresh', and `reload' commands. Open tables Number of tables that are open now. Memory in use Memory allocated directly by the `mysqld' code (only available when MySQL is compiled with -with-debug=full). Max memory used Maximum memory allocated directly by the `mysqld' code (only available when MySQL is compiled with -with-debug=full). If you do `myslqadmin shutdown' on a socket (in other words, on a the computer where `mysqld' is running), `mysqladmin' will wait until the MySQL `pid-file' is removed to ensure that the `mysqld' server has stopped properly. Using `mysqlcheck' for Table Maintenance and Crash Recovery ----------------------------------------------------------- Since MySQL version 3.23.38 you will be able to use a new checking and repairing tool for `MyISAM' tables. The difference to `myisamchk' is that `mysqlcheck' should be used when the `mysqld' server is running, where as `myisamchk' should be used when it is not. The benefit is that you no longer have to take the server down for checking or repairing your tables. `mysqlcheck' uses MySQL server commands `CHECK', `REPAIR', `ANALYZE' and `OPTIMIZE' in a convenient way for the user. There are three alternative ways to invoke `mysqlcheck': shell> mysqlcheck [OPTIONS] database [tables] shell> mysqlcheck [OPTIONS] --databases DB1 [DB2 DB3...] shell> mysqlcheck [OPTIONS] --all-databases So it can be used in a similar way as `mysqldump' when it comes to what databases and tables you want to choose. `mysqlcheck' does have a special feature compared to the other clients; the default behavior, checking tables (-c), can be changed by renaming the binary. So if you want to have a tool that repairs tables by default, you should just copy `mysqlcheck' to your harddrive with a new name, `mysqlrepair', or alternatively make a symbolic link to `mysqlrepair' and name the symbolic link as `mysqlrepair'. If you invoke `mysqlrepair' now, it will repair tables by default. The names that you can use to change `mysqlcheck' default behavior are here: mysqlrepair: The default option will be -r mysqlanalyze: The default option will be -a mysqloptimize: The default option will be -o The options available for `mysqlcheck' are listed here, please check what your version supports with `mysqlcheck --help'. `-A, --all-databases' Check all the databases. This will be same as -databases with all databases selected `-1, --all-in-1' Instead of making one query for each table, execute all queries in 1 query separately for each database. Table names will be in a comma separated list. `-a, --analyze' Analyze given tables. `--auto-repair' If a checked table is corrupted, automatically fix it. Repairing will be done after all tables have been checked, if corrupted ones were found. `-#, --debug=...' Output debug log. Often this is 'd:t:o,filename' `--character-sets-dir=...' Directory where character sets are `-c, --check' Check table for errors `-C, --check-only-changed' Check only tables that have changed since last check or haven't been closed properly. `--compress' Use compression in server/client protocol. `-?, --help' Display this help message and exit. `-B, --databases' To check several databases. Note the difference in usage; In this case no tables are given. All name arguments are regarded as database names. `--default-character-set=...' Set the default character set `-F, --fast' Check only tables that hasn't been closed properly `-f, --force' Continue even if we get an sql-error. `-e, --extended' If you are using this option with CHECK TABLE, it will ensure that the table is 100 percent consistent, but will take a long time. If you are using this option with REPAIR TABLE, it will run an extended repair on the table, which may not only take a long time to execute, but may produce a lot of garbage rows also! `-h, --host=...' Connect to host. `-m, --medium-check' Faster than extended-check, but only finds 99.99 percent of all errors. Should be good enough for most cases. `-o, --optimize' Optimize table `-p, --password[=...]' Password to use when connecting to server. If password is not given it's solicited on the tty. `-P, --port=...' Port number to use for connection. `-q, --quick' If you are using this option with CHECK TABLE, it prevents the check from scanning the rows to check for wrong links. This is the fastest check. If you are using this option with REPAIR TABLE, it will try to repair only the index tree. This is the fastest repair method for a table. `-r, --repair' Can fix almost anything except unique keys that aren't unique. `-s, --silent' Print only error messages. `-S, --socket=...' Socket file to use for connection. `--tables' Overrides option -databases (-B). `-u, --user=#' User for login if not current user. `-v, --verbose' Print info about the various stages. `-V, --version' Output version information and exit. mysqldump, Dumping Table Structure and Data ------------------------------------------- Utility to dump a database or a collection of database for backup or for transferring the data to another SQL server (not necessarily a MySQL server). The dump will contain SQL statements to create the table and/or populate the table. If you are doing a backup on the server, you should consider using the `mysqlhotcopy' instead. *Note `mysqlhotcopy': mysqlhotcopy. shell> mysqldump [OPTIONS] database [tables] OR mysqldump [OPTIONS] --databases [OPTIONS] DB1 [DB2 DB3...] OR mysqldump [OPTIONS] --all-databases [OPTIONS] If you don't give any tables or use the `--databases' or `--all-databases', the whole database(s) will be dumped. You can get a list of the options your version of `mysqldump' supports by executing `mysqldump --help'. Note that if you run `mysqldump' without `--quick' or `--opt', `mysqldump' will load the whole result set into memory before dumping the result. This will probably be a problem if you are dumping a big database. Note that if you are using a new copy of the `mysqldump' program and you are going to do a dump that will be read into a very old MySQL server, you should not use the `--opt' or `-e' options. `mysqldump' supports the following options: `--add-locks' Add `LOCK TABLES' before and `UNLOCK TABLE' after each table dump. (To get faster inserts into MySQL.) `--add-drop-table' Add a `drop table' before each create statement. `-A, --all-databases' Dump all the databases. This will be same as `--databases' with all databases selected. `-a, --all' Include all MySQL-specific create options. `--allow-keywords' Allow creation of column names that are keywords. This works by prefixing each column name with the table name. `-c, --complete-insert' Use complete insert statements (with column names). `-C, --compress' Compress all information between the client and the server if both support compression. `-B, --databases' To dump several databases. Note the difference in usage. In this case no tables are given. All name arguments are regarded as database names. `USE db_name;' will be included in the output before each new database. `--delayed' Insert rows with the `INSERT DELAYED' command. `-e, --extended-insert' Use the new multiline `INSERT' syntax. (Gives more compact and faster inserts statements.) `-#, --debug[=option_string]' Trace usage of the program (for debugging). `--help' Display a help message and exit. `--fields-terminated-by=...' `--fields-enclosed-by=...' `--fields-optionally-enclosed-by=...' `--fields-escaped-by=...' `--lines-terminated-by=...' These options are used with the `-T' option and have the same meaning as the corresponding clauses for `LOAD DATA INFILE'. *Note `LOAD DATA': LOAD DATA. `-F, --flush-logs' Flush log file in the MySQL server before starting the dump. `-f, --force,' Continue even if we get a SQL error during a table dump. `-h, --host=..' Dump data from the MySQL server on the named host. The default host is `localhost'. `-l, --lock-tables.' Lock all tables before starting the dump. The tables are locked with `READ LOCAL' to allow concurrent inserts in the case of `MyISAM' tables. `-n, --no-create-db' 'CREATE DATABASE /*!32312 IF NOT EXISTS*/ db_name;' will not be put in the output. The above line will be added otherwise, if -databases or -all-databases option was given. `-t, --no-create-info' Don't write table creation information (The `CREATE TABLE' statement.) `-d, --no-data' Don't write any row information for the table. This is very useful if you just want to get a dump of the structure for a table! `--opt' Same as `--quick --add-drop-table --add-locks --extended-insert --lock-tables'. Should give you the fastest possible dump for reading into a MySQL server. `-pyour_pass, --password[=your_pass]' The password to use when connecting to the server. If you specify no `=your_pass' part, `mysqldump' you will be prompted for a password. `-P port_num, --port=port_num' The TCP/IP port number to use for connecting to a host. (This is used for connections to hosts other than `localhost', for which Unix sockets are used.) `-q, --quick' Don't buffer query, dump directly to stdout. Uses `mysql_use_result()' to do this. `-r, --result-file=...' Direct output to a given file. This option should be used in MSDOS, because it prevents new line '\n' from being converted to '\n\r' (new line + carriage return). `-S /path/to/socket, --socket=/path/to/socket' The socket file to use when connecting to `localhost' (which is the default host). `--tables' Overrides option -databases (-B). `-T, --tab=path-to-some-directory' Creates a `table_name.sql' file, that contains the SQL CREATE commands, and a `table_name.txt' file, that contains the data, for each give table. *NOTE*: This only works if `mysqldump' is run on the same machine as the `mysqld' daemon. The format of the `.txt' file is made according to the `--fields-xxx' and `--lines--xxx' options. `-u user_name, --user=user_name' The MySQL user name to use when connecting to the server. The default value is your Unix login name. `-O var=option, --set-variable var=option' Set the value of a variable. The possible variables are listed below. `-v, --verbose' Verbose mode. Print out more information on what the program does. `-V, --version' Print version information and exit. `-w, --where='where-condition'' Dump only selected records. Note that QUOTES are mandatory: "--where=user='jimf'" "-wuserid>1" "-wuserid<1" `-O net_buffer_length=#, where # < 16M' When creating multi-row-insert statements (as with option `--extended-insert' or `--opt'), `mysqldump' will create rows up to `net_buffer_length' length. If you increase this variable, you should also ensure that the `max_allowed_packet' variable in the MySQL server is bigger than the `net_buffer_length'. The most normal use of `mysqldump' is probably for making a backup of whole databases. *Note Backup::. mysqldump --opt database > backup-file.sql You can read this back into MySQL with: mysql database < backup-file.sql or mysql -e "source /patch-to-backup/backup-file.sql" database However, it's also very useful to populate another MySQL server with information from a database: mysqldump --opt database | mysql ---host=remote-host -C database It is possible to dump several databases with one command: mysqldump --databases database1 [database2 database3...] > my_databases.sql If all the databases are wanted, one can use: mysqldump --all-databases > all_databases.sql mysqlhotcopy, Copying MySQL Databases and Tables ------------------------------------------------ `mysqlhotcopy' is a perl script that uses `LOCK TABLES', `FLUSH TABLES' and `cp' or `scp' to quickly make a backup of a database. It's the fastest way to make a backup of the database, of single tables but it can only be run on the same machine where the database directories are. mysqlhotcopy db_name [/path/to/new_directory] mysqlhotcopy db_name_1 ... db_name_n /path/to/new_directory mysqlhotcopy db_name./regex/ `mysqlhotcopy' supports the following options: `-?, --help' Display a help screen and exit `-u, --user=#' User for database login `-p, --password=#' Password to use when connecting to server `-P, --port=#' Port to use when connecting to local server `-S, --socket=#' Socket to use when connecting to local server `--allowold' Don't abort if target already exists (rename it _old) `--keepold' Don't delete previous (now renamed) target when done `--noindices' Don't include full index files in copy to make the backup smaller and faster The indexes can later be reconstructed with `myisamchk -rq.'. `--method=#' Method for copy (`cp' or `scp'). `-q, --quiet' Be silent except for errors `--debug' Enable debug `-n, --dryrun' Report actions without doing them `--regexp=#' Copy all databases with names matching regexp `--suffix=#' Suffix for names of copied databases `--checkpoint=#' Insert checkpoint entry into specified db.table `--flushlog' Flush logs once all tables are locked. `--tmpdir=#' Temporary directory (instead of /tmp). You can use `perldoc mysqlhotcopy' to get a more complete documentation for `mysqlhotcopy'. `mysqlhotcopy' reads the groups `[client]' and `[mysqlhotcopy]' from the option files. To be able to execute `mysqlhotcopy' you need write access to the backup directory, `SELECT' privilege to the tables you are about to copy and the MySQL `Reload' privilege (to be able to execute `FLUSH TABLES'). mysqlimport, Importing Data from Text Files ------------------------------------------- `mysqlimport' provides a command-line interface to the `LOAD DATA INFILE' SQL statement. Most options to `mysqlimport' correspond directly to the same options to `LOAD DATA INFILE'. *Note `LOAD DATA': LOAD DATA. `mysqlimport' is invoked like this: shell> mysqlimport [options] database textfile1 [textfile2....] For each text file named on the command line, `mysqlimport' strips any extension from the filename and uses the result to determine which table to import the file's contents into. For example, files named `patient.txt', `patient.text', and `patient' would all be imported into a table named `patient'. `mysqlimport' supports the following options: `-c, --columns=...' This option takes a comma-separated list of field names as an argument. The field list is used to create a proper `LOAD DATA INFILE' command, which is then passed to MySQL. *Note `LOAD DATA': LOAD DATA. `-C, --compress' Compress all information between the client and the server if both support compression. `-#, --debug[=option_string]' Trace usage of the program (for debugging). `-d, --delete' Empty the table before importing the text file. `--fields-terminated-by=...' `--fields-enclosed-by=...' `--fields-optionally-enclosed-by=...' `--fields-escaped-by=...' `--lines-terminated-by=...' These options have the same meaning as the corresponding clauses for `LOAD DATA INFILE'. *Note `LOAD DATA': LOAD DATA. `-f, --force' Ignore errors. For example, if a table for a text file doesn't exist, continue processing any remaining files. Without `--force', `mysqlimport' exits if a table doesn't exist. `--help' Display a help message and exit. `-h host_name, --host=host_name' Import data to the MySQL server on the named host. The default host is `localhost'. `-i, --ignore' See the description for the `--replace' option. `-l, --lock-tables' Lock *ALL* tables for writing before processing any text files. This ensures that all tables are synchronized on the server. `-L, --local' Read input files from the client. By default, text files are assumed to be on the server if you connect to `localhost' (which is the default host). `-pyour_pass, --password[=your_pass]' The password to use when connecting to the server. If you specify no `=your_pass' part, `mysqlimport' you will be prompted for a password. `-P port_num, --port=port_num' The TCP/IP port number to use for connecting to a host. (This is used for connections to hosts other than `localhost', for which Unix sockets are used.) `-r, --replace' The `--replace' and `--ignore' options control handling of input records that duplicate existing records on unique key values. If you specify `--replace', new rows replace existing rows that have the same unique key value. If you specify `--ignore', input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, an error occurs when a duplicate key value is found, and the rest of the text file is ignored. `-s, --silent' Silent mode. Write output only when errors occur. `-S /path/to/socket, --socket=/path/to/socket' The socket file to use when connecting to `localhost' (which is the default host). `-u user_name, --user=user_name' The MySQL user name to use when connecting to the server. The default value is your Unix login name. `-v, --verbose' Verbose mode. Print out more information what the program does. `-V, --version' Print version information and exit. Here is a sample run using `mysqlimport': $ mysql --version mysql Ver 9.33 Distrib 3.22.25, for pc-linux-gnu (i686) $ uname -a Linux xxx.com 2.2.5-15 #1 Mon Apr 19 22:21:09 EDT 1999 i586 unknown $ mysql -e 'CREATE TABLE imptest(id INT, n VARCHAR(30))' test $ ed a 100 Max Sydow 101 Count Dracula . w imptest.txt 32 q $ od -c imptest.txt 0000000 1 0 0 \t M a x S y d o w \n 1 0 0000020 1 \t C o u n t D r a c u l a \n 0000040 $ mysqlimport --local test imptest.txt test.imptest: Records: 2 Deleted: 0 Skipped: 0 Warnings: 0 $ mysql -e 'SELECT * FROM imptest' test +------+---------------+ | id | n | +------+---------------+ | 100 | Max Sydow | | 101 | Count Dracula | +------+---------------+ Showing Databases, Tables, and Columns -------------------------------------- `mysqlshow' can be used to quickly look at which databases exist, their tables, and the table's columns. With the `mysql' program you can get the same information with the `SHOW' commands. *Note SHOW::. `mysqlshow' is invoked like this: shell> mysqlshow [OPTIONS] [database [table [column]]] * If no database is given, all matching databases are shown. * If no table is given, all matching tables in the database are shown. * If no column is given, all matching columns and column types in the table are shown. Note that in newer MySQL versions, you only see those database/tables/columns for which you have some privileges. If the last argument contains a shell or SQL wild-card (`*', `?', `%' or `_') then only what's matched by the wild card is shown. This may cause some confusion when you try to display the columns for a table with a `_' as in this case `mysqlshow' only shows you the table names that match the pattern. This is easily fixed by adding an extra `%' last on the command line (as a separate argument). perror, Explaining Error Codes ------------------------------ `perror' can be used to print error message(s). `perror' can be invoked like this: shell> perror [OPTIONS] [ERRORCODE [ERRORCODE...]] For example: shell> perror 64 79 Error code 64: Machine is not on the network Error code 79: Can not access a needed shared library `perror' can be used to display a description for a system error code, or an MyISAM/ISAM table handler error code. The error messages are mostly system dependent. How to Run SQL Commands from a Text File ---------------------------------------- The `mysql' client typically is used interactively, like this: shell> mysql database However, it's also possible to put your SQL commands in a file and tell `mysql' to read its input from that file. To do so, create a text file `text_file' that contains the commands you wish to execute. Then invoke `mysql' as shown below: shell> mysql database < text_file You can also start your text file with a `USE db_name' statement. In this case, it is unnecessary to specify the database name on the command line: shell> mysql < text_file *Note Client-Side Scripts::. The MySQL Log Files =================== MySQL has several different log files that can help you find out what's going on inside `mysqld': The error log Problems encountering starting, running or stopping `mysqld'. The isam log Logs all changes to the ISAM tables. Used only for debugging the isam code. The query log Established connections and executed queries. The update log Deprecated: Stores all statements that changes data The binary log Stores all statements that changes something. Used also for replication The slow log Stores all queries that took more than `long_query_time' to execute or didn't use indexes. All logs can be found in the `mysqld' data directory. You can force `mysqld' to reopen the log files (or in some cases switch to a new log) by executing `FLUSH LOGS'. *Note FLUSH::. The Error Log ------------- `mysqld' writes all errors to the stderr, which the `safe_mysqld' script redirects to a file called `'hostname'.err'. (On Windows, `mysqld' writes this directly to `\mysql\data\mysql.err'). This contains information indicating when `mysqld' was started and stopped and also any critical errors found when running. If `mysqld' dies unexpectedly and `safe_mysqld' needs to restart `mysqld', `safe_mysqld' will write a `restarted mysqld' row in this file. This log also holds a warning if `mysqld' notices a table that needs to be automatically checked or repaired. On some operating systems, the error log will contain a stack trace for where `mysqld' died. This can be used to find out where `mysqld' died. *Note Using stack trace::. The General Query Log --------------------- If you want to know what happens within `mysqld', you should start it with `--log[=file]'. This will log all connections and queries to the log file (by default named `'hostname'.log'). This log can be very useful when you suspect an error in a client and want to know exactly what `mysqld' thought the client sent to it. By default, the `mysql.server' script starts the MySQL server with the `-l' option. If you need better performance when you start using MySQL in a production environment, you can remove the `-l' option from `mysql.server' or change it to `--log-bin'. The entries in this log are written as `mysqld' receives the questions. This may be different than the order in which the statements are executed. This is in contrast to the update log and the binary log which are written after the query is executed, but before any locks are released. The Update Log -------------- *NOTE*: The update log is replaced by the binary log. *Note Binary log::. With this you can do anything that you can do with the update log. When started with the `--log-update[=file_name]' option, `mysqld' writes a log file containing all SQL commands that update data. If no filename is given, it defaults to the name of the host machine. If a filename is given, but it doesn't contain a path, the file is written in the data directory. If `file_name' doesn't have an extension, `mysqld' will create log file names like so: `file_name.###', where `###' is a number that is incremented each time you execute `mysqladmin refresh', execute `mysqladmin flush-logs', execute the `FLUSH LOGS' statement, or restart the server. *NOTE:* For the above scheme to work, you should NOT create your own files with the same filename as the update log + some extensions that may be regarded as a number, in the directory used by the update log! If you use the `--log' or `-l' options, `mysqld' writes a general log with a filename of `hostname.log', and restarts and refreshes do not cause a new log file to be generated (although it is closed and reopened). In this case you can copy it (on Unix) by doing: mv hostname.log hostname-old.log mysqladmin flush-logs cp hostname-old.log to-backup-directory rm hostname-old.log Update logging is smart because it logs only statements that really update data. So an `UPDATE' or a `DELETE' with a `WHERE' that finds no rows is not written to the log. It even skips `UPDATE' statements that set a column to the value it already has. The update logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that the log will be logged in the execution order. If you want to update a database from update log files, you could do the following (assuming your update logs have names of the form `file_name.###'): shell> ls -1 -t -r file_name.[0-9]* | xargs cat | mysql `ls' is used to get all the log files in the right order. This can be useful if you have to revert to backup files after a crash and you want to redo the updates that occurred between the time of the backup and the crash. The Binary Update Log --------------------- The intention is that the binary log should replace the update log, so we recommend you to switch to this log format as soon as possible! The binary log contains all information that is available in the update log in a more efficient format. It also contains information about how long every query that updated the database took. The binary log is also used when you are replicating a slave from a master. *Note Replication::. When started with the `--log-bin[=file_name]' option, `mysqld' writes a log file containing all SQL commands that update data. If no file name is given, it defaults to the name of the host machine followed by `-bin'. If file name is given, but it doesn't contain a path, the file is written in the data directory. If you supply an extension to `--log-bin=filename.extension', the extension will be silenty removed. To the binary log filename `mysqld' will append an extension that is a number that is incremented each time you execute `mysqladmin refresh', execute `mysqladmin flush-logs', execute the `FLUSH LOGS' statement or restart the server. You can use the following options to `mysqld' to affect what is logged to the binary log: `binlog-do-db=database_name' Tells the master it should log updates for the specified database, and exclude all others not explicitly mentioned. (Example: `binlog-do-db=some_database') `binlog-ignore-db=database_name' Tells the master that updates to the given database should not be logged to the binary log (Example: `binlog-ignore-db=some_database') To be able to know which different binary log files have been used, `mysqld' will also create a binary log index file that contains the name of all used binary log files. By default this has the same name as the binary log file, with the extension `'.index''. You can change the name of the binary log index file with the `--log-bin-index=[filename]' option. If you are using replication, you should not delete old binary log files until you are sure that no slave will ever need to use them. One way to do this is to do `mysqladmin flush-logs' once a day and then remove any logs that are more than 3 days old. You can examine the binary log file with the `mysqlbinlog' command. For example, you can update a MySQL server from the binary log as follows: mysqlbinlog log-file | mysql -h server_name You can also use the `mysqlbinlog' program to read the binary log directly from a remote MySQL server! `mysqlbinlog --help' will give you more information of how to use this program! If you are using `BEGIN [WORK]' or `SET AUTOCOMMIT=0', you must use the MySQL binary log for backups instead of the old update log. The binary logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that the log will be logged in the execution order. All updates (`UPDATE', `DELETE' or `INSERT') that change a transactional table (like BDB tables) are cached until a `COMMIT'. Any updates to a non-transactional table are stored in the binary log at once. Every thread will, on start, allocate a buffer of `binlog_cache_size' to buffer queries. If a query is bigger than this, the thread will open a temporary file to handle the bigger cache. The temporary file will be deleted when the thread ends. The `max_binlog_cache_size' can be used to restrict the total size used to cache a multi-transaction query. If you are using the update or binary log, concurrent inserts will not work together with `CREATE ... INSERT' and `INSERT ... SELECT'. This is to ensure that you can recreate an exact copy of your tables by applying the log on a backup. The Slow Query Log ------------------ When started with the `--log-slow-queries[=file_name]' option, `mysqld' writes a log file containing all SQL commands that took more than `long_query_time' to execute. The time to get the initial table locks are not counted as execution time. The slow query log is logged after the query is executed and after all locks has been released. This may be different than the order in which the statements are executed. If no file name is given, it defaults to the name of the host machine suffixed with `-slow.log'. If a filename is given, but doesn't contain a path, the file is written in the data directory. The slow query log can be used to find queries that take a long time to execute and are thus candidates for optimization. With a large log, that can become a difficult task. You can pipe the slow query log through the `mysqldumpslow' command to get a summary of the queries which appear in the log. You are using `--log-long-format' then also queries that are not using indexes are printed. *Note Command-line options::. Log File Maintenance -------------------- MySQL has a lot of log files which make it easy to see what is going. *Note Log Files::. One must however from time to time clean up after `MysQL' to ensure that the logs don't take up too much disk space. When using MySQL with log files, you will, from time to time, want to remove/backup old log files and tell MySQL to start logging on new files. *Note Backup::. On a Linux (`Redhat') installation, you can use the `mysql-log-rotate' script for this. If you installed MySQL from an RPM distribution, the script should have been installed automatically. Note that you should be careful with this if you are using the log for replication! On other systems you must install a short script yourself that you start from `cron' to handle log files. You can force MySQL to start using new log files by using `mysqladmin flush-logs' or by using the SQL command `FLUSH LOGS'. If you are using MySQL Version 3.21 you must use `mysqladmin refresh'. The above command does the following: * If standard logging (`--log') or slow query logging (`--log-slow-queries') is used, closes and reopens the log file. (`mysql.log' and ``hostname`-slow.log' as default). * If update logging (`--log-update') is used, closes the update log and opens a new log file with a higher sequence number. If you are using only an update log, you only have to flush the logs and then move away the old update log files to a backup. If you are using the normal logging, you can do something like: shell> cd mysql-data-directory shell> mv mysql.log mysql.old shell> mysqladmin flush-logs and then take a backup and remove `mysql.old'. Replication in MySQL ==================== This chapter describes the various replication features in MySQL. It serves as a reference to the options available with replication. You will be introduced to replication and learn how to implement it. Towards the end, there are some frequently asked questions and descriptions of problems and how to solve them. Introduction ------------ One way replication can be used is to increase both robustness and speed. For robustness you can have two systems and can switch to the backup if you have problems with the master. The extra speed is achieved by sending a part of the non-updating queries to the replica server. Of course this only works if non-updating queries dominate, but that is the normal case. Starting in Version 3.23.15, MySQL supports one-way replication internally. One server acts as the master, while the other acts as the slave. Note that one server could play the roles of master in one pair and slave in the other. The master server keeps a binary log of updates (*Note Binary log::.) and an index file to binary logs to keep track of log rotation. The slave, upon connecting, informs the master where it left off since the last successfully propagated update, catches up on the updates, and then blocks and waits for the master to notify it of the new updates. Note that if you are replicating a database, all updates to this database should be done through the master! Another benefit of using replication is that one can get live backups of the system by doing a backup on a slave instead of doing it on the master. *Note Backup::. Replication Implementation Overview ----------------------------------- MySQL replication is based on the server keeping track of all changes to your database (updates, deletes, etc) in the binary log. (*Note Binary log::.) and the slave server(s) reading the saved queries from the master server's binary log so that the slave can execute the same queries on its copy of the data. It is *very important* to realize that the binary log is simply a record starting from a fixed point in time (the moment you enable binary logging). Any slaves which you set up will need copies of all the data from your master as it existed the moment that you enabled binary logging on the master. If you start your slaves with data that doesn't agree with what was on the master *when the binary log was started*, your slaves may fail. A future version (4.0) of MySQL will remove the need to keep a (possibly large) snapshot of data for new slaves that you might wish to set up through the live backup functionality with no locking required. However, at this time, it is necessary to block all writes either with a global read lock or by shutting down the master while taking a snapshot. Once a slave is properly configured and running, it will simply connect to the master and wait for updates to process. If the master goes away or the slave loses connectivity with your master, it will keep trying to connect every `master-connect-retry' seconds until it is able to reconnect and resume listening for updates. Each slave keeps track of where it left off. The master server has no knowledge of how many slaves there are or which ones are up-to-date at any given time. The next section explains the master/slave setup process in more detail. How To Set Up Replication ------------------------- Below is a quick description of how to set up complete replication on your current MySQL server. It assumes you want to replicate all your databases and have not configured replication before. You will need to shutdown your master server briefly to complete the steps outlined below. 1. Make sure you have a recent version of MySQL installed on the master and slave(s). Use Version 3.23.29 or higher. Previous releases used a different binary log format and had bugs which have been fixed in newer releases. Please, do not report bugs until you have verified that the problem is present in the latest release. 2. Set up special a replication user on the master with the `FILE' privilege and permission to connect from all the slaves. If the user is only doing replication (which is recommended), you don't need to grant any additional privileges. For example, to create a user named `repl' which can access your master from any host, you might use this command: GRANT FILE ON *.* TO repl@"%" IDENTIFIED BY '<password>'; 3. Shut down MySQL on the master. mysqladmin -u root -p<password> shutdown 4. Snapshot all the data on your master server. The easiest way to do this (on Unix) is to simply use *tar* to produce an archive of your entire data directory. The exact data directory location depends on your installation. tar -cvf /tmp/mysql-snapshot.tar /path/to/data-dir Windows users can use WinZip or similar software to create an archive of the data directory. 5. In `my.cnf' on the master add `log-bin' and `server-id=unique number' to the `[mysqld]' section and restart it. It is very important that the id of the slave is different from the id of the master. Think of `server-id' as something similar to the IP address - it uniquely identifies the server instance in the community of replication partners. [mysqld] log-bin server-id=1 6. Restart MySQL on the master. 7. Add the following to `my.cnf' on the slave(s): master-host=<hostname of the master> master-user=<replication user name> master-password=<replication user password> master-port=<TCP/IP port for master> server-id=<some unique number between 2 and 2^32-1> replacing the values in <> with what is relevant to your system. `server-id' must be different for each server participating in replication. If you don't specify a server-id, it will be set to 1 if you have not defined `master-host', else it will be set to 2. Note that in the case of `server-id' omission the master will refuse connections from all slaves, and the slave will refuse to connect to a master. Thus, omitting `server-id' is only good for backup with a binary log. 8. Copy the snapshot data into your data directory on your slave(s). Make sure that the privileges on the files and directories are correct. The user which MySQL runs as needs to be able to read and write to them, just as on the master. 9. Restart the slave(s). After you have done the above, the slave(s) should connect to the master and catch up on any updates which happened since the snapshot was taken. If you have forgotten to set `server-id' for the slave you will get the following error in the error log file: Warning: one should set server_id to a non-0 value if master_host is set. The server will not act as a slave. If you have forgot to do this for the master, the slaves will not be able to connect to the master. If a slave is not able to replicate for any reason, you will find error messages in the error log on the slave. Once a slave is replicating, you will find a file called `master.info' in the same directory as your error log. The `master.info' file is used by the slave to keep track of how much of the master's binary log is has processed. *Do not* remove or edit the file, unless you really know what you are doing. Even in that case, it is preferred that you use `CHANGE MASTER TO' command. Replication Features and Known Problems --------------------------------------- Below is an explanation of what is supported and what is not: * Replication will be done correctly with `AUTO_INCREMENT', `LAST_INSERT_ID', and `TIMESTAMP' values. * `RAND()' in updates does not replicate properly. Use `RAND(some_non_rand_expr)' if you are replicating updates with `RAND()'. You can, for example, use `UNIX_TIMESTAMP()' for the argument to `RAND()'. * You have to use the same character set (`--default-character-set') on the master and the slave. If not, you may get duplicate key errors on the slave, because a key that is regarded as unique on the master may not be that in the other character set. * `LOAD DATA INFILE' will be handled properly as long as the file still resides on the master server at the time of update propagation. `LOAD LOCAL DATA INFILE' will be skipped. * Update queries that use user variables are not replication-safe (yet). * `FLUSH' commands are not stored in the binary log and are because of this not replicated to the slaves. This is not normally a problem as `FLUSH' doesn't change anything. This does however mean that if you update the `MySQL' privilege tables directly without using `GRANT' statement and you replicate the `MySQL' privilege database, you must do a `FLUSH PRIVILEGES' on your slaves to put the new privileges into effect. * Temporary tables starting in 3.23.29 are replicated properly with the exception of the case when you shut down slave server ( not just slave thread), you have some temporary tables open, and the are used in subsequent updates. To deal with this problem, to shut down the slave, do `SLAVE STOP', then check `Slave_open_temp_tables' variable to see if it is 0, then issue `mysqladmin shutdown'. If the number is not 0, restart the slave thread with `SLAVE START' and see if you have better luck next time. There will be a cleaner solution, but it has to wait until version 4.0. In earlier versions temporary tables are not being replicated properly - we recommend that you either upgrade, or execute `SET SQL_LOG_BIN=0' on your clients before all queries with temp tables. * MySQL only supports one master and many slaves. We will in 4.x add a voting algorithm to automatically change master if something goes wrong with the current master. We will also introduce 'agent' processes to help doing load balancing by sending select queries to different slaves. * Starting in Version 3.23.26, it is safe to connect servers in a circular master-slave relationship with `log-slave-updates' enabled. Note, however, that many queries will not work right in this kind of setup unless your client code is written to take care of the potential problems that can happen from updates that occur in different sequence on different servers. This means that you can do a setup like the following: A -> B -> C -> A This setup will only works if you only do non conflicting updates between the tables. In other words, if you insert data in A and C, you should never insert a row in A that may have a conflicting key with a row insert in C. You should also not update the sam rows on two servers if the order in which the updates are applied matters. Note that the log format has changed in Version 3.23.26 so that pre-3.23.26 slaves will not be able to read it. * If the query on the slave gets an error, the slave thread will terminate, and a message will appear in the `.err' file. You should then connect to the slave manually, fix the cause of the error (for example, non-existent table), and then run `SLAVE START' sql command (available starting in Version 3.23.16). In Version 3.23.15, you will have to restart the server. * If connection to the master is lost, the slave will retry immediately, and then in case of failure every `master-connect-retry' (default 60) seconds. Because of this, it is safe to shut down the master, and then restart it after a while. The slave will also be able to deal with network connectivity outages. * Shutting down the slave (cleanly) is also safe, as it keeps track of where it left off. Unclean shutdowns might produce problems, especially if disk cache was not synced before the system died. Your system fault tolerance will be greatly increased if you have a good UPS. * If the master is listening on a non-standard port, you will also need to specify this with `master-port' parameter in `my.cnf' . * In Version 3.23.15, all of the tables and databases will be replicated. Starting in Version 3.23.16, you can restrict replication to a set of databases with `replicate-do-db' directives in `my.cnf' or just exclude a set of databases with `replicate-ignore-db'. Note that up until Version 3.23.23, there was a bug that did not properly deal with `LOAD DATA INFILE' if you did it in a database that was excluded from replication. * Starting in Version 3.23.16, `SET SQL_LOG_BIN = 0' will turn off replication (binary) logging on the master, and `SET SQL_LOG_BIN = 1' will turn in back on - you must have the process privilege to do this. * Starting in Version 3.23.19, you can clean up stale replication leftovers when something goes wrong and you want a clean start with `FLUSH MASTER' and `FLUSH SLAVE' commands. In Version 3.23.26 we have renamed them to `RESET MASTER' and `RESET SLAVE' respectively to clarify what they do. The old `FLUSH' variants still work, though, for compatibility. * Starting in Version 3.23.21, you can use `LOAD TABLE FROM MASTER' for network backup and to set up replication initially. We have recently received a number of bug reports concerning it that we are investigating, so we recommend that you use it only in testing until we make it more stable. * Starting in Version 3.23.23, you can change masters and adjust log position with `CHANGE MASTER TO'. * Starting in Version 3.23.23, you tell the master that updates in certain databases should not be logged to the binary log with `binlog-ignore-db'. * Starting in Version 3.23.26, you can use `replicate-rewrite-db' to tell the slave to apply updates from one database on the master to the one with a different name on the slave. * Starting in Version 3.23.28, you can use `PURGE MASTER LOGS TO 'log-name'' to get rid of old logs while the slave is running. Replication Options in my.cnf ----------------------------- If you are using replication, we recommend you to use MySQL Version 3.23.30 or later. Older versions work, but they do have some bugs and are missing some features. On both master and slave you need to use the `server-id' option. This sets an unique replication id. You should pick a unique value in the range between 1 to 2^32-1 for each master and slave. Example: `server-id=3' The following table has the options you can use for the *MASTER*: *Option* *Description* `log-bin=filename' Write to a binary update log to the specified location. Note that if you give it a parameter with an extension (for example, `log-bin=/mysql/logs/replication.log' ) versions up to 3.23.24 will not work right during replication if you do `FLUSH LOGS' . The problem is fixed in Version 3.23.25. If you are using this kind of log name, `FLUSH LOGS' will be ignored on binlog. To clear the log, run `FLUSH MASTER', and do not forget to run `FLUSH SLAVE' on all slaves. In Version 3.23.26 and in later versions you should use `RESET MASTER' and `RESET SLAVE' `log-bin-index=filename' Because the user could issue the `FLUSH LOGS' command, we need to know which log is currently active and which ones have been rotated out and in what sequence. This information is stored in the binary log index file. The default is `hostname`.index. You can use this option if you want to be a rebel. Example: `log-bin-index=db.index'. `sql-bin-update-same' If set, setting `SQL_LOG_BIN' to a value will automatically set `SQL_LOG_UPDATE' to the same value and vice versa. `binlog-do-db=database_name' Tells the master that it should log updates to the binary log if the current database is 'database_name'. All others database are ignored. Note that if you use this you should ensure that you only do updates in the current database. Example: `binlog-do-db=some_database'. `binlog-ignore-db=database_name' Tells the master that updates where the current database is 'database_name' should not be stored in the binary log. Note that if you use this you should ensure that you only do updates in the current database. Example: `binlog-ignore-db=some_database' The following table has the options you can use for the *SLAVE*: *Option* *Description* `master-host=host' Master hostname or IP address for replication. If not set, the slave thread will not be started. Example: `master-host=db-master.mycompany.com'. `master-user=username' The user the slave thread will us for authentication when connecting to the master. The user must have `FILE' privilege. If the master user is not set, user `test' is assumed. Example: `master-user=scott'. `master-password=password' The password the slave thread will authenticate with when connecting to the master. If not set, an empty password is assumed. Example: `master-password=tiger'. `master-port=portnumber' The port the master is listening on. If not set, the compiled setting of `MYSQL_PORT' is assumed. If you have not tinkered with `configure' options, this should be 3306. Example: `master-port=3306'. `master-connect-retry=seconds' The number of seconds the slave thread will sleep before retrying to connect to the master in case the master goes down or the connection is lost. Default is 60. Example: `master-connect-retry=60'. `master-info-file=filename' The location of the file that remembers where we left off on the master during the replication process. The default is master.info in the data directory. Sasha: The only reason I see for ever changing the default is the desire to be rebelious. Example: `master-info-file=master.info'. `replicate-do-table=db_name.table_name' Tells the slave thread to restrict replication to the specified table. To specify more than one table, use the directive multiple times, once for each table. This will work for cross-database updates, in contrast to `replicate-do-db'. Example: `replicate-do-table=some_db.some_table'. `replicate-ignore-table=db_name.table_name' Tells the slave thread to not replicate to the specified table. To specify more than one table to ignore, use the directive multiple times, once for each table. This will work for cross-datbase updates, in contrast to `replicate-ignore-db'. Example: `replicate-ignore-table=db_name.some_table'. `replicate-wild-do-table=db_name.table_name' Tells the slave thread to restrict replication to the tables that match the specified wildcard pattern. To specify more than one table, use the directive multiple times, once for each table. This will work for cross-database updates. Example: `replicate-wild-do-table=foo%.bar%' will replicate only updates to tables in all databases that start with foo and whose table names start with bar. `replicate-wild-ignore-table=db_name.table_name' Tells the slave thread to not replicate to the tables that match the given wild card pattern. To specify more than one table to ignore, use the directive multiple times, once for each table. This will work for cross-database updates. Example: `replicate-wild-ignore-table=foo%.bar%' will not do updates to tables in databases that start with foo and whose table names start with bar. `replicate-ignore-db=database_name' Tells the slave thread to not replicate to the specified database. To specify more than one database to ignore, use the directive multiple times, once for each database. This option will not work if you use cross database updates. If you need cross database updates to work, make sure you have 3.23.28 or later, and use `replicate-wild-ignore-table=db_name.%' Example: `replicate-ignore-db=some_db'. `replicate-do-db=database_name' Tells the slave thread to restrict replication to the specified database. To specify more than one database, use the directive multiple times, once for each database. Note that this will only work if you do not use cross-database queries such as `UPDATE some_db.some_table SET foo='bar'' while having selected a different or no database. If you need cross database updates to work, make sure you have 3.23.28 or later, and use `replicate-wild-do-table=db_name.%' Example: `replicate-do-db=some_db'. `log-slave-updates' Tells the slave to log the updates from the slave thread to the binary log. Off by default. You will need to turn it on if you plan to daisy-chain the slaves. `replicate-rewrite-db=from_name->to_name' Updates to a database with a different name than the original Example: `replicate-rewrite-db=master_db_name->slave_db_name'. `skip-slave-start' Tells the slave server not to start the slave on the startup. The user can start it later with `SLAVE START'. `slave_read_timeout=#' Number of seconds to wait for more data from the master before aborting the read. SQL Commands Related to Replication ----------------------------------- Replication can be controlled through the SQL interface. Below is the summary of commands: *Command* *Description* `SLAVE START' Starts the slave thread. (Slave) `SLAVE STOP' Stops the slave thread. (Slave) `SET SQL_LOG_BIN=0' Disables update logging if the user has process privilege. Ignored otherwise. (Master) `SET SQL_LOG_BIN=1' Re-enables update logging if the user has process privilege. Ignored otherwise. (Master) `SET Skip the next `n' events from the master. Only SQL_SLAVE_SKIP_COUNTER=n'valid when the slave thread is not running, otherwise, gives an error. Useful for recovering from replication glitches. `RESET MASTER' Deletes all binary logs listed in the index file, resetting the binlog index file to be empty. In pre-3.23.26 versions, `FLUSH MASTER' (Master) `RESET SLAVE' Makes the slave forget its replication position in the master logs. In pre 3.23.26 versions the command was called `FLUSH SLAVE'(Slave) `LOAD TABLE tblname Downloads a copy of the table from master to the FROM MASTER' slave. (Slave) `CHANGE MASTER TO Changes the master parameters to the values master_def_list' specified in `master_def_list' and restarts the slave thread. `master_def_list' is a comma-separated list of `master_def' where `master_def' is one of the following: `MASTER_HOST', `MASTER_USER', `MASTER_PASSWORD', `MASTER_PORT', `MASTER_CONNECT_RETRY', `MASTER_LOG_FILE', `MASTER_LOG_POS'. Example: CHANGE MASTER TO MASTER_HOST='master2.mycompany.com', MASTER_USER='replication', MASTER_PASSWORD='bigs3cret', MASTER_PORT=3306, MASTER_LOG_FILE='master2-bin.001', MASTER_LOG_POS=4; You only need to specify the values that need to be changed. The values that you omit will stay the same with the exception of when you change the host or the port. In that case, the slave will assume that since you are connecting to a different host or a different port, the master is different. Therefore, the old values of log and position are not applicable anymore, and will automatically be reset to an empty string and 0, respectively (the start values). Note that if you restart the slave, it will remember its last master. If this is not desirable, you should delete the `master.info' file before restarting, and the slave will read its master from `my.cnf' or the command line. (Slave) `SHOW MASTER STATUS' Provides status information on the binlog of the master. (Master) `SHOW SLAVE STATUS' Provides status information on essential parameters of the slave thread. (Slave) `SHOW MASTER LOGS' Only available starting in Version 3.23.28. Lists the binary logs on the master. You should use this command prior to `PURGE MASTER LOGS TO' to find out how far you should go. `PURGE MASTER LOGS TO Available starting in Version 3.23.28. Deletes 'logname'' all the replication logs that are listed in the log index as being prior to the specified log, and removed them from the log index, so that the given log now becomes first. Example: PURGE MASTER LOGS TO 'mysql-bin.010' This command will do nothing and fail with an error if you have an active slave that is currently reading one of the logs you are trying to delete. However, if you have a dormant slave, and happen to purge one of the logs it wants to read, the slave will be unable to replicate once it comes up. The command is safe to run while slaves are replicating - you do not need to stop them. You must first check all the slaves with `SHOW SLAVE STATUS' to see which log they are on, then do a listing of the logs on the master with `SHOW MASTER LOGS', find the earliest log among all the slaves (if all the slaves are up to date, this will be the last log on the list), backup all the logs you are about to delete (optional) and purge up to the target log. Replication FAQ --------------- *Q*: Why do I sometimes see more than one `Binlog_Dump' thread on the master after I have restarted the slave? *A*: `Binlog_Dump' is a continuous process that is handled by the server in the following way: * Catch up on the updates. * Once there are no more updates left, go into `pthread_cond_wait()', from which we can be awakened either by an update or a kill. * On wake up, check the reason. If we are not supposed to die, continue the `Binlog_dump' loop. * If there is some fatal error, such as detecting a dead client, terminate the loop. So if the slave thread stops on the slave, the corresponding `Binlog_Dump' thread on the master will not notice it until after at least one update to the master (or a kill), which is needed to wake it up from `pthread_cond_wait()'. In the meantime, the slave could have opened another connection, which resulted in another `Binlog_Dump' thread. The above problem should not be present in Version 3.23.26 and later versions. In Version 3.23.26 we added `server-id' to each replication server, and now all the old zombie threads are killed on the master when a new replication thread connects from the same slave *Q*: How do I rotate replication logs? *A*: In Version 3.23.28 you should use `PURGE MASTER LOGS TO' command after determining which logs can be deleted, and optionally backing them up first. In earlier versions the process is much more painful, and cannot be safely done without stopping all the slaves in the case that you plan to re-use log names. You will need to stop the slave threads, edit the binary log index file, delete all the old logs, restart the master, start slave threads, and then remove the old log files. *Q*: How do I upgrade on a hot replication setup? *A*: If you are upgrading pre-3.23.26 versions, you should just lock the master tables, let the slave catch up, then run `FLUSH MASTER' on the master, and `FLUSH SLAVE' on the slave to reset the logs, then restart new versions of the master and the slave. Note that the slave can stay down for some time - since the master is logging all the updates, the slave will be able to catch up once it is up and can connect. After 3.23.26, we have locked the replication protocol for modifications, so you can upgrade masters and slave on the fly to a newer 3.23 version and you can have different versions of MySQL running on the slave and the master, as long as they are both newer than 3.23.26. *Q*: What issues should I be aware of when setting up two-way replication? *A*: MySQL replication currently does not support any locking protocol between master and slave to guarantee the atomicity of a distributed (cross-server) update. In in other words, it is possible for client A to make an update to co-master 1, and in the meantime, before it propagates to co-master 2, client B could make an update to co-master 2 that will make the update of client A work differently than it did on co-master 1. Thus when the update of client A will make it to co-master 2, it will produce tables that will be different than what you have on co-master 1, even after all the updates from co-master 2 have also propagated. So you should not co-chain two servers in a two-way replication relationship, unless you are sure that you updates can safely happen in any order, or unless you take care of mis-ordered updates somehow in the client code. You must also realize that two-way replication actually does not improve performance very much, if at all, as far as updates are concerned. Both servers need to do the same amount of updates each, as you would have one server do. The only difference is that there will be a little less lock contention, because the updates originating on another server will be serialized in one slave thread. This benefit, though, might be offset by network delays. *Q*: How can I use replication to improve performance of my system? *A*: You should set up one server as the master, and direct all writes to it, and configure as many slaves as you have the money and rackspace for, distributing the reads among the master and the slaves. You can also start the slaves with `--skip-bdb', `--low-priority-updates' and `--delay-key-write-for-all-tables' to get speed improvements for the slave. In this case the slave will use non-transactional `MyISAM' tables instead of `BDB' tables to get more speed. *Q*: What should I do to prepare my client code to use performance-enhancing replication? *A*: If the part of your code that is responsible for database access has been properly abstracted/modularized, converting it to run with the replicated setup should be very smooth and easy - just change the implementation of your database access to read from some slave or the master, and to always write to the master. If your code does not have this level of abstraction, setting up a replicated system will give you an opportunity/motivation to it clean up. You should start by creating a wrapper library /module with the following functions: * `safe_writer_connect()' * `safe_reader_connect()' * `safe_reader_query()' * `safe_writer_query()' `safe_' means that the function will take care of handling all the error conditions. You should then convert your client code to use the wrapper library. It may be a painful and scary process at first, but it will pay off in the long run. All applications that follow the above pattern will be able to take advantage of one-master/many slaves solution. The code will be a lot easier to maintain, and adding troubleshooting options will be trivial. You will just need to modify one or two functions, for example, to log how long each query took, or which query, among your many thousands, gave you an error. If you have written a lot of code already, you may want to automate the conversion task by using Monty's `replace' utility, which comes with the standard distribution of MySQL, or just write your own Perl script. Hopefully, your code follows some recognizable pattern. If not, then you are probably better off re-writing it anyway, or at least going through and manually beating it into a pattern. Note that, of course, you can use different names for the functions. What is important is having unified interface for connecting for reads, connecting for writes, doing a read, and doing a write. *Q*: When and how much can MySQL replication improve the performance of my system? *A*: MySQL replication is most beneficial for a system with frequent reads and not so frequent writes. In theory, by using a one master/many slaves setup you can scale by adding more slaves until you either run out of network bandwidth, or your update load grows to the point that the master cannot handle it. In order to determine how many slaves you can get before the added benefits begin to level out, and how much you can improve performance of your site, you need to know your query patterns, and empirically (by benchmarking) determine the relationship between the throughput on reads (reads per second, or `max_reads') and on writes `max_writes') on a typical master and a typical slave. The example below will show you a rather simplified calculation of what you can get with replication for our imagined system. Let's say our system load consists of 10% writes and 90% reads, and we have determined that `max_reads' = 1200 - 2 * `max_writes', or in other words, our system can do 1200 reads per second with no writes, our average write is twice as slow as average read, and the relationship is linear. Let us suppose that our master and slave are of the same capacity, and we have N slaves and 1 master. Then we have for each server (master or slave): `reads = 1200 - 2 * writes' (from bencmarks) `reads = 9* writes / (N + 1) ' (reads split, but writes go to all servers) `9*writes/(N+1) + 2 * writes = 1200' `writes = 1200/(2 + 9/(N+1)' So if N = 0, which means we have no replication, our system can handle 1200/11, about 109 writes per second (which means we will have 9 times as many reads due to the nature of our application). If N = 1, we can get up to 184 writes per second. If N = 8, we get up to 400. If N = 17, 480 writes. Eventually as N approaches infinity (and our budget negative infinity), we can get very close to 600 writes per second, increasing system throughput about 5.5 times. However, with only 8 servers, we increased it almost 4 times already. Note that our computations assumed infinite network bandwidth, and neglected several other factors that could turn out to be significant on your system. In many cases, you may not be able to make a computation similar to the one above that will accurately predict what will happen on your system if you add N replication slaves. However, answering the following questions should help you decided whether and how much, if at all, the replication will improve the performance of your system: * What is the read/write ratio on your system? * How much more write load can one server handle if you reduce the reads? * How many slaves do you have bandwidth for on your network? *Q*: How can I use replication to provide redundancy/high availability? *A*: With the currently available features, you would have to set up a master and a slave (or several slaves), and write a script that will monitor the master to see if it is up, and instruct your applications and the slaves of the master change in case of failure. Some suggestions: * To tell a slave to change the master use the `CHANGE MASTER TO' command. * A good way to keep your applications informed where the master is by having a dynamic DNS entry for the master. With *bind* you can use `nsupdate' to dynamically update your DNS. * You should run your slaves with the `log-bin' option and without `log-slave-updates'. This way the slave will be ready to become a master as soon as you issue `STOP SLAVE'; `RESET MASTER', and `CHANGE MASTER TO' on the other slaves. It will also help you catch spurious updates that may happen because of misconfiguration of the slave (ideally, you want to configure access rights so that no client can update the slave, except for the slave thread) combined with the bugs in your client programs (they should never update the slave directly). We are currently working on integrating an automatic master election system into MySQL, but until it is ready, you will have to create your own monitoring tools. Troubleshooting Replication --------------------------- If you have followed the instructions, and your replication setup is not working, first eliminate the user error factor by checking the following: * Is the master logging to the binary log? Check with `SHOW MASTER STATUS'. If it is, `Position' will be non-zero. If not, verify that you have given the master `log-bin' option and have set `server-id'. * Is the slave running? Check with `SHOW SLAVE STATUS'. The answer is found in `Slave_running' column. If not, verify slave options and check the error log for messages. * If the slave is running, did it establish connection with the master? Do `SHOW PROCESSLIST', find the thread with `system user' value in `User' column and `none' in the `Host' column, and check the `State' column. If it says `connecting to master', verify the privileges for the replication user on the master, master host name, your DNS setup, whether the master is actually running, whether it is reachable from the slave, and if all that seems ok, read the error logs. * If the slave was running, but then stopped, look at SHOW SLAVE STATUS output and check the error logs. It usually happens when some query that succeeded on the master fails on the slave. This should never happen if you have taken a proper snapshot of the master, and never modify the data on the slave outside of the slave thread. If it does, it is a bug, read below on how to report it. * If a query on that succeeded on the master refuses to run on the slave, and a full database resync ( the proper thing to do ) does not seem feasible, try the following: - First see if there is some stray record in the way. Understand how it got there, then delete it and run `SLAVE START' - If the above does not work or does not apply, try to understand if it would be safe to make the update manually ( if needed) and then ignore the next query from the master. - If you have decided you can skip the next query, do `SET SQL_SLAVE_SKIP_COUNTER=1; SLAVE START;' to skip a query that does not use auto_increment, or last_insert_id or `SET SQL_SLAVE_SKIP_COUNTER=2; SLAVE START;' otherwise. The reason auto_increment/last_insert_id queries are different is that they take two events in the binary log of the master. - If you are sure the slave started out perfectly in sync with the master, and no one has updated the tables involved outside of slave thread, report the bug, so you will not have to do the above tricks again. * Make sure you are not running into an old bug by upgrading to the most recent version. * If all else fails, read the error logs. If they are big, `grep -i slave /path/to/your-log.err' on the slave. There is no generic pattern to search for on the master, as the only errors it logs are general system errors - if it can, it will send the error to the slave when things go wrong. When you have determined that there is no user error involved, and replication still either does not work at all or is unstable, it is time to start working on a bug report. We need to get as much info as possible from you to be able to track down the bug. Please do spend some time and effort preparing a good bug report. Ideally, we would like to have a test case in the format found in `mysql-test/t/rpl*' directory of the source tree. If you submit a test case like that, you can expect a patch within a day or two in most cases, although, of course, you mileage may vary depending on a number of factors. Second best option is a just program with easily configurable connection arguments for the master and the slave that will demonstrate the problem on our systems. You can write one in Perl or in C, depending on which language you know better. If you have one of the above ways to demonstrate the bug, use `mysqlbug' to prepare a bug report and send it to <bugs@lists.mysql.com>. If you have a phantom - a problem that does occur but you cannot duplicate "at will": * Verify that there is no user error involved. For example, if you update the slave outside of the slave thread, the data will be out of sync, and you can have unique key violations on updates, in which case the slave thread will stop and wait for you to clean up the tables manually to bring them in sync. * Run slave with `log-slave-updates' and `log-bin' - this will keep a log of all updates on the slave. * Save all evidence before resetting the replication. If we have no or only sketchy information, it would take us a while to track down the problem. The evidence you should collect is: - All binary logs on the master - All binary log on the slave - The output of `SHOW MASTER STATUS' on the master at the time you have discovered the problem - The output of `SHOW SLAVE STATUS' on the master at the time you have discovered the problem - Error logs on the master and on the slave * Use `mysqlbinlog' to examine the binary logs. The following should be helpful to find the trouble query, for example: mysqlbinlog -j pos_from_slave_status /path/to/log_from_slave_status | head Once you have collected the evidence on the phantom problem, try hard to isolate it into a separate test case first. Then report the problem to <bugs@lists.mysql.com> with as much info as possible. MySQL Optimization ****************** Optimization is a complicated task because it ultimately requires understanding of the whole system. While it may be possible to do some local optimizations with small knowledge of your system or application, the more optimal you want your system to become the more you will have to know about it. This chapter will try to explain and give some examples of different ways to optimize MySQL. Remember, however, that there are always some (increasingly harder) additional ways to make the system even faster. Optimization Overview ===================== The most important part for getting a system fast is of course the basic design. You also need to know what kinds of things your system will be doing, and what your bottlenecks are. The most common bottlenecks are: * Disk seeks. It takes time for the disk to find a piece of data. With modern disks in 1999, the mean time for this is usually lower than 10ms, so we can in theory do about 1000 seeks a second. This time improves slowly with new disks and is very hard to optimize for a single table. The way to optimize this is to spread the data on more than one disk. * Disk reading/writing. When the disk is at the correct position we need to read the data. With modern disks in 1999, one disk delivers something like 10-20Mb/s. This is easier to optimize than seeks because you can read in parallel from multiple disks. * CPU cycles. When we have the data in main memory (or if it already were there) we need to process it to get to our result. Having small tables compared to the memory is the most common limiting factor. But then, with small tables speed is usually not the problem. * Memory bandwidth. When the CPU needs more data than can fit in the CPU cache the main memory bandwidth becomes a bottleneck. This is an uncommon bottleneck for most systems, but one should be aware of it. MySQL Design Limitations/Tradeoffs ---------------------------------- Because MySQL uses extremely fast table locking (multiple readers / single writers) the biggest remaining problem is a mix of a steady stream of inserts and slow selects on the same table. We believe that for a huge number of systems the extremely fast performance in other cases make this choice a win. This case is usually also possible to solve by having multiple copies of the table, but it takes more effort and hardware. We are also working on some extensions to solve this problem for some common application niches. Portability ----------- Because all SQL servers implement different parts of SQL, it takes work to write portable SQL applications. For very simple selects/inserts it is very easy, but the more you need the harder it gets. If you want an application that is fast with many databases it becomes even harder! To make a complex application portable you need to choose a number of SQL servers that it should work with. You can use the MySQL crash-me program/web-page `http://www.mysql.com/information/crash-me.php' to find functions, types, and limits you can use with a selection of database servers. Crash-me now tests far from everything possible, but it is still comprehensive with about 450 things tested. For example, you shouldn't have column names longer than 18 characters if you want to be able to use Informix or DB2. Both the MySQL benchmarks and crash-me programs are very database-independent. By taking a look at how we have handled this, you can get a feeling for what you have to do to write your application database-independent. The benchmarks themselves can be found in the `sql-bench' directory in the MySQL source distribution. They are written in Perl with DBI database interface (which solves the access part of the problem). See `http://www.mysql.com/information/benchmarks.html' for the results from this benchmark. As you can see in these results, all databases have some weak points. That is, they have different design compromises that lead to different behavior. If you strive for database independence, you need to get a good feeling for each SQL server's bottlenecks. MySQL is VERY fast in retrieving and updating things, but will have a problem in mixing slow readers/writers on the same table. Oracle, on the other hand, has a big problem when you try to access rows that you have recently updated (until they are flushed to disk). Transaction databases in general are not very good at generating summary tables from log tables, as in this case row locking is almost useless. To get your application _really_ database-independent, you need to define an easy extendable interface through which you manipulate your data. As C++ is available on most systems, it makes sense to use a C++ classes interface to the databases. If you use some specific feature for some database (like the `REPLACE' command in MySQL), you should code a method for the other SQL servers to implement the same feature (but slower). With MySQL you can use the `/*! */' syntax to add MySQL-specific keywords to a query. The code inside `/**/' will be treated as a comment (ignored) by most other SQL servers. If REAL high performance is more important than exactness, as in some Web applications, a possibility is to create an application layer that caches all results to give you even higher performance. By letting old results 'expire' after a while, you can keep the cache reasonably fresh. This is quite nice in case of extremely high load, in which case you can dynamically increase the cache and set the expire timeout higher until things get back to normal. In this case the table creation information should contain information of the initial size of the cache and how often the table should normally be refreshed. What Have We Used MySQL For? ---------------------------- During MySQL initial development, the features of MySQL were made to fit our largest customer. They handle data warehousing for a couple of the biggest retailers in Sweden. From all stores, we get weekly summaries of all bonus card transactions, and we are expected to provide useful information for the store owners to help them find how their advertisement campaigns are affecting their customers. The data is quite huge (about 7 million summary transactions per month), and we have data for 4-10 years that we need to present to the users. We got weekly requests from the customers that they want to get 'instant' access to new reports from this data. We solved this by storing all information per month in compressed 'transaction' tables. We have a set of simple macros (script) that generates summary tables grouped by different criteria (product group, customer id, store ...) from the transaction tables. The reports are Web pages that are dynamically generated by a small Perl script that parses a Web page, executes the SQL statements in it, and inserts the results. We would have used PHP or mod_perl instead but they were not available at that time. For graphical data we wrote a simple tool in `C' that can produce GIFs based on the result of a SQL query (with some processing of the result). This is also dynamically executed from the Perl script that parses the `HTML' files. In most cases a new report can simply be done by copying an existing script and modifying the SQL query in it. In some cases, we will need to add more fields to an existing summary table or generate a new one, but this is also quite simple, as we keep all transactions tables on disk. (Currently we have at least 50G of transactions tables and 200G of other customer data.) We also let our customers access the summary tables directly with ODBC so that the advanced users can themselves experiment with the data. We haven't had any problems handling this with quite modest Sun Ultra SPARCstation (2x200 Mhz). We recently upgraded one of our servers to a 2 CPU 400 Mhz UltraSPARC, and we are now planning to start handling transactions on the product level, which would mean a ten-fold increase of data. We think we can keep up with this by just adding more disk to our systems. We are also experimenting with Intel-Linux to be able to get more CPU power cheaper. Now that we have the binary portable database format (new in Version 3.23), we will start to use this for some parts of the application. Our initial feelings are that Linux will perform much better on low-to-medium load and Solaris will perform better when you start to get a high load because of extreme disk IO, but we don't yet have anything conclusive about this. After some discussion with a Linux Kernel developer, this might be a side effect of Linux giving so much resources to the batch job that the interactive performance gets very low. This makes the machine feel very slow and unresponsive while big batches are going. Hopefully this will be better handled in future Linux Kernels. The MySQL Benchmark Suite ------------------------- This should contain a technical description of the MySQL benchmark suite (and `crash-me'), but that description is not written yet. Currently, you can get a good idea of the benchmark by looking at the code and results in the `sql-bench' directory in any MySQL source distributions. This benchmark suite is meant to be a benchmark that will tell any user what things a given SQL implementation performs well or poorly at. Note that this benchmark is single threaded, so it measures the minimum time for the operations. We plan to in the future add a lot of multi-threaded tests to the benchmark suite. For example, (run on the same NT 4.0 machine): *Reading 2000000 rows by index* *Seconds* *Seconds* mysql 367 249 mysql_odbc 464 db2_odbc 1206 informix_odbc 121126 ms-sql_odbc 1634 oracle_odbc 20800 solid_odbc 877 sybase_odbc 17614 *Inserting (350768) rows* *Seconds* *Seconds* mysql 381 206 mysql_odbc 619 db2_odbc 3460 informix_odbc 2692 ms-sql_odbc 4012 oracle_odbc 11291 solid_odbc 1801 sybase_odbc 4802 In the above test MySQL was run with a 8M index cache. We have gather some more benchmark results at `http://www.mysql.com/information/benchmarks.html'. Note that Oracle is not included because they asked to be removed. All Oracle benchmarks have to be passed by Oracle! We believe that makes Oracle benchmarks *VERY* biased because the above benchmarks are supposed to show what a standard installation can do for a single client. To run the benchmark suite, you have to download a MySQL source distribution, install the perl DBI driver, the perl DBD driver for the database you want to test and then do: cd sql-bench perl run-all-tests --server=# where # is one of supported servers. You can get a list of all options and supported servers by doing `run-all-tests --help'. `crash-me' tries to determine what features a database supports and what its capabilities and limitations are by actually running queries. For example, it determines: * What column types are supported * How many indexes are supported * What functions are supported * How big a query can be * How big a `VARCHAR' column can be We can find the result from crash-me on a lot of different databases at `http://www.mysql.com/information/crash-me.php'. Using Your Own Benchmarks ------------------------- You should definitely benchmark your application and database to find out where the bottlenecks are. By fixing it (or by replacing the bottleneck with a 'dummy module') you can then easily identify the next bottleneck (and so on). Even if the overall performance for your application is sufficient, you should at least make a plan for each bottleneck, and decide how to solve it if someday you really need the extra performance. For an example of portable benchmark programs, look at the MySQL benchmark suite. *Note MySQL Benchmarks: MySQL Benchmarks. You can take any program from this suite and modify it for your needs. By doing this, you can try different solutions to your problem and test which is really the fastest solution for you. It is very common that some problems only occur when the system is very heavily loaded. We have had many customers who contact us when they have a (tested) system in production and have encountered load problems. In every one of these cases so far, it has been problems with basic design (table scans are NOT good at high load) or OS/Library issues. Most of this would be a *LOT* easier to fix if the systems were not already in production. To avoid problems like this, you should put some effort into benchmarking your whole application under the worst possible load! You can use Super Smack for this, and it is available at: `http://www.mysql.com/Downloads/super-smack/super-smack-1.0.tar.gz'. As the name suggests, it can bring your system down to its knees if you ask it, so make sure to use it only on your development systems. Optimizing `SELECT's and Other Queries ====================================== First, one thing that affects all queries: The more complex permission system setup you have, the more overhead you get. If you do not have any `GRANT' statements done, MySQL will optimize the permission checking somewhat. So if you have a very high volume it may be worth the time to avoid grants. Otherwise more permission check results in a larger overhead. If your problem is with some explicit MySQL function, you can always time this in the MySQL client: mysql> select benchmark(1000000,1+1); +------------------------+ | benchmark(1000000,1+1) | +------------------------+ | 0 | +------------------------+ 1 row in set (0.32 sec) The above shows that MySQL can execute 1,000,000 `+' expressions in 0.32 seconds on a `PentiumII 400MHz'. All MySQL functions should be very optimized, but there may be some exceptions, and the `benchmark(loop_count,expression)' is a great tool to find out if this is a problem with your query. `EXPLAIN' Syntax (Get Information About a `SELECT') --------------------------------------------------- EXPLAIN tbl_name or EXPLAIN SELECT select_options `EXPLAIN tbl_name' is a synonym for `DESCRIBE tbl_name' or `SHOW COLUMNS FROM tbl_name'. When you precede a `SELECT' statement with the keyword `EXPLAIN', MySQL explains how it would process the `SELECT', providing information about how tables are joined and in which order. With the help of `EXPLAIN', you can see when you must add indexes to tables to get a faster `SELECT' that uses indexes to find the records. You can also see if the optimizer joins the tables in an optimal order. To force the optimizer to use a specific join order for a `SELECT' statement, add a `STRAIGHT_JOIN' clause. For non-simple joins, `EXPLAIN' returns a row of information for each table used in the `SELECT' statement. The tables are listed in the order they would be read. MySQL resolves all joins using a single-sweep multi-join method. This means that MySQL reads a row from the first table, then finds a matching row in the second table, then in the third table and so on. When all tables are processed, it outputs the selected columns and backtracks through the table list until a table is found for which there are more matching rows. The next row is read from this table and the process continues with the next table. Output from `EXPLAIN' includes the following columns: `table' The table to which the row of output refers. `type' The join type. Information about the various types is given below. `possible_keys' The `possible_keys' column indicates which indexes MySQL could use to find the rows in this table. Note that this column is totally independent of the order of the tables. That means that some of the keys in possible_keys may not be usable in practice with the generated table order. If this column is empty, there are no relevant indexes. In this case, you may be able to improve the performance of your query by examining the `WHERE' clause to see if it refers to some column or columns that would be suitable for indexing. If so, create an appropriate index and check the query with `EXPLAIN' again. *Note ALTER TABLE::. To see what indexes a table has, use `SHOW INDEX FROM tbl_name'. `key' The `key' column indicates the key that MySQL actually decided to use. The key is `NULL' if no index was chosen. If MySQL chooses the wrong index, you can probably force MySQL to use another index by using `myisamchk --analyze', *Note myisamchk syntax::, or by using `USE INDEX/IGNORE INDEX'. *Note JOIN::. `key_len' The `key_len' column indicates the length of the key that MySQL decided to use. The length is `NULL' if the `key' is `NULL'. Note that this tells us how many parts of a multi-part key MySQL will actually use. `ref' The `ref' column shows which columns or constants are used with the `key' to select rows from the table. `rows' The `rows' column indicates the number of rows MySQL believes it must examine to execute the query. `Extra' This column contains additional information of how MySQL will resolve the query. Here is an explanation of the different text strings that can be found in this column: `Distinct' MySQL will not continue searching for more rows for the current row combination after it has found the first matching row. `Not exists' MySQL was able to do a `LEFT JOIN' optimization on the query and will not examine more rows in this table for the previous row combination after it finds one row that matches the `LEFT JOIN' criteria. Here is an example for this: SELECT * FROM t1 LEFT JOIN t2 ON t1.id=t2.id WHERE t2.id IS NULL; Assume that `t2.id' is defined with `NOT NULL'. In this case MySQL will scan `t1' and look up the rows in `t2' through `t1.id'. If MySQL finds a matching row in `t2', it knows that `t2.id' can never be `NULL', and will not scan through the rest of the rows in `t2' that has the same `id'. In other words, for each row in `t1', MySQL only needs to do a single lookup in `t2', independent of how many matching rows there are in `t2'. ``range checked for each record (index map: #)'' MySQL didn't find a real good index to use. It will, instead, for each row combination in the preceding tables, do a check on which index to use (if any), and use this index to retrieve the rows from the table. This isn't very fast but is faster than having to do a join without an index. `Using filesort' MySQL will need to do an extra pass to find out how to retrieve the rows in sorted order. The sort is done by going through all rows according to the `join type' and storing the sort key + pointer to the row for all rows that match the `WHERE'. Then the keys are sorted. Finally the rows are retrieved in sorted order. `Using index' The column information is retrieved from the table using only information in the index tree without having to do an additional seek to read the actual row. This can be done when all the used columns for the table are part of the same index. `Using temporary' To resolve the query MySQL will need to create a temporary table to hold the result. This typically happens if you do an `ORDER BY' on a different column set than you did a `GROUP BY' on. `Where used' A `WHERE' clause will be used to restrict which rows will be matched against the next table or sent to the client. If you don't have this information and the table is of type `ALL' or `index', you may have something wrong in your query (if you don't intend to fetch/examine all rows from the table). If you want to get your queries as fast as possible, you should look out for `Using filesort' and `Using temporary'. The different join types are listed below, ordered from best to worst type: `system' The table has only one row (= system table). This is a special case of the `const' join type. `const' The table has at most one matching row, which will be read at the start of the query. Because there is only one row, values from the column in this row can be regarded as constants by the rest of the optimizer. `const' tables are very fast as they are read only once! `eq_ref' One row will be read from this table for each combination of rows from the previous tables. This is the best possible join type, other than the `const' types. It is used when all parts of an index are used by the join and the index is `UNIQUE' or a `PRIMARY KEY'. `ref' All rows with matching index values will be read from this table for each combination of rows from the previous tables. `ref' is used if the join uses only a leftmost prefix of the key, or if the key is not `UNIQUE' or a `PRIMARY KEY' (in other words, if the join cannot select a single row based on the key value). If the key that is used matches only a few rows, this join type is good. `range' Only rows that are in a given range will be retrieved, using an index to select the rows. The `key' column indicates which index is used. The `key_len' contains the longest key part that was used. The `ref' column will be NULL for this type. `index' This is the same as `ALL', except that only the index tree is scanned. This is usually faster than `ALL', as the index file is usually smaller than the data file. `ALL' A full table scan will be done for each combination of rows from the previous tables. This is normally not good if the table is the first table not marked `const', and usually *very* bad in all other cases. You normally can avoid `ALL' by adding more indexes, so that the row can be retrieved based on constant values or column values from earlier tables. You can get a good indication of how good a join is by multiplying all values in the `rows' column of the `EXPLAIN' output. This should tell you roughly how many rows MySQL must examine to execute the query. This number is also used when you restrict queries with the `max_join_size' variable. *Note Server parameters::. The following example shows how a `JOIN' can be optimized progressively using the information provided by `EXPLAIN'. Suppose you have the `SELECT' statement shown below, that you examine using `EXPLAIN': EXPLAIN SELECT tt.TicketNumber, tt.TimeIn, tt.ProjectReference, tt.EstimatedShipDate, tt.ActualShipDate, tt.ClientID, tt.ServiceCodes, tt.RepetitiveID, tt.CurrentProcess, tt.CurrentDPPerson, tt.RecordVolume, tt.DPPrinted, et.COUNTRY, et_1.COUNTRY, do.CUSTNAME FROM tt, et, et AS et_1, do WHERE tt.SubmitTime IS NULL AND tt.ActualPC = et.EMPLOYID AND tt.AssignedPC = et_1.EMPLOYID AND tt.ClientID = do.CUSTNMBR; For this example, assume that: * The columns being compared have been declared as follows: *Table* *Column* *Column type* `tt' `ActualPC' `CHAR(10)' `tt' `AssignedPC' `CHAR(10)' `tt' `ClientID' `CHAR(10)' `et' `EMPLOYID' `CHAR(15)' `do' `CUSTNMBR' `CHAR(15)' * The tables have the indexes shown below: *Table* *Index* `tt' `ActualPC' `tt' `AssignedPC' `tt' `ClientID' `et' `EMPLOYID' (primary key) `do' `CUSTNMBR' (primary key) * The `tt.ActualPC' values aren't evenly distributed. Initially, before any optimizations have been performed, the `EXPLAIN' statement produces the following information: table type possible_keys key key_len ref rows Extra et ALL PRIMARY NULL NULL NULL 74 do ALL PRIMARY NULL NULL NULL 2135 et_1 ALL PRIMARY NULL NULL NULL 74 tt ALL AssignedPC,ClientID,ActualPC NULL NULL NULL 3872 range checked for each record (key map: 35) Because `type' is `ALL' for each table, this output indicates that MySQL is doing a full join for all tables! This will take quite a long time, as the product of the number of rows in each table must be examined! For the case at hand, this is `74 * 2135 * 74 * 3872 = 45,268,558,720' rows. If the tables were bigger, you can only imagine how long it would take. One problem here is that MySQL can't (yet) use indexes on columns efficiently if they are declared differently. In this context, `VARCHAR' and `CHAR' are the same unless they are declared as different lengths. Because `tt.ActualPC' is declared as `CHAR(10)' and `et.EMPLOYID' is declared as `CHAR(15)', there is a length mismatch. To fix this disparity between column lengths, use `ALTER TABLE' to lengthen `ActualPC' from 10 characters to 15 characters: mysql> ALTER TABLE tt MODIFY ActualPC VARCHAR(15); Now `tt.ActualPC' and `et.EMPLOYID' are both `VARCHAR(15)'. Executing the `EXPLAIN' statement again produces this result: table type possible_keys key key_len ref rows Extra tt ALL AssignedPC,ClientID,ActualPC NULL NULL NULL 3872 where used do ALL PRIMARY NULL NULL NULL 2135 range checked for each record (key map: 1) et_1 ALL PRIMARY NULL NULL NULL 74 range checked for each record (key map: 1) et eq_ref PRIMARY PRIMARY 15 tt.ActualPC 1 This is not perfect, but is much better (the product of the `rows' values is now less by a factor of 74). This version is executed in a couple of seconds. A second alteration can be made to eliminate the column length mismatches for the `tt.AssignedPC = et_1.EMPLOYID' and `tt.ClientID = do.CUSTNMBR' comparisons: mysql> ALTER TABLE tt MODIFY AssignedPC VARCHAR(15), MODIFY ClientID VARCHAR(15); Now `EXPLAIN' produces the output shown below: table type possible_keys key key_len ref rows Extra et ALL PRIMARY NULL NULL NULL 74 tt ref AssignedPC,ClientID,ActualPC ActualPC 15 et.EMPLOYID 52 where used et_1 eq_ref PRIMARY PRIMARY 15 tt.AssignedPC 1 do eq_ref PRIMARY PRIMARY 15 tt.ClientID 1 This is almost as good as it can get. The remaining problem is that, by default, MySQL assumes that values in the `tt.ActualPC' column are evenly distributed, and that isn't the case for the `tt' table. Fortunately, it is easy to tell MySQL about this: shell> myisamchk --analyze PATH_TO_MYSQL_DATABASE/tt shell> mysqladmin refresh Now the join is perfect, and `EXPLAIN' produces this result: table type possible_keys key key_len ref rows Extra tt ALL AssignedPC,ClientID,ActualPC NULL NULL NULL 3872 where used et eq_ref PRIMARY PRIMARY 15 tt.ActualPC 1 et_1 eq_ref PRIMARY PRIMARY 15 tt.AssignedPC 1 do eq_ref PRIMARY PRIMARY 15 tt.ClientID 1 Note that the `rows' column in the output from `EXPLAIN' is an educated guess from the MySQL join optimizer. To optimize a query, you should check if the numbers are even close to the truth. If not, you may get better performance by using `STRAIGHT_JOIN' in your `SELECT' statement and trying to list the tables in a different order in the `FROM' clause. Estimating Query Performance ---------------------------- In most cases you can estimate the performance by counting disk seeks. For small tables, you can usually find the row in 1 disk seek (as the index is probably cached). For bigger tables, you can estimate that (using B++ tree indexes) you will need: `log(row_count) / log(index_block_length / 3 * 2 / (index_length + data_pointer_length)) + 1' seeks to find a row. In MySQL an index block is usually 1024 bytes and the data pointer is usually 4 bytes. A 500,000 row table with an index length of 3 (medium integer) gives you: `log(500,000)/log(1024/3*2/(3+4)) + 1' = 4 seeks. As the above index would require about 500,000 * 7 * 3/2 = 5.2M, (assuming that the index buffers are filled to 2/3, which is typical) you will probably have much of the index in memory and you will probably only need 1-2 calls to read data from the OS to find the row. For writes, however, you will need 4 seek requests (as above) to find where to place the new index and normally 2 seeks to update the index and write the row. Note that the above doesn't mean that your application will slowly degenerate by N log N! As long as everything is cached by the OS or SQL server things will only go marginally slower while the table gets bigger. After the data gets too big to be cached, things will start to go much slower until your applications is only bound by disk-seeks (which increase by N log N). To avoid this, increase the index cache as the data grows. *Note Server parameters::. Speed of `SELECT' Queries ------------------------- In general, when you want to make a slow `SELECT ... WHERE' faster, the first thing to check is whether or not you can add an index. *Note MySQL indexes: MySQL indexes. All references between different tables should usually be done with indexes. You can use the `EXPLAIN' command to determine which indexes are used for a `SELECT'. *Note `EXPLAIN': EXPLAIN. Some general tips: * To help MySQL optimize queries better, run `myisamchk --analyze' on a table after it has been loaded with relevant data. This updates a value for each index part that indicates the average number of rows that have the same value. (For unique indexes, this is always 1, of course.). MySQL will use this to decide which index to choose when you connect two tables with 'a non-constant expression'. You can check the result from the `analyze' run by doing `SHOW INDEX FROM table_name' and examining the `Cardinality' column. * To sort an index and data according to an index, use `myisamchk --sort-index --sort-records=1' (if you want to sort on index 1). If you have a unique index from which you want to read all records in order according to that index, this is a good way to make that faster. Note, however, that this sorting isn't written optimally and will take a long time for a large table! How MySQL Optimizes `WHERE' Clauses ----------------------------------- The `WHERE' optimizations are put in the `SELECT' part here because they are mostly used with `SELECT', but the same optimizations apply for `WHERE' in `DELETE' and `UPDATE' statements. Also note that this section is incomplete. MySQL does many optimizations, and we have not had time to document them all. Some of the optimizations performed by MySQL are listed below: * Removal of unnecessary parentheses: ((a AND b) AND c OR (((a AND b) AND (c AND d)))) -> (a AND b AND c) OR (a AND b AND c AND d) * Constant folding: (a<b AND b=c) AND a=5 -> b>5 AND b=c AND a=5 * Constant condition removal (needed because of constant folding): (B>=5 AND B=5) OR (B=6 AND 5=5) OR (B=7 AND 5=6) -> B=5 OR B=6 * Constant expressions used by indexes are evaluated only once. * `COUNT(*)' on a single table without a `WHERE' is retrieved directly from the table information. This is also done for any `NOT NULL' expression when used with only one table. * Early detection of invalid constant expressions. MySQL quickly detects that some `SELECT' statements are impossible and returns no rows. * `HAVING' is merged with `WHERE' if you don't use `GROUP BY' or group functions (`COUNT()', `MIN()'...). * For each sub-join, a simpler `WHERE' is constructed to get a fast `WHERE' evaluation for each sub-join and also to skip records as soon as possible. * All constant tables are read first, before any other tables in the query. A constant table is: - An empty table or a table with 1 row. - A table that is used with a `WHERE' clause on a `UNIQUE' index, or a `PRIMARY KEY', where all index parts are used with constant expressions and the index parts are defined as `NOT NULL'. All the following tables are used as constant tables: mysql> SELECT * FROM t WHERE primary_key=1; mysql> SELECT * FROM t1,t2 WHERE t1.primary_key=1 AND t2.primary_key=t1.id; * The best join combination to join the tables is found by trying all possibilities. If all columns in `ORDER BY' and in `GROUP BY' come from the same table, then this table is preferred first when joining. * If there is an `ORDER BY' clause and a different `GROUP BY' clause, or if the `ORDER BY' or `GROUP BY' contains columns from tables other than the first table in the join queue, a temporary table is created. * If you use `SQL_SMALL_RESULT', MySQL will use an in-memory temporary table. * Each table index is queried, and the best index that spans fewer than 30% of the rows is used. If no such index can be found, a quick table scan is used. * In some cases, MySQL can read rows from the index without even consulting the data file. If all columns used from the index are numeric, then only the index tree is used to resolve the query. * Before each record is output, those that do not match the `HAVING' clause are skipped. Some examples of queries that are very fast: mysql> SELECT COUNT(*) FROM tbl_name; mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name; mysql> SELECT MAX(key_part2) FROM tbl_name WHERE key_part_1=constant; mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,... LIMIT 10; mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10; The following queries are resolved using only the index tree (assuming the indexed columns are numeric): mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val; mysql> SELECT COUNT(*) FROM tbl_name WHERE key_part1=val1 AND key_part2=val2; mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1; The following queries use indexing to retrieve the rows in sorted order without a separate sorting pass: mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,... ; mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,... ; How MySQL Optimizes `DISTINCT' ------------------------------ `DISTINCT' is converted to a `GROUP BY' on all columns, `DISTINCT' combined with `ORDER BY' will in many cases also need a temporary table. When combining `LIMIT #' with `DISTINCT', MySQL will stop as soon as it finds `#' unique rows. If you don't use columns from all used tables, MySQL will stop the scanning of the not used tables as soon as it has found the first match. SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a; In the case, assuming t1 is used before t2 (check with `EXPLAIN'), then MySQL will stop reading from t2 (for that particular row in t1) when the first row in t2 is found. How MySQL Optimizes `LEFT JOIN' and `RIGHT JOIN' ------------------------------------------------ `A LEFT JOIN B' in MySQL is implemented as follows: * The table `B' is set to be dependent on table `A' and all tables that `A' is dependent on. * The table `A' is set to be dependent on all tables (except `B') that are used in the `LEFT JOIN' condition. * All `LEFT JOIN' conditions are moved to the `WHERE' clause. * All standard join optimizations are done, with the exception that a table is always read after all tables it is dependent on. If there is a circular dependence then MySQL will issue an error. * All standard `WHERE' optimizations are done. * If there is a row in `A' that matches the `WHERE' clause, but there wasn't any row in `B' that matched the `LEFT JOIN' condition, then an extra `B' row is generated with all columns set to `NULL'. * If you use `LEFT JOIN' to find rows that don't exist in some table and you have the following test: `column_name IS NULL' in the `WHERE' part, where column_name is a column that is declared as `NOT NULL', then MySQL will stop searching after more rows (for a particular key combination) after it has found one row that matches the `LEFT JOIN' condition. `RIGHT JOIN' is implemented analogously as `LEFT JOIN'. The table read order forced by `LEFT JOIN' and `STRAIGHT JOIN' will help the join optimizer (which calculates in which order tables should be joined) to do its work much more quickly, as there are fewer table permutations to check. Note that the above means that if you do a query of type: SELECT * FROM a,b LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key MySQL will do a full scan on `b' as the `LEFT JOIN' will force it to be read before `d'. The fix in this case is to change the query to: SELECT * FROM b,a LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key How MySQL Optimizes `LIMIT' --------------------------- In some cases MySQL will handle the query differently when you are using `LIMIT #' and not using `HAVING': * If you are selecting only a few rows with `LIMIT', MySQL will use indexes in some cases when it normally would prefer to do a full table scan. * If you use `LIMIT #' with `ORDER BY', MySQL will end the sorting as soon as it has found the first `#' lines instead of sorting the whole table. * When combining `LIMIT #' with `DISTINCT', MySQL will stop as soon as it finds `#' unique rows. * In some cases a `GROUP BY' can be resolved by reading the key in order (or do a sort on the key) and then calculate summaries until the key value changes. In this case `LIMIT #' will not calculate any unnecessary `GROUP BY''s. * As soon as MySQL has sent the first `#' rows to the client, it will abort the query. * `LIMIT 0' will always quickly return an empty set. This is useful to check the query and to get the column types of the result columns. * The size of temporary tables uses the `LIMIT #' to calculate how much space is needed to resolve the query. Speed of `INSERT' Queries ------------------------- The time to insert a record consists approximately of: * Connect: (3) * Sending query to server: (2) * Parsing query: (2) * Inserting record: (1 x size of record) * Inserting indexes: (1 x number of indexes) * Close: (1) where the numbers are somewhat proportional to the overall time. This does not take into consideration the initial overhead to open tables (which is done once for each concurrently running query). The size of the table slows down the insertion of indexes by N log N (B-trees). Some ways to speed up inserts: * If you are inserting many rows from the same client at the same time, use multiple value lists `INSERT' statements. This is much faster (many times in some cases) than using separate `INSERT' statements. * If you are inserting a lot of rows from different clients, you can get higher speed by using the `INSERT DELAYED' statement. *Note `INSERT': INSERT. * Note that with `MyISAM' you can insert rows at the same time `SELECT's are running if there are no deleted rows in the tables. * When loading a table from a text file, use `LOAD DATA INFILE'. This is usually 20 times faster than using a lot of `INSERT' statements. *Note `LOAD DATA': LOAD DATA. * It is possible with some extra work to make `LOAD DATA INFILE' run even faster when the table has many indexes. Use the following procedure: 1. Optionally create the table with `CREATE TABLE'. For example, using `mysql' or Perl-DBI. 2. Execute a `FLUSH TABLES' statement or the shell command `mysqladmin flush-tables'. 3. Use `myisamchk --keys-used=0 -rq /path/to/db/tbl_name'. This will remove all usage of all indexes from the table. 4. Insert data into the table with `LOAD DATA INFILE'. This will not update any indexes and will therefore be very fast. 5. If you are going to only read the table in the future, run `myisampack' on it to make it smaller. *Note Compressed format::. 6. Re-create the indexes with `myisamchk -r -q /path/to/db/tbl_name'. This will create the index tree in memory before writing it to disk, which is much faster because it avoids lots of disk seeks. The resulting index tree is also perfectly balanced. 7. Execute a `FLUSH TABLES' statement or the shell command `mysqladmin flush-tables'. This procedure will be built into `LOAD DATA INFILE' in some future version of MySQL. * You can speed up insertions by locking your tables: mysql> LOCK TABLES a WRITE; mysql> INSERT INTO a VALUES (1,23),(2,34),(4,33); mysql> INSERT INTO a VALUES (8,26),(6,29); mysql> UNLOCK TABLES; The main speed difference is that the index buffer is flushed to disk only once, after all `INSERT' statements have completed. Normally there would be as many index buffer flushes as there are different `INSERT' statements. Locking is not needed if you can insert all rows with a single statement. Locking will also lower the total time of multi-connection tests, but the maximum wait time for some threads will go up (because they wait for locks). For example: thread 1 does 1000 inserts thread 2, 3, and 4 does 1 insert thread 5 does 1000 inserts If you don't use locking, 2, 3, and 4 will finish before 1 and 5. If you use locking, 2, 3, and 4 probably will not finish before 1 or 5, but the total time should be about 40% faster. As `INSERT', `UPDATE', and `DELETE' operations are very fast in MySQL, you will obtain better overall performance by adding locks around everything that does more than about 5 inserts or updates in a row. If you do very many inserts in a row, you could do a `LOCK TABLES' followed by an `UNLOCK TABLES' once in a while (about each 1000 rows) to allow other threads access to the table. This would still result in a nice performance gain. Of course, `LOAD DATA INFILE' is much faster for loading data. To get some more speed for both `LOAD DATA INFILE' and `INSERT', enlarge the key buffer. *Note Server parameters::. Speed of `UPDATE' Queries ------------------------- Update queries are optimized as a `SELECT' query with the additional overhead of a write. The speed of the write is dependent on the size of the data that is being updated and the number of indexes that are updated. Indexes that are not changed will not be updated. Also, another way to get fast updates is to delay updates and then do many updates in a row later. Doing many updates in a row is much quicker than doing one at a time if you lock the table. Note that, with dynamic record format, updating a record to a longer total length may split the record. So if you do this often, it is very important to `OPTIMIZE TABLE' sometimes. *Note `OPTIMIZE TABLE': OPTIMIZE TABLE. Speed of `DELETE' Queries ------------------------- If you want to delete all rows in the table, you should use `TRUNCATE TABLE table_name'. *Note TRUNCATE::. The time to delete a record is exactly proportional to the number of indexes. To delete records more quickly, you can increase the size of the index cache. *Note Server parameters::. Other Optimization Tips ----------------------- Unsorted tips for faster systems: * Use persistent connections to the database to avoid the connection overhead. If you can't use persistent connections and you are doing a lot of new connections to the database, you may want to change the value of the `thread_cache_size' variable. *Note Server parameters::. * Always check that all your queries really use the indexes you have created in the tables. In MySQL you can do this with the `EXPLAIN' command. *Note Explain: (manual)EXPLAIN. * Try to avoid complex `SELECT' queries on tables that are updated a lot. This is to avoid problems with table locking. * The new `MyISAM' tables can insert rows in a table without deleted rows at the same time another table is reading from it. If this is important for you, you should consider methods where you don't have to delete rows or run `OPTIMIZE TABLE' after you have deleted a lot of rows. * Use `ALTER TABLE ... ORDER BY expr1,expr2...' if you mostly retrieve rows in expr1,expr2.. order. By using this option after big changes to the table, you may be able to get higher performance. * In some cases it may make sense to introduce a column that is 'hashed' based on information from other columns. If this column is short and reasonably unique it may be much faster than a big index on many columns. In MySQL it's very easy to use this extra column: `SELECT * FROM table_name WHERE hash=MD5(concat(col1,col2)) AND col_1='constant' AND col_2='constant'' * For tables that change a lot you should try to avoid all `VARCHAR' or `BLOB' columns. You will get dynamic row length as soon as you are using a single `VARCHAR' or `BLOB' column. *Note Table types::. * It's not normally useful to split a table into different tables just because the rows gets 'big'. To access a row, the biggest performance hit is the disk seek to find the first byte of the row. After finding the data most new disks can read the whole row fast enough for most applications. The only cases where it really matters to split up a table is if it's a dynamic row size table (see above) that you can change to a fixed row size, or if you very often need to scan the table and don't need most of the columns. *Note Table types::. * If you very often need to calculate things based on information from a lot of rows (like counts of things), it's probably much better to introduce a new table and update the counter in real time. An update of type `UPDATE table set count=count+1 where index_column=constant' is very fast! This is really important when you use databases like MySQL that only have table locking (multiple readers / single writers). This will also give better performance with most databases, as the row locking manager in this case will have less to do. * If you need to collect statistics from big log tables, use summary tables instead of scanning the whole table. Maintaining the summaries should be much faster than trying to do statistics 'live'. It's much faster to regenerate new summary tables from the logs when things change (depending on business decisions) than to have to change the running application! * If possible, one should classify reports as 'live' or 'statistical', where data needed for statistical reports are only generated based on summary tables that are generated from the actual data. * Take advantage of the fact that columns have default values. Insert values explicitly only when the value to be inserted differs from the default. This reduces the parsing that MySQL need to do and improves the insert speed. * In some cases it's convenient to pack and store data into a blob. In this case you have to add some extra code in your application to pack/unpack things in the blob, but this may save a lot of accesses at some stage. This is practical when you have data that doesn't conform to a static table structure. * Normally you should try to keep all data non-redundant (what is called 3rd normal form in database theory), but you should not be afraid of duplicating things or creating summary tables if you need these to gain more speed. * Stored procedures or UDF (user-defined functions) may be a good way to get more performance. In this case you should, however, always have a way to do this some other (slower) way if you use some database that doesn't support this. * You can always gain something by caching queries/answers in your application and trying to do many inserts/updates at the same time. If your database supports lock tables (like MySQL and Oracle), this should help to ensure that the index cache is only flushed once after all updates. * Use `INSERT /*! DELAYED */' when you do not need to know when your data is written. This speeds things up because many records can be written with a single disk write. * Use `INSERT /*! LOW_PRIORITY */' when you want your selects to be more important. * Use `SELECT /*! HIGH_PRIORITY */' to get selects that jump the queue. That is, the select is done even if there is somebody waiting to do a write. * Use the multi-line `INSERT' statement to store many rows with one SQL command (many SQL servers supports this). * Use `LOAD DATA INFILE' to load bigger amounts of data. This is faster than normal inserts and will be even faster when `myisamchk' is integrated in `mysqld'. * Use `AUTO_INCREMENT' columns to make unique values. * Use `OPTIMIZE TABLE' once in a while to avoid fragmentation when using dynamic table format. *Note `OPTIMIZE TABLE': OPTIMIZE TABLE. * Use `HEAP' tables to get more speed when possible. *Note Table types::. * When using a normal Web server setup, images should be stored as files. That is, store only a file reference in the database. The main reason for this is that a normal Web server is much better at caching files than database contents. So it it's much easier to get a fast system if you are using files. * Use in memory tables for non-critical data that are accessed often (like information about the last shown banner for users that don't have cookies). * Columns with identical information in different tables should be declared identical and have identical names. Before Version 3.23 you got slow joins otherwise. Try to keep the names simple (use `name' instead of `customer_name' in the customer table). To make your names portable to other SQL servers you should keep them shorter than 18 characters. * If you need REALLY high speed, you should take a look at the low-level interfaces for data storage that the different SQL servers support! For example, by accessing the MySQL `MyISAM' directly, you could get a speed increase of 2-5 times compared to using the SQL interface. To be able to do this the data must be on the same server as the application, and usually it should only be accessed by one process (because external file locking is really slow). One could eliminate the above problems by introducing low-level `MyISAM' commands in the MySQL server (this could be one easy way to get more performance if needed). By carefully designing the database interface, it should be quite easy to support this types of optimization. * In many cases it's faster to access data from a database (using a live connection) than accessing a text file, just because the database is likely to be more compact than the text file (if you are using numerical data), and this will involve fewer disk accesses. You will also save code because you don't have to parse your text files to find line and column boundaries. * You can also use replication to speed things up. *Note Replication::. * Declaring a table with `DELAY_KEY_WRITE=1' will make the updating of indexes faster, as these are not logged to disk until the file is closed. The downside is that you should run `myisamchk' on these tables before you start `mysqld' to ensure that they are okay if something killed `mysqld' in the middle. As the key information can always be generated from the data, you should not lose anything by using `DELAY_KEY_WRITE'. Locking Issues ============== How MySQL Locks Tables ---------------------- You can find a discussion about different locking methods in the appendix. *Note Locking methods::. All locking in MySQL is deadlock-free. This is managed by always requesting all needed locks at once at the beginning of a query and always locking the tables in the same order. The locking method MySQL uses for `WRITE' locks works as follows: * If there are no locks on the table, put a write lock on it. * Otherwise, put the lock request in the write lock queue. The locking method MySQL uses for `READ' locks works as follows: * If there are no write locks on the table, put a read lock on it. * Otherwise, put the lock request in the read lock queue. When a lock is released, the lock is made available to the threads in the write lock queue, then to the threads in the read lock queue. This means that if you have many updates on a table, `SELECT' statements will wait until there are no more updates. To work around this for the case where you want to do many `INSERT' and `SELECT' operations on a table, you can insert rows in a temporary table and update the real table with the records from the temporary table once in a while. This can be done with the following code: mysql> LOCK TABLES real_table WRITE, insert_table WRITE; mysql> insert into real_table select * from insert_table; mysql> TRUNCATE TABLE insert_table; mysql> UNLOCK TABLES; You can use the `LOW_PRIORITY' options with `INSERT', `UPDATE' or `DELETE' or `HIGH_PRIORITY' with `SELECT' if you want to prioritize retrieval in some specific cases. You can also start `mysqld' with `--low-priority-updates' to get the same behaveour. Using `SQL_BUFFER_RESULT' can also help making table locks shorter. *Note SELECT::. You could also change the locking code in `mysys/thr_lock.c' to use a single queue. In this case, write locks and read locks would have the same priority, which might help some applications. Table Locking Issues -------------------- The table locking code in MySQL is deadlock free. MySQL uses table locking (instead of row locking or column locking) on all table types, except `BDB' tables, to achieve a very high lock speed. For large tables, table locking is MUCH better than row locking for most applications, but there are, of course, some pitfalls. For `BDB' and `InnoDB' tables, MySQL only uses table locking if you explicitely lock the table with `LOCK TABLES' or execute a command that will modify every row in the table, like `ALTER TABLE'. For these table types we recommend you to not use `LOCK TABLES' at all. In MySQL Version 3.23.7 and above, you can insert rows into `MyISAM' tables at the same time other threads are reading from the table. Note that currently this only works if there are no holes after deleted rows in the table at the time the insert is made. When all holes has been filled with new data, concurrent inserts will automatically be enabled again. Table locking enables many threads to read from a table at the same time, but if a thread wants to write to a table, it must first get exclusive access. During the update, all other threads that want to access this particular table will wait until the update is ready. As updates on tables normally are considered to be more important than `SELECT', all statements that update a table have higher priority than statements that retrieve information from a table. This should ensure that updates are not 'starved' because one issues a lot of heavy queries against a specific table. (You can change this by using LOW_PRIORITY with the statement that does the update or `HIGH_PRIORITY' with the `SELECT' statement.) Starting from MySQL Version 3.23.7 one can use the `max_write_lock_count' variable to force MySQL to temporary give all `SELECT' statements, that wait for a table, a higher priority after a specific number of inserts on a table. Table locking is, however, not very good under the following senario: * A client issues a `SELECT' that takes a long time to run. * Another client then issues an `UPDATE' on a used table. This client will wait until the `SELECT' is finished. * Another client issues another `SELECT' statement on the same table. As `UPDATE' has higher priority than `SELECT', this `SELECT' will wait for the `UPDATE' to finish. It will also wait for the first `SELECT' to finish! * A thread is waiting for something like `full disk', in which case all threads that wants to access the problem table will also be put in a waiting state until more disk space is made available. Some possible solutions to this problem are: * Try to get the `SELECT' statements to run faster. You may have to create some summary tables to do this. * Start `mysqld' with `--low-priority-updates'. This will give all statements that update (modify) a table lower priority than a `SELECT' statement. In this case the last `SELECT' statement in the previous scenario would execute before the `INSERT' statement. * You can give a specific `INSERT', `UPDATE', or `DELETE' statement lower priority with the `LOW_PRIORITY' attribute. * Start `mysqld' with a low value for *max_write_lock_count* to give `READ' locks after a certain number of `WRITE' locks. * You can specify that all updates from a specific thread should be done with low priority by using the SQL command: `SET SQL_LOW_PRIORITY_UPDATES=1'. *Note `SET OPTION': SET OPTION. * You can specify that a specific `SELECT' is very important with the `HIGH_PRIORITY' attribute. *Note `SELECT': SELECT. * If you have problems with `INSERT' combined with `SELECT', switch to use the new `MyISAM' tables as these support concurrent `SELECT's and `INSERT's. * If you mainly mix `INSERT' and `SELECT' statements, the `DELAYED' attribute to `INSERT' will probably solve your problems. *Note `INSERT': INSERT. * If you have problems with `SELECT' and `DELETE', the `LIMIT' option to `DELETE' may help. *Note `DELETE': DELETE. Optimizing Database Structure ============================= Design Choices -------------- MySQL keeps row data and index data in separate files. Many (almost all) other databases mix row and index data in the same file. We believe that the MySQL choice is better for a very wide range of modern systems. Another way to store the row data is to keep the information for each column in a separate area (examples are SDBM and Focus). This will cause a performance hit for every query that accesses more than one column. Because this degenerates so quickly when more than one column is accessed, we believe that this model is not good for general purpose databases. The more common case is that the index and data are stored together (like in Oracle/Sybase et al). In this case you will find the row information at the leaf page of the index. The good thing with this layout is that it, in many cases, depending on how well the index is cached, saves a disk read. The bad things with this layout are: * Table scanning is much slower because you have to read through the indexes to get at the data. * You can't use only the index table to retrieve data for a query. * You lose a lot of space, as you must duplicate indexes from the nodes (as you can't store the row in the nodes). * Deletes will degenerate the table over time (as indexes in nodes are usually not updated on delete). * It's harder to cache ONLY the index data. Get Your Data as Small as Possible ---------------------------------- One of the most basic optimization is to get your data (and indexes) to take as little space on the disk (and in memory) as possible. This can give huge improvements because disk reads are faster and normally less main memory will be used. Indexing also takes less resources if done on smaller columns. MySQL supports a lot of different table types and row formats. Choosing the right table format may give you a big performance gain. *Note Table types::. You can get better performance on a table and minimize storage space using the techniques listed below: * Use the most efficient (smallest) types possible. MySQL has many specialized types that save disk space and memory. * Use the smaller integer types if possible to get smaller tables. For example, `MEDIUMINT' is often better than `INT'. * Declare columns to be `NOT NULL' if possible. It makes everything faster and you save one bit per column. Note that if you really need `NULL' in your application you should definitely use it. Just avoid having it on all columns by default. * If you don't have any variable-length columns (`VARCHAR', `TEXT', or `BLOB' columns), a fixed-size record format is used. This is faster but unfortunately may waste some space. *Note MyISAM table formats::. * The primary index of a table should be as short as possible. This makes identification of one row easy and efficient. * For each table, you have to decide which storage/index method to use. *Note Table types::. * Only create the indexes that you really need. Indexes are good for retrieval but bad when you need to store things fast. If you mostly access a table by searching on a combination of columns, make an index on them. The first index part should be the most used column. If you are ALWAYS using many columns, you should use the column with more duplicates first to get better compression of the index. * If it's very likely that a column has a unique prefix on the first number of characters, it's better to only index this prefix. MySQL supports an index on a part of a character column. Shorter indexes are faster not only because they take less disk space but also because they will give you more hits in the index cache and thus fewer disk seeks. *Note Server parameters::. * In some circumstances it can be beneficial to split into two a table that is scanned very often. This is especially true if it is a dynamic format table and it is possible to use a smaller static format table that can be used to find the relevant rows when scanning the table. How MySQL Uses Indexes ---------------------- Indexes are used to find rows with a specific value of one column fast. Without an index MySQL has to start with the first record and then read through the whole table until it finds the relevant rows. The bigger the table, the more this costs. If the table has an index for the columns in question, MySQL can quickly get a position to seek to in the middle of the data file without having to look at all the data. If a table has 1000 rows, this is at least 100 times faster than reading sequentially. Note that if you need to access almost all 1000 rows it is faster to read sequentially because we then avoid disk seeks. All MySQL indexes (`PRIMARY', `UNIQUE', and `INDEX') are stored in B-trees. Strings are automatically prefix- and end-space compressed. *Note `CREATE INDEX': CREATE INDEX. Indexes are used to: * Quickly find the rows that match a `WHERE' clause. * Retrieve rows from other tables when performing joins. * Find the `MAX()' or `MIN()' value for a specific indexed column. This is optimized by a preprocessor that checks if you are using `WHERE' key_part_# = constant on all key parts < N. In this case MySQL will do a single key lookup and replace the `MIN()' expression with a constant. If all expressions are replaced with constants, the query will return at once: SELECT MIN(key_part2),MAX(key_part2) FROM table_name where key_part1=10 * Sort or group a table if the sorting or grouping is done on a leftmost prefix of a usable key (for example, `ORDER BY key_part_1,key_part_2 '). The key is read in reverse order if all key parts are followed by `DESC'. The index can also be used even if the `ORDER BY' doesn't match the index exactly, as long as all the unused index parts and all the extra are `ORDER BY' columns are constants in the `WHERE' clause. The following queries will use the index to resolve the `ORDER BY' part: SELECT * FROM foo ORDER BY key_part1,key_part2,key_part3; SELECT * FROM foo WHERE column=constant ORDER BY column, key_part1; SELECT * FROM foo WHERE key_part1=const GROUP BY key_part2; * In some cases a query can be optimized to retrieve values without consulting the data file. If all used columns for some table are numeric and form a leftmost prefix for some key, the values may be retrieved from the index tree for greater speed: SELECT key_part3 FROM table_name WHERE key_part1=1 Suppose you issue the following `SELECT' statement: mysql> SELECT * FROM tbl_name WHERE col1=val1 AND col2=val2; If a multiple-column index exists on `col1' and `col2', the appropriate rows can be fetched directly. If separate single-column indexes exist on `col1' and `col2', the optimizer tries to find the most restrictive index by deciding which index will find fewer rows and using that index to fetch the rows. If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to find rows. For example, if you have a three-column index on `(col1,col2,col3)', you have indexed search capabilities on `(col1)', `(col1,col2)', and `(col1,col2,col3)'. MySQL can't use a partial index if the columns don't form a leftmost prefix of the index. Suppose you have the `SELECT' statements shown below: mysql> SELECT * FROM tbl_name WHERE col1=val1; mysql> SELECT * FROM tbl_name WHERE col2=val2; mysql> SELECT * FROM tbl_name WHERE col2=val2 AND col3=val3; If an index exists on `(col1,col2,col3)', only the first query shown above uses the index. The second and third queries do involve indexed columns, but `(col2)' and `(col2,col3)' are not leftmost prefixes of `(col1,col2,col3)'. MySQL also uses indexes for `LIKE' comparisons if the argument to `LIKE' is a constant string that doesn't start with a wild-card character. For example, the following `SELECT' statements use indexes: mysql> select * from tbl_name where key_col LIKE "Patrick%"; mysql> select * from tbl_name where key_col LIKE "Pat%_ck%"; In the first statement, only rows with `"Patrick" <= key_col < "Patricl"' are considered. In the second statement, only rows with `"Pat" <= key_col < "Pau"' are considered. The following `SELECT' statements will not use indexes: mysql> select * from tbl_name where key_col LIKE "%Patrick%"; mysql> select * from tbl_name where key_col LIKE other_col; In the first statement, the `LIKE' value begins with a wild-card character. In the second statement, the `LIKE' value is not a constant. Searching using `column_name IS NULL' will use indexes if column_name is an index. MySQL normally uses the index that finds the least number of rows. An index is used for columns that you compare with the following operators: `=', `>', `>=', `<', `<=', `BETWEEN', and a `LIKE' with a non-wild-card prefix like `'something%''. Any index that doesn't span all `AND' levels in the `WHERE' clause is not used to optimize the query. In other words: To be able to use an index, a prefix of the index must be used in every `AND' group. The following `WHERE' clauses use indexes: ... WHERE index_part1=1 AND index_part2=2 AND other_column=3 ... WHERE index=1 OR A=10 AND index=2 /* index = 1 OR index = 2 */ ... WHERE index_part1='hello' AND index_part_3=5 /* optimized like "index_part1='hello'" */ ... WHERE index1=1 and index2=2 or index1=3 and index3=3; /* Can use index on index1 but not on index2 or index 3 */ These `WHERE' clauses do *NOT* use indexes: ... WHERE index_part2=1 AND index_part3=2 /* index_part_1 is not used */ ... WHERE index=1 OR A=10 /* Index is not used in both AND parts */ ... WHERE index_part1=1 OR index_part2=10 /* No index spans all rows */ Note that in some cases MySQL will not use an index, even if one would be available. Some of the cases where this happens are: * If the use of the index would require MySQL to access more than 30 % of the rows in the table. (In this case a table scan is probably much faster, as this will require us to do much fewer seeks). Note that if such a query uses `LIMIT' to only retrieve part of the rows, MySQL will use an index anyway, as it can much more quickly find the few rows to return in the result. Column Indexes -------------- All MySQL column types can be indexed. Use of indexes on the relevant columns is the best way to improve the performance of `SELECT' operations. The maximum number of keys and the maximum index length is defined per table handler. *Note Table types::. You can with all table handlers have at least 16 keys and a total index length of at least 256 bytes. For `CHAR' and `VARCHAR' columns, you can index a prefix of a column. This is much faster and requires less disk space than indexing the whole column. The syntax to use in the `CREATE TABLE' statement to index a column prefix looks like this: KEY index_name (col_name(length)) The example below creates an index for the first 10 characters of the `name' column: mysql> CREATE TABLE test ( name CHAR(200) NOT NULL, KEY index_name (name(10))); For `BLOB' and `TEXT' columns, you must index a prefix of the column. You cannot index the entire column. In MySQL Version 3.23.23 or later, you can also create special *FULLTEXT* indexes. They are used for full-text search. Only the `MyISAM' table type supports `FULLTEXT' indexes. They can be created only from `VARCHAR' and `TEXT' columns. Indexing always happens over the entire column and partial indexing is not supported. See *Note Fulltext Search:: for details. Multiple-Column Indexes ----------------------- MySQL can create indexes on multiple columns. An index may consist of up to 15 columns. (On `CHAR' and `VARCHAR' columns you can also use a prefix of the column as a part of an index). A multiple-column index can be considered a sorted array containing values that are created by concatenating the values of the indexed columns. MySQL uses multiple-column indexes in such a way that queries are fast when you specify a known quantity for the first column of the index in a `WHERE' clause, even if you don't specify values for the other columns. Suppose a table is created using the following specification: mysql> CREATE TABLE test ( id INT NOT NULL, last_name CHAR(30) NOT NULL, first_name CHAR(30) NOT NULL, PRIMARY KEY (id), INDEX name (last_name,first_name)); Then the index `name' is an index over `last_name' and `first_name'. The index will be used for queries that specify values in a known range for `last_name', or for both `last_name' and `first_name'. Therefore, the `name' index will be used in the following queries: mysql> SELECT * FROM test WHERE last_name="Widenius"; mysql> SELECT * FROM test WHERE last_name="Widenius" AND first_name="Michael"; mysql> SELECT * FROM test WHERE last_name="Widenius" AND (first_name="Michael" OR first_name="Monty"); mysql> SELECT * FROM test WHERE last_name="Widenius" AND first_name >="M" AND first_name < "N"; However, the `name' index will NOT be used in the following queries: mysql> SELECT * FROM test WHERE first_name="Michael"; mysql> SELECT * FROM test WHERE last_name="Widenius" OR first_name="Michael"; For more information on the manner in which MySQL uses indexes to improve query performance, see *Note MySQL indexes: MySQL indexes. How MySQL Opens and Closes Tables --------------------------------- `table_cache', `max_connections', and `max_tmp_tables' affect the maximum number of files the server keeps open. If you increase one or both of these values, you may run up against a limit imposed by your operating system on the per-process number of open file descriptors. However, you can increase the limit on many systems. Consult your OS documentation to find out how to do this, because the method for changing the limit varies widely from system to system. `table_cache' is related to `max_connections'. For example, for 200 concurrent running connections, you should have a table cache of at least `200 * n', where `n' is the maximum number of tables in a join. You also need to reserve some extra file descriptors for temporary tables and files. The cache of open tables can grow to a maximum of `table_cache' (default 64; this can be changed with the `-O table_cache=#' option to `mysqld'). A table is never closed, except when the cache is full and another thread tries to open a table or if you use `mysqladmin refresh' or `mysqladmin flush-tables'. When the table cache fills up, the server uses the following procedure to locate a cache entry to use: * Tables that are not currently in use are released, in least-recently-used order. * If the cache is full and no tables can be released, but a new table needs to be opened, the cache is temporarily extended as necessary. * If the cache is in a temporarily-extended state and a table goes from in-use to not-in-use state, the table is closed and released from the cache. A table is opened for each concurrent access. This means that if you have two threads accessing the same table or access the table twice in the same query (with `AS') the table needs to be opened twice. The first open of any table takes two file descriptors; each additional use of the table takes only one file descriptor. The extra descriptor for the first open is used for the index file; this descriptor is shared among all threads. You can check if your table cache is too small by checking the mysqld variable `opened_tables'. If this is quite big, even if you haven't done a lot of `FLUSH TABLES', you should increase your table cache. *Note SHOW STATUS::. Drawbacks to Creating Large Numbers of Tables in the Same Database ------------------------------------------------------------------ If you have many files in a directory, open, close, and create operations will be slow. If you execute `SELECT' statements on many different tables, there will be a little overhead when the table cache is full, because for every table that has to be opened, another must be closed. You can reduce this overhead by making the table cache larger. Why So Many Open tables? ------------------------ When you run `mysqladmin status', you'll see something like this: Uptime: 426 Running threads: 1 Questions: 11082 Reloads: 1 Open tables: 12 This can be somewhat perplexing if you only have 6 tables. MySQL is multithreaded, so it may have many queries on the same table simultaneously. To minimize the problem with two threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory and one extra file descriptor for the data file. The index file descriptor is shared between all threads. Optimizing the MySQL Server =========================== System/Compile Time and Startup Parameter Tuning ------------------------------------------------ We start with the system level things since some of these decisions have to be made very early. In other cases a fast look at this part may suffice because it not that important for the big gains. However, it is always nice to have a feeling about how much one could gain by changing things at this level. The default OS to use is really important! To get the most use of multiple CPU machines one should use Solaris (because the threads works really nice) or Linux (because the 2.2 kernel has really good SMP support). Also on 32-bit machines Linux has a 2G file size limit by default. Hopefully this will be fixed soon when new filesystems are released (XFS/Reiserfs). If you have a desperate need for files bigger than 2G on Linux-intel 32 bit, you should get the LFS patch for the ext2 file system. Because we have not run MySQL in production on that many platforms, we advice you to test your intended platform before choosing it, if possible. Other tips: * If you have enough RAM, you could remove all swap devices. Some operating systems will use a swap device in some contexts even if you have free memory. * Use the `--skip-locking' MySQL option to avoid external locking. Note that this will not impact MySQL's functionality as long as you only run one server. Just remember to take down the server (or lock relevant parts) before you run `myisamchk'. On some system this switch is mandatory because the external locking does not work in any case. The `--skip-locking' option is on by default when compiling with MIT-pthreads, because `flock()' isn't fully supported by MIT-pthreads on all platforms. It's also on default for Linux as Linux file locking are not yet safe. The only case when you can't use `--skip-locking' is if you run multiple MySQL _servers_ (not clients) on the same data, or run `myisamchk' on the table without first flushing and locking the `mysqld' server tables first. You can still use `LOCK TABLES'/`UNLOCK TABLES' even if you are using `--skip-locking' Tuning Server Parameters ------------------------ You can get the default buffer sizes used by the `mysqld' server with this command: shell> mysqld --help This command produces a list of all `mysqld' options and configurable variables. The output includes the default values and looks something like this: Possible variables for option --set-variable (-O) are: back_log current value: 5 bdb_cache_size current value: 1048540 binlog_cache_size current_value: 32768 connect_timeout current value: 5 delayed_insert_timeout current value: 300 delayed_insert_limit current value: 100 delayed_queue_size current value: 1000 flush_time current value: 0 interactive_timeout current value: 28800 join_buffer_size current value: 131072 key_buffer_size current value: 1048540 lower_case_table_names current value: 0 long_query_time current value: 10 max_allowed_packet current value: 1048576 max_binlog_cache_size current_value: 4294967295 max_connections current value: 100 max_connect_errors current value: 10 max_delayed_threads current value: 20 max_heap_table_size current value: 16777216 max_join_size current value: 4294967295 max_sort_length current value: 1024 max_tmp_tables current value: 32 max_write_lock_count current value: 4294967295 myisam_sort_buffer_size current value: 8388608 net_buffer_length current value: 16384 net_retry_count current value: 10 net_read_timeout current value: 30 net_write_timeout current value: 60 query_buffer_size current value: 0 record_buffer current value: 131072 record_rnd_buffer current value: 131072 slow_launch_time current value: 2 sort_buffer current value: 2097116 table_cache current value: 64 thread_concurrency current value: 10 tmp_table_size current value: 1048576 thread_stack current value: 131072 wait_timeout current value: 28800 If there is a `mysqld' server currently running, you can see what values it actually is using for the variables by executing this command: shell> mysqladmin variables You can find a full description for all variables in the `SHOW VARIABLES' section in this manual. *Note SHOW VARIABLES::. You can also see some statistics from a running server by issuing the command `SHOW STATUS'. *Note SHOW STATUS::. MySQL uses algorithms that are very scalable, so you can usually run with very little memory. If you, however, give MySQL more memory, you will normally also get better performance. When tuning a MySQL server, the two most important variables to use are `key_buffer_size' and `table_cache'. You should first feel confident that you have these right before trying to change any of the other variables. If you have much memory (>=256M) and many tables and want maximum performance with a moderate number of clients, you should use something like this: shell> safe_mysqld -O key_buffer=64M -O table_cache=256 \ -O sort_buffer=4M -O record_buffer=1M & If you have only 128M and only a few tables, but you still do a lot of sorting, you can use something like: shell> safe_mysqld -O key_buffer=16M -O sort_buffer=1M If you have little memory and lots of connections, use something like this: shell> safe_mysqld -O key_buffer=512k -O sort_buffer=100k \ -O record_buffer=100k & or even: shell> safe_mysqld -O key_buffer=512k -O sort_buffer=16k \ -O table_cache=32 -O record_buffer=8k -O net_buffer=1K & If you are doing a `GROUP BY' or `ORDER BY' on files that are much bigger than your available memory you should increase the value of `record_rnd_buffer' to speed up the reading of rows after the sorting is done. When you have installed MySQL, the `support-files' directory will contain some different `my.cnf' example files, `my-huge.cnf', `my-large.cnf', `my-medium.cnf', and `my-small.cnf', you can use as a base to optimize your system. If there are very many connections, "swapping problems" may occur unless `mysqld' has been configured to use very little memory for each connection. `mysqld' performs better if you have enough memory for all connections, of course. Note that if you change an option to `mysqld', it remains in effect only for that instance of the server. To see the effects of a parameter change, do something like this: shell> mysqld -O key_buffer=32m --help Make sure that the `--help' option is last; otherwise, the effect of any options listed after it on the command line will not be reflected in the output. How Compiling and Linking Affects the Speed of MySQL ---------------------------------------------------- Most of the following tests are done on Linux with the MySQL benchmarks, but they should give some indication for other operating systems and workloads. You get the fastest executable when you link with `-static'. On Linux, you will get the fastest code when compiling with `pgcc' and `-O3'. To compile `sql_yacc.cc' with these options, you need about 200M memory because `gcc/pgcc' needs a lot of memory to make all functions inline. You should also set `CXX=gcc' when configuring MySQL to avoid inclusion of the `libstdc++' library (it is not needed). Note that with some versions of `pgcc', the resulting code will only run on true Pentium processors, even if you use the compiler option that you want the resulting code to be working on all x586 type processors (like AMD). By just using a better compiler and/or better compiler options you can get a 10-30 % speed increase in your application. This is particularly important if you compile the SQL server yourself! We have tested both the Cygnus CodeFusion and Fujitsu compilers, but when we tested them, neither was sufficiently bug free to allow MySQL to be compiled with optimizations on. When you compile MySQL you should only include support for the character sets that you are going to use. (Option `--with-charset=xxx'). The standard MySQL binary distributions are compiled with support for all character sets. Here is a list of some measurements that we have done: * If you use `pgcc' and compile everything with `-O6', the `mysqld' server is 1% faster than with `gcc' 2.95.2. * If you link dynamically (without `-static'), the result is 13% slower on Linux. Note that you still can use a dynamic linked MySQL library. It is only the server that is critical for performance. * If you strip your `mysqld' binary with `strip libexec/mysqld', the resulting binary can be up to 4 % faster. * If you connect using TCP/IP rather than Unix sockets, the result is 7.5% slower on the same computer. (If you are connection to `localhost', MySQL will, by default, use sockets). * If you connect using TCP/IP from another computer over a 100M Ethernet, things will be 8-11 % slower. * If you compile with `--with-debug=full', then you will loose 20 % for most queries, but some queries may take substantially longer (The MySQL benchmarks ran 35 % slower) If you use `--with-debug', then you will only loose 15 %. By starting a `mysqld' version compiled with `--with-debug=full' with `--skip-safemalloc' the end result should be close to when configuring with `--with-debug'. * On a Sun SPARCstation 20, SunPro C++ 4.2 is 5 % faster than `gcc' 2.95.2. * Compiling with `gcc' 2.95.2 for ultrasparc with the option `-mcpu=v8 -Wa,-xarch=v8plusa' gives 4 % more performance. * On Solaris 2.5.1, MIT-pthreads is 8-12% slower than Solaris native threads on a single processor. With more load/CPUs the difference should get bigger. * Running with `--log-bin' makes *[MySQL* 1 % slower. * Compiling on Linux-x86 using gcc without frame pointers `-fomit-frame-pointer' or `-fomit-frame-pointer -ffixed-ebp' `mysqld' 1-4% faster. The MySQL-Linux distribution provided by MySQL AB used to be compiled with `pgcc', but we had to go back to regular gcc because of a bug in `pgcc' that would generate the code that does not run on AMD. We will continue using gcc until that bug is resolved. In the meantime, if you have a non-AMD machine, you can get a faster binary by compiling with `pgcc'. The standard MySQL Linux binary is linked statically to get it faster and more portable. How MySQL Uses Memory --------------------- The list below indicates some of the ways that the `mysqld' server uses memory. Where applicable, the name of the server variable relevant to the memory use is given: * The key buffer (variable `key_buffer_size') is shared by all threads; Other buffers used by the server are allocated as needed. *Note Server parameters::. * Each connection uses some thread-specific space: A stack (default 64K, variable `thread_stack'), a connection buffer (variable `net_buffer_length'), and a result buffer (variable `net_buffer_length'). The connection buffer and result buffer are dynamically enlarged up to `max_allowed_packet' when needed. When a query is running, a copy of the current query string is also allocated. * All threads share the same base memory. * Only the compressed ISAM / MyISAM tables are memory mapped. This is because the 32-bit memory space of 4GB is not large enough for most big tables. When systems with a 64-bit address space become more common we may add general support for memory mapping. * Each request doing a sequential scan over a table allocates a read buffer (variable `record_buffer'). * When reading rows in 'random' order (for example after a sort) a random-read buffer is allocated to avoid disk seeks. (variable `record_rnd_buffer'). * All joins are done in one pass, and most joins can be done without even using a temporary table. Most temporary tables are memory-based (HEAP) tables. Temporary tables with a big record length (calculated as the sum of all column lengths) or that contain `BLOB' columns are stored on disk. One problem in MySQL versions before Version 3.23.2 is that if a HEAP table exceeds the size of `tmp_table_size', you get the error `The table tbl_name is full'. In newer versions this is handled by automatically changing the in-memory (HEAP) table to a disk-based (MyISAM) table as necessary. To work around this problem, you can increase the temporary table size by setting the `tmp_table_size' option to `mysqld', or by setting the SQL option `SQL_BIG_TABLES' in the client program. *Note `SET OPTION': SET OPTION. In MySQL Version 3.20, the maximum size of the temporary table was `record_buffer*16', so if you are using this version, you have to increase the value of `record_buffer'. You can also start `mysqld' with the `--big-tables' option to always store temporary tables on disk. However, this will affect the speed of many complicated queries. * Most requests doing a sort allocates a sort buffer and 0-2 temporary files depending on the result set size. *Note Temporary files::. * Almost all parsing and calculating is done in a local memory store. No memory overhead is needed for small items and the normal slow memory allocation and freeing is avoided. Memory is allocated only for unexpectedly large strings (this is done with `malloc()' and `free()'). * Each index file is opened once and the data file is opened once for each concurrently running thread. For each concurrent thread, a table structure, column structures for each column, and a buffer of size `3 * n' is allocated (where `n' is the maximum row length, not counting `BLOB' columns). A `BLOB' uses 5 to 8 bytes plus the length of the `BLOB' data. The `ISAM'/`MyISAM' table handlers will use one extra row buffer for internal usage. * For each table having `BLOB' columns, a buffer is enlarged dynamically to read in larger `BLOB' values. If you scan a table, a buffer as large as the largest `BLOB' value is allocated. * Table handlers for all in-use tables are saved in a cache and managed as a FIFO. Normally the cache has 64 entries. If a table has been used by two running threads at the same time, the cache contains two entries for the table. *Note Table cache::. * A `mysqladmin flush-tables' command closes all tables that are not in use and marks all in-use tables to be closed when the currently executing thread finishes. This will effectively free most in-use memory. `ps' and other system status programs may report that `mysqld' uses a lot of memory. This may be caused by thread-stacks on different memory addresses. For example, the Solaris version of `ps' counts the unused memory between stacks as used memory. You can verify this by checking available swap with `swap -s'. We have tested `mysqld' with commercial memory-leakage detectors, so there should be no memory leaks. How MySQL uses DNS ------------------ When a new thread connects to `mysqld', `mysqld' will span a new thread to handle the request. This thread will first check if the hostname is in the hostname cache. If not the thread will call `gethostbyaddr_r()' and `gethostbyname_r()' to resolve the hostname. If the operating system doesn't support the above thread-safe calls, the thread will lock a mutex and call `gethostbyaddr()' and `gethostbyname()' instead. Note that in this case no other thread can resolve other hostnames that is not in the hostname cache until the first thread is ready. You can disable DNS host lookup by starting `mysqld' with `--skip-name-resolve'. In this case you can however only use IP names in the MySQL privilege tables. If you have a very slow DNS and many hosts, you can get more performance by either disabling DNS lookop with `--skip-name-resolve' or by increasing the `HOST_CACHE_SIZE' define (default: 128) and recompile `mysqld'. You can disable the hostname cache with `--skip-host-cache'. You can clear the hostname cache with `FLUSH HOSTS' or `mysqladmin flush-hosts'. If you don't want to allow connections over `TCP/IP', you can do this by starting `mysqld' with `--skip-networking'. `SET' Syntax ------------ SET [OPTION] SQL_VALUE_OPTION= value, ... `SET OPTION' sets various options that affect the operation of the server or your client. Any option you set remains in effect until the current session ends, or until you set the option to a different value. `CHARACTER SET character_set_name | DEFAULT' This maps all strings from and to the client with the given mapping. Currently the only option for `character_set_name' is `cp1251_koi8', but you can easily add new mappings by editing the `sql/convert.cc' file in the MySQL source distribution. The default mapping can be restored by using a `character_set_name' value of `DEFAULT'. Note that the syntax for setting the `CHARACTER SET' option differs from the syntax for setting the other options. `PASSWORD = PASSWORD('some password')' Set the password for the current user. Any non-anonymous user can change his own password! `PASSWORD FOR user = PASSWORD('some password')' Set the password for a specific user on the current server host. Only a user with access to the `mysql' database can do this. The user should be given in `user@hostname' format, where `user' and `hostname' are exactly as they are listed in the `User' and `Host' columns of the `mysql.user' table entry. For example, if you had an entry with `User' and `Host' fields of `'bob'' and `'%.loc.gov'', you would write: mysql> SET PASSWORD FOR bob@"%.loc.gov" = PASSWORD("newpass"); or mysql> UPDATE mysql.user SET password=PASSWORD("newpass") where user="bob' and host="%.loc.gov"; `SQL_AUTO_IS_NULL = 0 | 1' If set to `1' (default) then one can find the last inserted row for a table with an auto_increment row with the following construct: `WHERE auto_increment_column IS NULL'. This is used by some ODBC programs like Access. `AUTOCOMMIT= 0 | 1' If set to `1' all changes to a table will be done at once. To start a multi-command transaction, you have to use the `BEGIN' statement. *Note COMMIT::. If set to `0' you have to use `COMMIT' / `ROLLBACK' to accept/revoke that transaction. *Note COMMIT::. Note that when you change from not `AUTOCOMMIT' mode to `AUTOCOMMIT' mode, MySQL will do an automatic `COMMIT' on any open transactions. `SQL_BIG_TABLES = 0 | 1' If set to `1', all temporary tables are stored on disk rather than in memory. This will be a little slower, but you will not get the error `The table tbl_name is full' for big `SELECT' operations that require a large temporary table. The default value for a new connection is `0' (that is, use in-memory temporary tables). `SQL_BIG_SELECTS = 0 | 1' If set to `0', MySQL will abort if a `SELECT' is attempted that probably will take a very long time. This is useful when an inadvisable `WHERE' statement has been issued. A big query is defined as a `SELECT' that probably will have to examine more than `max_join_size' rows. The default value for a new connection is `1' (which will allow all `SELECT' statements). `SQL_BUFFER_RESULT = 0 | 1' `SQL_BUFFER_RESULT' will force the result from `SELECT''s to be put into a temporary table. This will help MySQL free the table locks early and will help in cases where it takes a long time to send the result set to the client. `SQL_LOW_PRIORITY_UPDATES = 0 | 1' If set to `1', all `INSERT', `UPDATE', `DELETE', and and `LOCK TABLE WRITE' statements wait until there is no pending `SELECT' or `LOCK TABLE READ' on the affected table. `SQL_MAX_JOIN_SIZE = value | DEFAULT' Don't allow `SELECT's that will probably need to examine more than `value' row combinations. By setting this value, you can catch `SELECT's where keys are not used properly and that would probably take a long time. Setting this to a value other than `DEFAULT' will reset the `SQL_BIG_SELECTS' flag. If you set the `SQL_BIG_SELECTS' flag again, the `SQL_MAX_JOIN_SIZE' variable will be ignored. You can set a default value for this variable by starting `mysqld' with `-O max_join_size=#'. `SQL_SAFE_UPDATES = 0 | 1' If set to `1', MySQL will abort if an `UPDATE' or `DELETE' is attempted that doesn't use a key or `LIMIT' in the `WHERE' clause. This makes it possible to catch wrong updates when creating SQL commands by hand. `SQL_SELECT_LIMIT = value | DEFAULT' The maximum number of records to return from `SELECT' statements. If a `SELECT' has a `LIMIT' clause, the `LIMIT' takes precedence over the value of `SQL_SELECT_LIMIT'. The default value for a new connection is "unlimited." If you have changed the limit, the default value can be restored by using a `SQL_SELECT_LIMIT' value of `DEFAULT'. `SQL_LOG_OFF = 0 | 1' If set to `1', no logging will be done to the standard log for this client, if the client has the *process* privilege. This does not affect the update log! `SQL_LOG_UPDATE = 0 | 1' If set to `0', no logging will be done to the update log for the client, if the client has the *process* privilege. This does not affect the standard log! `SQL_QUOTE_SHOW_CREATE = 0 | 1' If set to `1', `SHOW CREATE TABLE' will quote table and column names. This is *on* by default, for replication of tables with fancy column names to work. *Note `SHOW CREATE TABLE': SHOW CREATE TABLE. `TIMESTAMP = timestamp_value | DEFAULT' Set the time for this client. This is used to get the original timestamp if you use the update log to restore rows. `timestamp_value' should be a UNIX Epoch timestamp, not a MySQL timestamp. `LAST_INSERT_ID = #' Set the value to be returned from `LAST_INSERT_ID()'. This is stored in the update log when you use `LAST_INSERT_ID()' in a command that updates a table. `INSERT_ID = #' Set the value to be used by the following `INSERT' or `ALTER TABLE' command when inserting an `AUTO_INCREMENT' value. This is mainly used with the update log. Disk Issues =========== * As mentioned before, disks seeks are a big performance bottleneck. This problems gets more and more apparent when the data starts to grow so large that effective caching becomes impossible. For large databases, where you access data more or less randomly, you can be sure that you will need at least one disk seek to read and a couple of disk seeks to write things. To minimize this problem, use disks with low seek times. * Increase the number of available disk spindles (and thereby reduce the seek overhead) by either symlink files to different disks or striping the disks. *Using symbolic links* This means that you symlink the index and/or data file(s) from the normal data directory to another disk (that may also be striped). This makes both the seek and read times better (if the disks are not used for other things). *Note Symbolic links::. *Striping* Striping means that you have many disks and put the first block on the first disk, the second block on the second disk, and the Nth on the (N mod number_of_disks) disk, and so on. This means if your normal data size is less than the stripe size (or perfectly aligned) you will get much better performance. Note that striping is very dependent on the OS and stripe-size. So benchmark your application with different stripe-sizes. *Note Custom Benchmarks::. Note that the speed difference for striping is *very* dependent on the parameters. Depending on how you set the striping parameters and number of disks you may get a difference in orders of magnitude. Note that you have to choose to optimize for random or sequential access. * For reliability you may want to use RAID 0+1 (striping + mirroring), but in this case you will need 2*N drives to hold N drives of data. This is probably the best option if you have the money for it! You may, however, also have to invest in some volume-management software to handle it efficiently. * A good option is to have semi-important data (that can be regenerated) on RAID 0 disk while storing really important data (like host information and logs) on a RAID 0+1 or RAID N disk. RAID N can be a problem if you have many writes because of the time to update the parity bits. * You may also set the parameters for the file system that the database uses. One easy change is to mount the file system with the noatime option. That makes it skip the updating of the last access time in the inode and by this will avoid some disk seeks. * On Linux, you can get much more performance (up to 100 % under load is not uncommon) by using hdpram to configure your disk's interface! The following should be quite good hdparm options for MySQL (and probably many other applications): hdparm -m 16 -d 1 Note that the performance/reliability when using the above depends on your hardware, so we strongly suggest that you test your system thoroughly after using `hdparm'! Please consult the `hdparm' man page for more information! If `hdparm' is not used wisely, filesystem corruption may result. Backup everything before experimenting! * On many operating systems you can mount the disks with the 'async' flag to set the file system to be updated asynchronously. If your computer is reasonable stable, this should give you more performance without sacrificing too much reliability. (This flag is on by default on Linux.) * If you don't need to know when a file was last accessed (which is not really useful on a database server), you can mount your file systems with the noatime flag. Using Symbolic Links -------------------- You can move tables and databases from the database directory to other locations and replace them with symbolic links to the new locations. You might want to do this, for example, to move a database to a file system with more free space or increase the speed of your system by spreading your tables to different disk. The recommended may to do this, is to just symlink databases to different disk and only symlink tables as a last resort. Using Symbolic Links for Databases .................................. The way to symlink a database is to first create a directory on some disk where you have free space and then create a symlink to it from the MySQL database directory. shell> mkdir /dr1/databases/test shell> ln -s /dr1/databases/test mysqld-datadir MySQL doesn't support that you link one directory to multiple databases. Replacing a database directory with a symbolic link will work fine as long as you don't make a symbolic link between databases. Suppose you have a database `db1' under the MySQL data directory, and then make a symlink `db2' that points to `db1': shell> cd /path/to/datadir shell> ln -s db1 db2 Now, for any table `tbl_a' in `db1', there also appears to be a table `tbl_a' in `db2'. If one thread updates `db1.tbl_a' and another thread updates `db2.tbl_a', there will be problems. If you really need this, you must change the following code in `mysys/mf_format.c': if (flag & 32 || (!lstat(to,&stat_buff) && S_ISLNK(stat_buff.st_mode))) to if (1) On Windows you can use internal symbolic links to directories by compiling MySQL with `-DUSE_SYMDIR'. This allows you to put different databases on different disks. *Note Windows symbolic links::. Using Symbolic Links for Tables ............................... Before MySQL 4.0 you should not symlink tables, if you are not very carefully with them. The problem is that if you run `ALTER TABLE', `REPAIR TABLE' or `OPTIMIZE TABLE' on a symlinked table, the symlinks will be removed and replaced by the original files. This happens because the above command works by creating a temporary file in the database directory and when the command is complete, replace the original file with the temporary file. You should not symlink tables on system that doesn't have a fully working `realpath()' call. (At least Linux and Solaris support `realpath()') In MySQL 4.0 symlinks is only fully supported for `MyISAM' tables. For other table types you will probably get strange problems when doing any of the above mentioned commands. The handling of symbolic links in MySQL 4.0 works the following way (this is mostly relevant only for `MyISAM' tables). * In the data directory you will always have the table definition file and the data/index files. * You can symlink the index file and the data file to different directories independent of the other. * The symlinking can be done from the operating system (if `mysqld' is not running) or with the `INDEX/DATA DIRECTORY="path-to-dir"' command in `CREATE TABLE'. *Note CREATE TABLE::. * `myisamchk' will not replace a symlink with the index/file but work directly on the files the symlinks points to. Any temporary files will be created in the same directory where the data/index file is. * When you drop a table that is using symlinks, both the symlink and the file the symlink points to is dropped. This is a good reason to why you should NOT run `mysqld' as root and not allow persons to have write access to the MySQL database directories. * If you rename a table with `ALTER TABLE RENAME' and you don't change database, the symlink in the database directory will be renamed to the new name and the data/index file will be renamed accordingly. * If you use `ALTER TABLE RENAME' to move a table to another database, then the table will be moved to the other database directory and the old symlinks and the files they pointed to will be deleted. * If you are not using symlinks you should use the `--skip-symlink' option to `mysqld' to ensure that no one can drop or rename a file outside of the `mysqld' data directory. Things that are not yet supported: * `ALTER TABLE' ignores all `INDEX/DATA DIRECTORY="path"' options. * `CREATE TABLE' doesn't report if the table has symbolic links. * `mysqldump' doesn't include the symbolic links information in the output. * `BACKUP TABLE' and `RESTORE TABLE' don't respect symbolic links. MySQL Language Reference ************************ MySQL has a very complex, but intuitive and easy to learn SQL interface. This chapter describes the various commands, types, and functions you will need to know in order to use MySQL efficiently and effectively. This chapter also serves as a reference to all functionality included in MySQL. In order to use this chapter effectively, you may find it useful to refer to the various indexes. Language Structure ================== Literals: How to Write Strings and Numbers ------------------------------------------ This section describes the various ways to write strings and numbers in MySQL. It also covers the various nuances and "gotchas" that you may run into when dealing with these basic types in MySQL. Strings ....... A string is a sequence of characters, surrounded by either single quote (`'') or double quote (`"') characters (only the single quote if you run in ANSI mode). Examples: 'a string' "another string" Within a string, certain sequences have special meaning. Each of these sequences begins with a backslash (`\'), known as the _escape character_. MySQL recognizes the following escape sequences: `\0' An ASCII 0 (`NUL') character. `\'' A single quote (`'') character. `\"' A double quote (`"') character. `\b' A backspace character. `\n' A newline character. `\r' A carriage return character. `\t' A tab character. `\z' ASCII(26) (Control-Z). This character can be encoded to allow you to go around the problem that ASCII(26) stands for END-OF-FILE on Windows. (ASCII(26) will cause problems if you try to use `mysql database < filename'). `\\' A backslash (`\') character. `\%' A `%' character. This is used to search for literal instances of `%' in contexts where `%' would otherwise be interpreted as a wild-card character. *Note String comparison functions::. `\_' A `_' character. This is used to search for literal instances of `_' in contexts where `_' would otherwise be interpreted as a wild-card character. *Note String comparison functions::. Note that if you use `\%' or `\_' in some string contexts, these will return the strings `\%' and `\_' and not `%' and `_'. There are several ways to include quotes within a string: * A `'' inside a string quoted with `'' may be written as `'''. * A `"' inside a string quoted with `"' may be written as `""'. * You can precede the quote character with an escape character (`\'). * A `'' inside a string quoted with `"' needs no special treatment and need not be doubled or escaped. In the same way, `"' inside a string quoted with `'' needs no special treatment. The `SELECT' statements shown below demonstrate how quoting and escaping work: mysql> SELECT 'hello', '"hello"', '""hello""', 'hel''lo', '\'hello'; +-------+---------+-----------+--------+--------+ | hello | "hello" | ""hello"" | hel'lo | 'hello | +-------+---------+-----------+--------+--------+ mysql> SELECT "hello", "'hello'", "''hello''", "hel""lo", "\"hello"; +-------+---------+-----------+--------+--------+ | hello | 'hello' | ''hello'' | hel"lo | "hello | +-------+---------+-----------+--------+--------+ mysql> SELECT "This\nIs\nFour\nlines"; +--------------------+ | This Is Four lines | +--------------------+ If you want to insert binary data into a `BLOB' column, the following characters must be represented by escape sequences: `NUL' ASCII 0. You should represent this by `\0' (a backslash and an ASCII `0' character). `\' ASCII 92, backslash. Represent this by `\\'. `'' ASCII 39, single quote. Represent this by `\''. `"' ASCII 34, double quote. Represent this by `\"'. If you write C code, you can use the C API function `mysql_escape_string()' to escape characters for the `INSERT' statement. *Note C API function overview::. In Perl, you can use the `quote' method of the `DBI' package to convert special characters to the proper escape sequences. *Note Perl `DBI' Class: Perl DBI Class. You should use an escape function on any string that might contain any of the special characters listed above! Numbers ....... Integers are represented as a sequence of digits. Floats use `.' as a decimal separator. Either type of number may be preceded by `-' to indicate a negative value. Examples of valid integers: 1221 0 -32 Examples of valid floating-point numbers: 294.42 -32032.6809e+10 148.00 An integer may be used in a floating-point context; it is interpreted as the equivalent floating-point number. Hexadecimal Values .................. MySQL supports hexadecimal values. In number context these act like an integer (64-bit precision). In string context these act like a binary string where each pair of hex digits is converted to a character: mysql> SELECT 0xa+0; -> 10 mysql> select 0x5061756c; -> Paul Hexadecimal strings are often used by ODBC to give values for BLOB columns. `NULL' Values ............. The `NULL' value means "no data" and is different from values such as `0' for numeric types or the empty string for string types. *Note Problems with `NULL': Problems with NULL. `NULL' may be represented by `\N' when using the text file import or export formats (`LOAD DATA INFILE', `SELECT ... INTO OUTFILE'). *Note `LOAD DATA': LOAD DATA. Database, Table, Index, Column, and Alias Names ----------------------------------------------- Database, table, index, column, and alias names all follow the same rules in MySQL. Note that the rules changed starting with MySQL Version 3.23.6 when we introduced quoting of identifiers (database, table, and column names) with ``'. `"' will also work to quote identifiers if you run in ANSI mode. *Note ANSI mode::. *Identifier**Max *Allowed characters* length* Database 64 Any character that is allowed in a directory name except `/' or `.'. Table 64 Any character that is allowed in a file name, except `/' or `.'. Column 64 All characters. Alias 255 All characters. Note that in addition to the above, you can't have ASCII(0) or ASCII(255) or the quoting character in an identifier. Note that if the identifier is a restricted word or contains special characters you must always quote it with ``' when you use it: SELECT * from `select` where `select`.id > 100; In previous versions of MySQL, the name rules are as follows: * A name may consist of alphanumeric characters from the current character set and also `_' and `$'. The default character set is ISO-8859-1 Latin1; this may be changed with the `--default-character-set' option to `mysqld'. *Note Character sets::. * A name may start with any character that is legal in a name. In particular, a name may start with a number (this differs from many other database systems!). However, a name cannot consist _only_ of numbers. * You cannot use the `.' character in names because it is used to extend the format by which you can refer to columns (see immediately below). It is recommended that you do not use names like `1e', because an expression like `1e+1' is ambiguous. It may be interpreted as the expression `1e + 1' or as the number `1e+1'. In MySQL you can refer to a column using any of the following forms: *Column reference* *Meaning* `col_name' Column `col_name' from whichever table used in the query contains a column of that name. `tbl_name.col_name' Column `col_name' from table `tbl_name' of the current database. `db_name.tbl_name.col_name'Column `col_name' from table `tbl_name' of the database `db_name'. This form is available in MySQL Version 3.22 or later. ``column_name`' A column that is a keyword or contains special characters. You need not specify a `tbl_name' or `db_name.tbl_name' prefix for a column reference in a statement unless the reference would be ambiguous. For example, suppose tables `t1' and `t2' each contain a column `c', and you retrieve `c' in a `SELECT' statement that uses both `t1' and `t2'. In this case, `c' is ambiguous because it is not unique among the tables used in the statement, so you must indicate which table you mean by writing `t1.c' or `t2.c'. Similarly, if you are retrieving from a table `t' in database `db1' and from a table `t' in database `db2', you must refer to columns in those tables as `db1.t.col_name' and `db2.t.col_name'. The syntax `.tbl_name' means the table `tbl_name' in the current database. This syntax is accepted for ODBC compatibility, because some ODBC programs prefix table names with a `.' character. Case Sensitivity in Names ------------------------- In MySQL, databases and tables correspond to directories and files within those directories. Consequently, the case sensitivity of the underlying operating system determines the case sensitivity of database and table names. This means database and table names are case sensitive in Unix and case insensitive in Windows. *Note Extensions to ANSI::. *NOTE:* Although database and table names are case insensitive for Windows, you should not refer to a given database or table using different cases within the same query. The following query would not work because it refers to a table both as `my_table' and as `MY_TABLE': mysql> SELECT * FROM my_table WHERE MY_TABLE.col=1; Column names are case insensitive in all cases. Aliases on tables are case sensitive. The following query would not work because it refers to the alias both as `a' and as `A': mysql> SELECT col_name FROM tbl_name AS a WHERE a.col_name = 1 OR A.col_name = 2; Aliases on columns are case insensitive. If you have a problem remembering the used cases for a table names, adopt a consistent convention, such as always creating databases and tables using lowercase names. One way to avoid this problem is to start `mysqld' with `-O lower_case_table_names=1'. By default this option is 1 on Windows and 0 on Unix. If `lower_case_table_names' is 1 MySQL will convert all table names to lower case on storage and lookup. Note that if you change this option, you need to first convert your old table names to lower case before starting `mysqld'. User Variables -------------- MySQL supports thread-specific variables with the `@variablename' syntax. A variable name may consist of alphanumeric characters from the current character set and also `_', `$', and `.' . The default character set is ISO-8859-1 Latin1; this may be changed with the `--default-character-set' option to `mysqld'. *Note Character sets::. Variables don't have to be initialized. They contain `NULL' by default and can store an integer, real, or string value. All variables for a thread are automatically freed when the thread exits. You can set a variable with the `SET' syntax: SET @variable= { integer expression | real expression | string expression } [,@variable= ...]. You can also set a variable in an expression with the `@variable:=expr' syntax: select @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3; +----------------------+------+------+------+ | @t1:=(@t2:=1)+@t3:=4 | @t1 | @t2 | @t3 | +----------------------+------+------+------+ | 5 | 5 | 1 | 4 | +----------------------+------+------+------+ (We had to use the `:=' syntax here, because `=' was reserved for comparisons.) User variables may be used where expressions are allowed. Note that this does not currently include use in contexts where a number is explicitly required, such as in the `LIMIT' clause of a `SELECT' statement, or the `IGNORE number LINES' clause of a `LOAD DATA' statement. *NOTE:* In a `SELECT' statement, each expression is only evaluated when it's sent to the client. This means that in the `HAVING', `GROUP BY', or `ORDER BY' clause, you can't refer to an expression that involves variables that are set in the `SELECT' part. For example, the following statement will NOT work as expected: SELECT (@aa:=id) AS a, (@aa+3) AS b FROM table_name HAVING b=5; The reason is that `@aa' will not contain the value of the current row, but the value of `id' for the previous accepted row. Comment Syntax -------------- The MySQL server supports the `# to end of line', `-- to end of line' and `/* in-line or multiple-line */' comment styles: mysql> select 1+1; # This comment continues to the end of line mysql> select 1+1; -- This comment continues to the end of line mysql> select 1 /* this is an in-line comment */ + 1; mysql> select 1+ /* this is a multiple-line comment */ 1; Note that the `--' comment style requires you to have at least one space after the `--'! Although the server understands the comment syntax just described, there are some limitations on the way that the `mysql' client parses `/* ... */' comments: * Single-quote and double-quote characters are taken to indicate the beginning of a quoted string, even within a comment. If the quote is not matched by a second quote within the comment, the parser doesn't realize the comment has ended. If you are running `mysql' interactively, you can tell that it has gotten confused like this because the prompt changes from `mysql>' to `'>' or `">'. * A semicolon is taken to indicate the end of the current SQL statement and anything following it to indicate the beginning of the next statement. These limitations apply both when you run `mysql' interactively and when you put commands in a file and tell `mysql' to read its input from that file with `mysql < some-file'. MySQL doesn't support the `--' ANSI SQL comment style. *Note Missing comments::. Is MySQL Picky About Reserved Words? ------------------------------------ A common problem stems from trying to create a table with column names that use the names of datatypes or functions built into MySQL, such as `TIMESTAMP' or `GROUP'. You're allowed to do it (for example, `ABS' is an allowed column name), but whitespace is not allowed between a function name and the `(' when using functions whose names are also column names. The following words are explicitly reserved in MySQL. Most of them are forbidden by ANSI SQL92 as column and/or table names (for example, `group'). A few are reserved because MySQL needs them and is (currently) using a `yacc' parser: `action' `add' `aggregate' `all' `alter' `after' `and' `as' `asc' `avg' `avg_row_length' `auto_increment' `between' `bigint' `bit' `binary' `blob' `bool' `both' `by' `cascade' `case' `char' `character' `change' `check' `checksum' `column' `columns' `comment' `constraint' `create' `cross' `current_date' `current_time' `current_timestamp' `data' `database' `databases' `date' `datetime' `day' `day_hour' `day_minute' `day_second' `dayofmonth' `dayofweek' `dayofyear' `dec' `decimal' `default' `delayed' `delay_key_write' `delete' `desc' `describe' `distinct' `distinctrow' `double' `drop' `end' `else' `escape' `escaped' `enclosed' `enum' `explain' `exists' `fields' `file' `first' `float' `float4' `float8' `flush' `foreign' `from' `for' `full' `function' `global' `grant' `grants' `group' `having' `heap' `high_priority' `hour' `hour_minute' `hour_second' `hosts' `identified' `ignore' `in' `index' `infile' `inner' `insert' `insert_id' `int' `integer' `interval' `int1' `int2' `int3' `int4' `int8' `into' `if' `is' `isam' `join' `key' `keys' `kill' `last_insert_id' `leading' `left' `length' `like' `lines' `limit' `load' `local' `lock' `logs' `long' `longblob' `longtext' `low_priority' `max' `max_rows' `match' `mediumblob' `mediumtext' `mediumint' `middleint' `min_rows' `minute' `minute_second' `modify' `month' `monthname' `myisam' `natural' `numeric' `no' `not' `null' `on' `optimize' `option' `optionally' `or' `order' `outer' `outfile' `pack_keys' `partial' `password' `precision' `primary' `procedure' `process' `processlist' `privileges' `read' `real' `references' `reload' `regexp' `rename' `replace' `restrict' `returns' `revoke' `rlike' `row' `rows' `second' `select' `set' `show' `shutdown' `smallint' `soname' `sql_big_tables' `sql_big_selects' `sql_low_priority_updates'`sql_log_off' `sql_log_update' `sql_select_limit' `sql_small_result' `sql_big_result' `sql_warnings' `straight_join' `starting' `status' `string' `table' `tables' `temporary' `terminated' `text' `then' `time' `timestamp' `tinyblob' `tinytext' `tinyint' `trailing' `to' `type' `use' `using' `unique' `unlock' `unsigned' `update' `usage' `values' `varchar' `variables' `varying' `varbinary' `with' `write' `when' `where' `year' `year_month' `zerofill' The following symbols (from the table above) are disallowed by ANSI SQL but allowed by MySQL as column/table names. This is because some of these names are very natural names and a lot of people have already used them. * `ACTION' * `BIT' * `DATE' * `ENUM' * `NO' * `TEXT' * `TIME' * `TIMESTAMP' Column Types ============ MySQL supports a number of column types, which may be grouped into three categories: numeric types, date and time types, and string (character) types. This section first gives an overview of the types available and summarizes the storage requirements for each column type, then provides a more detailed description of the properties of the types in each category. The overview is intentionally brief. The more detailed descriptions should be consulted for additional information about particular column types, such as the allowable formats in which you can specify values. The column types supported by MySQL are listed below. The following code letters are used in the descriptions: `M' Indicates the maximum display size. The maximum legal display size is 255. `D' Applies to floating-point types and indicates the number of digits following the decimal point. The maximum possible value is 30, but should be no greater than `M'-2. Square brackets (`[' and `]') indicate parts of type specifiers that are optional. Note that if you specify `ZEROFILL' for a column, MySQL will automatically add the `UNSIGNED' attribute to the column. `TINYINT[(M)] [UNSIGNED] [ZEROFILL]' A very small integer. The signed range is `-128' to `127'. The unsigned range is `0' to `255'. `SMALLINT[(M)] [UNSIGNED] [ZEROFILL]' A small integer. The signed range is `-32768' to `32767'. The unsigned range is `0' to `65535'. `MEDIUMINT[(M)] [UNSIGNED] [ZEROFILL]' A medium-size integer. The signed range is `-8388608' to `8388607'. The unsigned range is `0' to `16777215'. `INT[(M)] [UNSIGNED] [ZEROFILL]' A normal-size integer. The signed range is `-2147483648' to `2147483647'. The unsigned range is `0' to `4294967295'. `INTEGER[(M)] [UNSIGNED] [ZEROFILL]' This is a synonym for `INT'. `BIGINT[(M)] [UNSIGNED] [ZEROFILL]' A large integer. The signed range is `-9223372036854775808' to `9223372036854775807'. The unsigned range is `0' to `18446744073709551615'. Some things you should be aware about `BIGINT' columns: * As all arithmetic is done using signed `BIGINT' or `DOUBLE' values, so you shouldn't use unsigned big integers larger than `9223372036854775807' (63 bits) except with bit functions! If you do that, some of the last digits in the result may be wrong because of rounding errors when converting the `BIGINT' to a `DOUBLE'. * You can always store an exact integer value in a `BIGINT' column by storing it as a string, as there is in this case there will be no intermediate double representation. * `-', `+', and `*' will use `BIGINT' arithmetic when both arguments are `INTEGER' values! This means that if you multiply two big integers (or results from functions that return integers) you may get unexpected results when the result is larger than `9223372036854775807'. `FLOAT(precision) [ZEROFILL]' A floating-point number. Cannot be unsigned. `precision' can be `<=24' for a single-precision floating-point number and between 25 and 53 for a double-precision floating-point number. These types are like the `FLOAT' and `DOUBLE' types described immediately below. `FLOAT(X)' has the same range as the corresponding `FLOAT' and `DOUBLE' types, but the display size and number of decimals is undefined. In MySQL Version 3.23, this is a true floating-point value. In earlier MySQL versions, `FLOAT(precision)' always has 2 decimals. Note that using `FLOAT' may give you some unexpected problems as all calculation in MySQL is done with double precision. *Note No matching rows::. This syntax is provided for ODBC compatibility. `FLOAT[(M,D)] [ZEROFILL]' A small (single-precision) floating-point number. Cannot be unsigned. Allowable values are `-3.402823466E+38' to `-1.175494351E-38', `0', and `1.175494351E-38' to `3.402823466E+38'. The M is the display width and D is the number of decimals. `FLOAT' without an argument or with an argument of <= 24 stands for a single-precision floating-point number. `DOUBLE[(M,D)] [ZEROFILL]' A normal-size (double-precision) floating-point number. Cannot be unsigned. Allowable values are `-1.7976931348623157E+308' to `-2.2250738585072014E-308', `0', and `2.2250738585072014E-308' to `1.7976931348623157E+308'. The M is the display width and D is the number of decimals. `DOUBLE' without an argument or `FLOAT(X)' where 25 <= X <= 53 stands for a double-precision floating-point number. `DOUBLE PRECISION[(M,D)] [ZEROFILL]' `REAL[(M,D)] [ZEROFILL]' These are synonyms for `DOUBLE'. `DECIMAL[(M[,D])] [ZEROFILL]' An unpacked floating-point number. Cannot be unsigned. Behaves like a `CHAR' column: "unpacked" means the number is stored as a string, using one character for each digit of the value. The decimal point and, for negative numbers, the `-' sign, are not counted in M (but space for these are reserved). If `D' is 0, values will have no decimal point or fractional part. The maximum range of `DECIMAL' values is the same as for `DOUBLE', but the actual range for a given `DECIMAL' column may be constrained by the choice of `M' and `D'. If `D' is left out it's set to 0. If `M' is left out it's set to 10. Note that in MySQL Version 3.22 the `M' argument had to includes the space needed for the sign and the decimal point. `NUMERIC(M,D) [ZEROFILL]' This is a synonym for `DECIMAL'. `DATE' A date. The supported range is `'1000-01-01'' to `'9999-12-31''. MySQL displays `DATE' values in `'YYYY-MM-DD'' format, but allows you to assign values to `DATE' columns using either strings or numbers. *Note DATETIME::. `DATETIME' A date and time combination. The supported range is `'1000-01-01 00:00:00'' to `'9999-12-31 23:59:59''. MySQL displays `DATETIME' values in `'YYYY-MM-DD HH:MM:SS'' format, but allows you to assign values to `DATETIME' columns using either strings or numbers. *Note DATETIME::. `TIMESTAMP[(M)]' A timestamp. The range is `'1970-01-01 00:00:00'' to sometime in the year `2037'. MySQL displays `TIMESTAMP' values in `YYYYMMDDHHMMSS', `YYMMDDHHMMSS', `YYYYMMDD', or `YYMMDD' format, depending on whether `M' is `14' (or missing), `12', `8', or `6', but allows you to assign values to `TIMESTAMP' columns using either strings or numbers. A `TIMESTAMP' column is useful for recording the date and time of an `INSERT' or `UPDATE' operation because it is automatically set to the date and time of the most recent operation if you don't give it a value yourself. You can also set it to the current date and time by assigning it a `NULL' value. *Note Date and time types::. A `TIMESTAMP' is always stored in 4 bytes. The `M' argument only affects how the `TIMESTAMP' column is displayed. Note that `TIMESTAMP(X)' columns where X is 8 or 14 are reported to be numbers while other `TIMESTAMP(X)' columns are reported to be strings. This is just to ensure that one can reliably dump and restore the table with these types! *Note DATETIME::. `TIME' A time. The range is `'-838:59:59'' to `'838:59:59''. MySQL displays `TIME' values in `'HH:MM:SS'' format, but allows you to assign values to `TIME' columns using either strings or numbers. *Note TIME::. `YEAR[(2|4)]' A year in 2- or 4-digit format (default is 4-digit). The allowable values are `1901' to `2155', `0000' in the 4-digit year format, and 1970-2069 if you use the 2-digit format (70-69). MySQL displays `YEAR' values in `YYYY' format, but allows you to assign values to `YEAR' columns using either strings or numbers. (The `YEAR' type is new in MySQL Version 3.22.). *Note YEAR::. `[NATIONAL] CHAR(M) [BINARY]' A fixed-length string that is always right-padded with spaces to the specified length when stored. The range of `M' is 1 to 255 characters. Trailing spaces are removed when the value is retrieved. `CHAR' values are sorted and compared in case-insensitive fashion according to the default character set unless the `BINARY' keyword is given. `NATIONAL CHAR' (short form `NCHAR') is the ANSI SQL way to define that a CHAR column should use the default CHARACTER set. This is the default in MySQL. `CHAR' is a shorthand for `CHARACTER'. MySQL allows you to create a column of type `CHAR(0)'. This is mainly useful when you have to be compliant with some old applications that depend on the existence of a column but that do not actually use the value. This is also quite nice when you need a column that only can take 2 values: A `CHAR(0)', that is not defined as `NOT NULL', will only occupy one bit and can only take 2 values: `NULL' or `""'. *Note CHAR::. `BIT' `BOOL' `CHAR' These three are synonyms for `CHAR(1)'. `[NATIONAL] VARCHAR(M) [BINARY]' A variable-length string. *NOTE:* Trailing spaces are removed when the value is stored (this differs from the ANSI SQL specification). The range of `M' is 1 to 255 characters. `VARCHAR' values are sorted and compared in case-insensitive fashion unless the `BINARY' keyword is given. *Note Silent column changes::. `VARCHAR' is a shorthand for `CHARACTER VARYING'. *Note CHAR::. `TINYBLOB' `TINYTEXT' A `BLOB' or `TEXT' column with a maximum length of 255 (2^8 - 1) characters. *Note Silent column changes::. *Note BLOB::. `BLOB' `TEXT' A `BLOB' or `TEXT' column with a maximum length of 65535 (2^16 - 1) characters. *Note Silent column changes::. *Note BLOB::. `MEDIUMBLOB' `MEDIUMTEXT' A `BLOB' or `TEXT' column with a maximum length of 16777215 (2^24 - 1) characters. *Note Silent column changes::. *Note BLOB::. `LONGBLOB' `LONGTEXT' A `BLOB' or `TEXT' column with a maximum length of 4294967295 (2^32 - 1) characters. *Note Silent column changes::. Note that because the server/client protocol and MyISAM tables has currently a limit of 16M per communication packet / table row, you can't yet use this the whole range of this type. *Note BLOB::. `ENUM('value1','value2',...)' An enumeration. A string object that can have only one value, chosen from the list of values `'value1'', `'value2'', `...', `NULL' or the special `""' error value. An `ENUM' can have a maximum of 65535 distinct values. *Note ENUM::. `SET('value1','value2',...)' A set. A string object that can have zero or more values, each of which must be chosen from the list of values `'value1'', `'value2'', `...' A `SET' can have a maximum of 64 members. *Note SET::. Numeric Types ------------- MySQL supports all of the ANSI/ISO SQL92 numeric types. These types include the exact numeric data types (`NUMERIC', `DECIMAL', `INTEGER', and `SMALLINT'), as well as the approximate numeric data types (`FLOAT', `REAL', and `DOUBLE PRECISION'). The keyword `INT' is a synonym for `INTEGER', and the keyword `DEC' is a synonym for `DECIMAL'. The `NUMERIC' and `DECIMAL' types are implemented as the same type by MySQL, as permitted by the SQL92 standard. They are used for values for which it is important to preserve exact precision, for example with monetary data. When declaring a column of one of these types the precision and scale can be (and usually is) specified; for example: salary DECIMAL(9,2) In this example, `9' (`precision') represents the number of significant decimal digits that will be stored for values, and `2' (`scale') represents the number of digits that will be stored following the decimal point. In this case, therefore, the range of values that can be stored in the `salary' column is from `-9999999.99' to `9999999.99'. (MySQL can actually store numbers up to `9999999.99' in this column because it doesn't have to store the sign for positive numbers) In ANSI/ISO SQL92, the syntax `DECIMAL(p)' is equivalent to `DECIMAL(p,0)'. Similarly, the syntax `DECIMAL' is equivalent to `DECIMAL(p,0)', where the implementation is allowed to decide the value of `p'. MySQL does not currently support either of these variant forms of the `DECIMAL'/`NUMERIC' data types. This is not generally a serious problem, as the principal benefits of these types derive from the ability to control both precision and scale explicitly. `DECIMAL' and `NUMERIC' values are stored as strings, rather than as binary floating-point numbers, in order to preserve the decimal precision of those values. One character is used for each digit of the value, the decimal point (if `scale' > 0), and the `-' sign (for negative numbers). If `scale' is 0, `DECIMAL' and `NUMERIC' values contain no decimal point or fractional part. The maximum range of `DECIMAL' and `NUMERIC' values is the same as for `DOUBLE', but the actual range for a given `DECIMAL' or `NUMERIC' column can be constrained by the `precision' or `scale' for a given column. When such a column is assigned a value with more digits following the decimal point than are allowed by the specified `scale', the value is rounded to that `scale'. When a `DECIMAL' or `NUMERIC' column is assigned a value whose magnitude exceeds the range implied by the specified (or defaulted) `precision' and `scale', MySQL stores the value representing the corresponding end point of that range. As an extension to the ANSI/ISO SQL92 standard, MySQL also supports the integral types `TINYINT', `MEDIUMINT', and `BIGINT' as listed in the tables above. Another extension is supported by MySQL for optionally specifying the display width of an integral value in parentheses following the base keyword for the type (for example, `INT(4)'). This optional width specification is used to left-pad the display of values whose width is less than the width specified for the column, but does not constrain the range of values that can be stored in the column, nor the number of digits that will be displayed for values whose width exceeds that specified for the column. When used in conjunction with the optional extension attribute `ZEROFILL', the default padding of spaces is replaced with zeroes. For example, for a column declared as `INT(5) ZEROFILL', a value of `4' is retrieved as `00004'. Note that if you store larger values than the display width in an integer column, you may experience problems when MySQL generates temporary tables for some complicated joins, as in these cases MySQL trusts that the data did fit into the original column width. All integral types can have an optional (non-standard) attribute `UNSIGNED'. Unsigned values can be used when you want to allow only positive numbers in a column and you need a little bigger numeric range for the column. The `FLOAT' type is used to represent approximate numeric data types. The ANSI/ISO SQL92 standard allows an optional specification of the precision (but not the range of the exponent) in bits following the keyword `FLOAT' in parentheses. The MySQL implementation also supports this optional precision specification. When the keyword `FLOAT' is used for a column type without a precision specification, MySQL uses four bytes to store the values. A variant syntax is also supported, with two numbers given in parentheses following the `FLOAT' keyword. With this option, the first number continues to represent the storage requirements for the value in bytes, and the second number specifies the number of digits to be stored and displayed following the decimal point (as with `DECIMAL' and `NUMERIC'). When MySQL is asked to store a number for such a column with more decimal digits following the decimal point than specified for the column, the value is rounded to eliminate the extra digits when the value is stored. The `REAL' and `DOUBLE PRECISION' types do not accept precision specifications. As an extension to the ANSI/ISO SQL92 standard, MySQL recognizes `DOUBLE' as a synonym for the `DOUBLE PRECISION' type. In contrast with the standard's requirement that the precision for `REAL' be smaller than that used for `DOUBLE PRECISION', MySQL implements both as 8-byte double-precision floating-point values (when not running in "ANSI mode"). For maximum portability, code requiring storage of approximate numeric data values should use `FLOAT' or `DOUBLE PRECISION' with no specification of precision or number of decimal points. When asked to store a value in a numeric column that is outside the column type's allowable range, MySQL clips the value to the appropriate endpoint of the range and stores the resulting value instead. For example, the range of an `INT' column is `-2147483648' to `2147483647'. If you try to insert `-9999999999' into an `INT' column, the value is clipped to the lower endpoint of the range, and `-2147483648' is stored instead. Similarly, if you try to insert `9999999999', `2147483647' is stored instead. If the `INT' column is `UNSIGNED', the size of the column's range is the same but its endpoints shift up to `0' and `4294967295'. If you try to store `-9999999999' and `9999999999', the values stored in the column become `0' and `4294967296'. Conversions that occur due to clipping are reported as "warnings" for `ALTER TABLE', `LOAD DATA INFILE', `UPDATE', and multi-row `INSERT' statements. Date and Time Types ------------------- The date and time types are `DATETIME', `DATE', `TIMESTAMP', `TIME', and `YEAR'. Each of these has a range of legal values, as well as a "zero" value that is used when you specify a really illegal value. Note that MySQL allows you to store certain 'not strictly' legal date values, for example `1999-11-31'. The reason for this is that we think it's the responsibility of the application to handle date checking, not the SQL servers. To make the date checking 'fast', MySQL only checks that the month is in the range of 0-12 and the day is in the range of 0-31. The above ranges are defined this way because MySQL allows you to store, in a `DATE' or `DATETIME' column, dates where the day or month-day is zero. This is extremely useful for applications that need to store a birth-date for which you don't know the exact date. In this case you simply store the date like `1999-00-00' or `1999-01-00'. (You cannot expect to get a correct value from functions like `DATE_SUB()' or `DATE_ADD' for dates like these.) Here are some general considerations to keep in mind when working with date and time types: * MySQL retrieves values for a given date or time type in a standard format, but it attempts to interpret a variety of formats for values that you supply (for example, when you specify a value to be assigned to or compared to a date or time type). Nevertheless, only the formats described in the following sections are supported. It is expected that you will supply legal values, and unpredictable results may occur if you use values in other formats. * Although MySQL tries to interpret values in several formats, it always expects the year part of date values to be leftmost. Dates must be given in year-month-day order (for example, `'98-09-04''), rather than in the month-day-year or day-month-year orders commonly used elsewhere (for example, `'09-04-98'', `'04-09-98''). * MySQL automatically converts a date or time type value to a number if the value is used in a numeric context, and vice versa. * When MySQL encounters a value for a date or time type that is out of range or otherwise illegal for the type (see the start of this section), it converts the value to the "zero" value for that type. (The exception is that out-of-range `TIME' values are clipped to the appropriate endpoint of the `TIME' range.) The table below shows the format of the "zero" value for each type: *Column type* *"Zero" value* `DATETIME' `'0000-00-00 00:00:00'' `DATE' `'0000-00-00'' `TIMESTAMP' `00000000000000' (length depends on display size) `TIME' `'00:00:00'' `YEAR' `0000' * The "zero" values are special, but you can store or refer to them explicitly using the values shown in the table. You can also do this using the values `'0'' or `0', which are easier to write. * "Zero" date or time values used through *MyODBC* are converted automatically to `NULL' in *MyODBC* Version 2.50.12 and above, because ODBC can't handle such values. Y2K Issues and Date Types ......................... MySQL itself is Y2K-safe (*note Year 2000 compliance::), but input values presented to MySQL may not be. Any input containing 2-digit year values is ambiguous, because the century is unknown. Such values must be interpreted into 4-digit form because MySQL stores years internally using four digits. For `DATETIME', `DATE', `TIMESTAMP', and `YEAR' types, MySQL interprets dates with ambiguous year values using the following rules: * Year values in the range `00-69' are converted to `2000-2069'. * Year values in the range `70-99' are converted to `1970-1999'. Remember that these rules provide only reasonable guesses as to what your data mean. If the heuristics used by MySQL don't produce the correct values, you should provide unambiguous input containing 4-digit year values. `ORDER BY' will sort 2-digit `YEAR/DATE/DATETIME' types properly. Note also that some functions like `MIN()' and `MAX()' will convert a `TIMESTAMP/DATE' to a number. This means that a timestamp with a 2-digit year will not work properly with these functions. The fix in this case is to convert the `TIMESTAMP/DATE' to 4-digit year format or use something like `MIN(DATE_ADD(timestamp,INTERVAL 0 DAYS))'. The `DATETIME', `DATE', and `TIMESTAMP' Types ............................................. The `DATETIME', `DATE', and `TIMESTAMP' types are related. This section describes their characteristics, how they are similar, and how they differ. The `DATETIME' type is used when you need values that contain both date and time information. MySQL retrieves and displays `DATETIME' values in `'YYYY-MM-DD HH:MM:SS'' format. The supported range is `'1000-01-01 00:00:00'' to `'9999-12-31 23:59:59''. ("Supported" means that although earlier values might work, there is no guarantee that they will.) The `DATE' type is used when you need only a date value, without a time part. MySQL retrieves and displays `DATE' values in `'YYYY-MM-DD'' format. The supported range is `'1000-01-01'' to `'9999-12-31''. The `TIMESTAMP' column type provides a type that you can use to automatically mark `INSERT' or `UPDATE' operations with the current date and time. If you have multiple `TIMESTAMP' columns, only the first one is updated automatically. Automatic updating of the first `TIMESTAMP' column occurs under any of the following conditions: * The column is not specified explicitly in an `INSERT' or `LOAD DATA INFILE' statement. * The column is not specified explicitly in an `UPDATE' statement and some other column changes value. (Note that an `UPDATE' that sets a column to the value it already has will not cause the `TIMESTAMP' column to be updated, because if you set a column to its current value, MySQL ignores the update for efficiency.) * You explicitly set the `TIMESTAMP' column to `NULL'. `TIMESTAMP' columns other than the first may also be set to the current date and time. Just set the column to `NULL' or to `NOW()'. You can set any `TIMESTAMP' column to a value different than the current date and time by setting it explicitly to the desired value. This is true even for the first `TIMESTAMP' column. You can use this property if, for example, you want a `TIMESTAMP' to be set to the current date and time when you create a row, but not to be changed whenever the row is updated later: * Let MySQL set the column when the row is created. This will initialize it to the current date and time. * When you perform subsequent updates to other columns in the row, set the `TIMESTAMP' column explicitly to its current value. On the other hand, you may find it just as easy to use a `DATETIME' column that you initialize to `NOW()' when the row is created and leave alone for subsequent updates. `TIMESTAMP' values may range from the beginning of 1970 to sometime in the year 2037, with a resolution of one second. Values are displayed as numbers. The format in which MySQL retrieves and displays `TIMESTAMP' values depends on the display size, as illustrated by the table below. The `full' `TIMESTAMP' format is 14 digits, but `TIMESTAMP' columns may be created with shorter display sizes: *Column type* *Display format* `TIMESTAMP(14)' `YYYYMMDDHHMMSS' `TIMESTAMP(12)' `YYMMDDHHMMSS' `TIMESTAMP(10)' `YYMMDDHHMM' `TIMESTAMP(8)' `YYYYMMDD' `TIMESTAMP(6)' `YYMMDD' `TIMESTAMP(4)' `YYMM' `TIMESTAMP(2)' `YY' All `TIMESTAMP' columns have the same storage size, regardless of display size. The most common display sizes are 6, 8, 12, and 14. You can specify an arbitrary display size at table creation time, but values of 0 or greater than 14 are coerced to 14. Odd-valued sizes in the range from 1 to 13 are coerced to the next higher even number. You can specify `DATETIME', `DATE', and `TIMESTAMP' values using any of a common set of formats: * As a string in either `'YYYY-MM-DD HH:MM:SS'' or `'YY-MM-DD HH:MM:SS'' format. A "relaxed" syntax is allowed--any punctuation character may be used as the delimiter between date parts or time parts. For example, `'98-12-31 11:30:45'', `'98.12.31 11+30+45'', `'98/12/31 11*30*45'', and `'98@12@31 11^30^45'' are equivalent. * As a string in either `'YYYY-MM-DD'' or `'YY-MM-DD'' format. A "relaxed" syntax is allowed here, too. For example, `'98-12-31'', `'98.12.31'', `'98/12/31'', and `'98@12@31'' are equivalent. * As a string with no delimiters in either `'YYYYMMDDHHMMSS'' or `'YYMMDDHHMMSS'' format, provided that the string makes sense as a date. For example, `'19970523091528'' and `'970523091528'' are interpreted as `'1997-05-23 09:15:28'', but `'971122129015'' is illegal (it has a nonsensical minute part) and becomes `'0000-00-00 00:00:00''. * As a string with no delimiters in either `'YYYYMMDD'' or `'YYMMDD'' format, provided that the string makes sense as a date. For example, `'19970523'' and `'970523'' are interpreted as `'1997-05-23'', but `'971332'' is illegal (it has nonsensical month and day parts) and becomes `'0000-00-00''. * As a number in either `YYYYMMDDHHMMSS' or `YYMMDDHHMMSS' format, provided that the number makes sense as a date. For example, `19830905132800' and `830905132800' are interpreted as `'1983-09-05 13:28:00''. * As a number in either `YYYYMMDD' or `YYMMDD' format, provided that the number makes sense as a date. For example, `19830905' and `830905' are interpreted as `'1983-09-05''. * As the result of a function that returns a value that is acceptable in a `DATETIME', `DATE', or `TIMESTAMP' context, such as `NOW()' or `CURRENT_DATE'. Illegal `DATETIME', `DATE', or `TIMESTAMP' values are converted to the "zero" value of the appropriate type (`'0000-00-00 00:00:00'', `'0000-00-00'', or `00000000000000'). For values specified as strings that include date part delimiters, it is not necessary to specify two digits for month or day values that are less than `10'. `'1979-6-9'' is the same as `'1979-06-09''. Similarly, for values specified as strings that include time part delimiters, it is not necessary to specify two digits for hour, month, or second values that are less than `10'. `'1979-10-30 1:2:3'' is the same as `'1979-10-30 01:02:03''. Values specified as numbers should be 6, 8, 12, or 14 digits long. If the number is 8 or 14 digits long, it is assumed to be in `YYYYMMDD' or `YYYYMMDDHHMMSS' format and that the year is given by the first 4 digits. If the number is 6 or 12 digits long, it is assumed to be in `YYMMDD' or `YYMMDDHHMMSS' format and that the year is given by the first 2 digits. Numbers that are not one of these lengths are interpreted as though padded with leading zeros to the closest length. Values specified as non-delimited strings are interpreted using their length as given. If the string is 8 or 14 characters long, the year is assumed to be given by the first 4 characters. Otherwise the year is assumed to be given by the first 2 characters. The string is interpreted from left to right to find year, month, day, hour, minute, and second values, for as many parts as are present in the string. This means you should not use strings that have fewer than 6 characters. For example, if you specify `'9903'', thinking that will represent March, 1999, you will find that MySQL inserts a "zero" date into your table. This is because the year and month values are `99' and `03', but the day part is missing (zero), so the value is not a legal date. `TIMESTAMP' columns store legal values using the full precision with which the value was specified, regardless of the display size. This has several implications: * Always specify year, month, and day, even if your column types are `TIMESTAMP(4)' or `TIMESTAMP(2)'. Otherwise, the value will not be a legal date and `0' will be stored. * If you use `ALTER TABLE' to widen a narrow `TIMESTAMP' column, information will be displayed that previously was "hidden". * Similarly, narrowing a `TIMESTAMP' column does not cause information to be lost, except in the sense that less information is shown when the values are displayed. * Although `TIMESTAMP' values are stored to full precision, the only function that operates directly on the underlying stored value is `UNIX_TIMESTAMP()'. Other functions operate on the formatted retrieved value. This means you cannot use functions such as `HOUR()' or `SECOND()' unless the relevant part of the `TIMESTAMP' value is included in the formatted value. For example, the `HH' part of a `TIMESTAMP' column is not displayed unless the display size is at least 10, so trying to use `HOUR()' on shorter `TIMESTAMP' values produces a meaningless result. You can to some extent assign values of one date type to an object of a different date type. However, there may be some alteration of the value or loss of information: * If you assign a `DATE' value to a `DATETIME' or `TIMESTAMP' object, the time part of the resulting value is set to `'00:00:00'', because the `DATE' value contains no time information. * If you assign a `DATETIME' or `TIMESTAMP' value to a `DATE' object, the time part of the resulting value is deleted, because the `DATE' type stores no time information. * Remember that although `DATETIME', `DATE', and `TIMESTAMP' values all can be specified using the same set of formats, the types do not all have the same range of values. For example, `TIMESTAMP' values cannot be earlier than `1970' or later than `2037'. This means that a date such as `'1968-01-01'', while legal as a `DATETIME' or `DATE' value, is not a valid `TIMESTAMP' value and will be converted to `0' if assigned to such an object. Be aware of certain pitfalls when specifying date values: * The relaxed format allowed for values specified as strings can be deceiving. For example, a value such as `'10:11:12'' might look like a time value because of the `:' delimiter, but if used in a date context will be interpreted as the year `'2010-11-12''. The value `'10:45:15'' will be converted to `'0000-00-00'' because `'45'' is not a legal month. * Year values specified as two digits are ambiguous, because the century is unknown. MySQL interprets 2-digit year values using the following rules: - Year values in the range `00-69' are converted to `2000-2069'. - Year values in the range `70-99' are converted to `1970-1999'. The `TIME' Type ............... MySQL retrieves and displays `TIME' values in `'HH:MM:SS'' format (or `'HHH:MM:SS'' format for large hours values). `TIME' values may range from `'-838:59:59'' to `'838:59:59''. The reason the hours part may be so large is that the `TIME' type may be used not only to represent a time of day (which must be less than 24 hours), but also elapsed time or a time interval between two events (which may be much greater than 24 hours, or even negative). You can specify `TIME' values in a variety of formats: * As a string in `'D HH:MM:SS.fraction'' format. (Note that MySQL doesn't yet store the fraction for the time column). One can also use one of the following "relaxed" syntax: `HH:MM:SS.fraction', `HH:MM:SS', `HH:MM', `D HH:MM:SS', `D HH:MM', `D HH' or `SS'. Here `D' is days between 0-33. * As a string with no delimiters in `'HHMMSS'' format, provided that it makes sense as a time. For example, `'101112'' is understood as `'10:11:12'', but `'109712'' is illegal (it has a nonsensical minute part) and becomes `'00:00:00''. * As a number in `HHMMSS' format, provided that it makes sense as a time. For example, `101112' is understood as `'10:11:12''. The following alternative formats are also understood: `SS', `MMSS',`HHMMSS', `HHMMSS.fraction'. Note that MySQL doesn't yet store the fraction part. * As the result of a function that returns a value that is acceptable in a `TIME' context, such as `CURRENT_TIME'. For `TIME' values specified as strings that include a time part delimiter, it is not necessary to specify two digits for hours, minutes, or seconds values that are less than `10'. `'8:3:2'' is the same as `'08:03:02''. Be careful about assigning "short" `TIME' values to a `TIME' column. Without semicolon, MySQL interprets values using the assumption that the rightmost digits represent seconds. (MySQL interprets `TIME' values as elapsed time rather than as time of day.) For example, you might think of `'1112'' and `1112' as meaning `'11:12:00'' (12 minutes after 11 o'clock), but MySQL interprets them as `'00:11:12'' (11 minutes, 12 seconds). Similarly, `'12'' and `12' are interpreted as `'00:00:12''. `TIME' values with semicolon, instead, are always treated as time of the day. That is `'11:12'' will mean `'11:12:00'', not `'00:11:12''. Values that lie outside the `TIME' range but are otherwise legal are clipped to the appropriate endpoint of the range. For example, `'-850:00:00'' and `'850:00:00'' are converted to `'-838:59:59'' and `'838:59:59''. Illegal `TIME' values are converted to `'00:00:00''. Note that because `'00:00:00'' is itself a legal `TIME' value, there is no way to tell, from a value of `'00:00:00'' stored in a table, whether the original value was specified as `'00:00:00'' or whether it was illegal. The `YEAR' Type ............... The `YEAR' type is a 1-byte type used for representing years. MySQL retrieves and displays `YEAR' values in `YYYY' format. The range is `1901' to `2155'. You can specify `YEAR' values in a variety of formats: * As a four-digit string in the range `'1901'' to `'2155''. * As a four-digit number in the range `1901' to `2155'. * As a two-digit string in the range `'00'' to `'99''. Values in the ranges `'00'' to `'69'' and `'70'' to `'99'' are converted to `YEAR' values in the ranges `2000' to `2069' and `1970' to `1999'. * As a two-digit number in the range `1' to `99'. Values in the ranges `1' to `69' and `70' to `99' are converted to `YEAR' values in the ranges `2001' to `2069' and `1970' to `1999'. Note that the range for two-digit numbers is slightly different than the range for two-digit strings, because you cannot specify zero directly as a number and have it be interpreted as `2000'. You _must_ specify it as a string `'0'' or `'00'' or it will be interpreted as `0000'. * As the result of a function that returns a value that is acceptable in a `YEAR' context, such as `NOW()'. Illegal `YEAR' values are converted to `0000'. String Types ------------ The string types are `CHAR', `VARCHAR', `BLOB', `TEXT', `ENUM', and `SET'. This section describes how these types work, their storage requirements, and how to use them in your queries. The `CHAR' and `VARCHAR' Types .............................. The `CHAR' and `VARCHAR' types are similar, but differ in the way they are stored and retrieved. The length of a `CHAR' column is fixed to the length that you declare when you create the table. The length can be any value between 1 and 255. (As of MySQL Version 3.23, the length of `CHAR' may be 0 to 255.) When `CHAR' values are stored, they are right-padded with spaces to the specified length. When `CHAR' values are retrieved, trailing spaces are removed. Values in `VARCHAR' columns are variable-length strings. You can declare a `VARCHAR' column to be any length between 1 and 255, just as for `CHAR' columns. However, in contrast to `CHAR', `VARCHAR' values are stored using only as many characters as are needed, plus one byte to record the length. Values are not padded; instead, trailing spaces are removed when values are stored. (This space removal differs from the ANSI SQL specification.) If you assign a value to a `CHAR' or `VARCHAR' column that exceeds the column's maximum length, the value is truncated to fit. The table below illustrates the differences between the two types of columns by showing the result of storing various string values into `CHAR(4)' and `VARCHAR(4)' columns: *Value* `CHAR(4)' *Storage `VARCHAR(4)' *Storage required* required* `''' `' '' 4 bytes `''' 1 byte `'ab'' `'ab '' 4 bytes `'ab'' 3 bytes `'abcd'' `'abcd'' 4 bytes `'abcd'' 5 bytes `'abcdefgh'' `'abcd'' 4 bytes `'abcd'' 5 bytes The values retrieved from the `CHAR(4)' and `VARCHAR(4)' columns will be the same in each case, because trailing spaces are removed from `CHAR' columns upon retrieval. Values in `CHAR' and `VARCHAR' columns are sorted and compared in case-insensitive fashion, unless the `BINARY' attribute was specified when the table was created. The `BINARY' attribute means that column values are sorted and compared in case-sensitive fashion according to the ASCII order of the machine where the MySQL server is running. `BINARY' doesn't affect how the column is stored or retrieved. The `BINARY' attribute is sticky. This means that if a column marked `BINARY' is used in an expression, the whole expression is compared as a `BINARY' value. MySQL may silently change the type of a `CHAR' or `VARCHAR' column at table creation time. *Note Silent column changes::. The `BLOB' and `TEXT' Types ........................... A `BLOB' is a binary large object that can hold a variable amount of data. The four `BLOB' types `TINYBLOB', `BLOB', `MEDIUMBLOB', and `LONGBLOB' differ only in the maximum length of the values they can hold. *Note Storage requirements::. The four `TEXT' types `TINYTEXT', `TEXT', `MEDIUMTEXT', and `LONGTEXT' correspond to the four `BLOB' types and have the same maximum lengths and storage requirements. The only difference between `BLOB' and `TEXT' types is that sorting and comparison is performed in case-sensitive fashion for `BLOB' values and case-insensitive fashion for `TEXT' values. In other words, a `TEXT' is a case-insensitive `BLOB'. If you assign a value to a `BLOB' or `TEXT' column that exceeds the column type's maximum length, the value is truncated to fit. In most respects, you can regard a `TEXT' column as a `VARCHAR' column that can be as big as you like. Similarly, you can regard a `BLOB' column as a `VARCHAR BINARY' column. The differences are: * You can have indexes on `BLOB' and `TEXT' columns with MySQL Version 3.23.2 and newer. Older versions of MySQL did not support this. * There is no trailing-space removal for `BLOB' and `TEXT' columns when values are stored, as there is for `VARCHAR' columns. * `BLOB' and `TEXT' columns cannot have `DEFAULT' values. *MyODBC* defines `BLOB' values as `LONGVARBINARY' and `TEXT' values as `LONGVARCHAR'. Because `BLOB' and `TEXT' values may be extremely long, you may run up against some constraints when using them: * If you want to use `GROUP BY' or `ORDER BY' on a `BLOB' or `TEXT' column, you must convert the column value into a fixed-length object. The standard way to do this is with the `SUBSTRING' function. For example: mysql> select comment from tbl_name,substring(comment,20) as substr ORDER BY substr; If you don't do this, only the first `max_sort_length' bytes of the column are used when sorting. The default value of `max_sort_length' is 1024; this value can be changed using the `-O' option when starting the `mysqld' server. You can group on an expression involving `BLOB' or `TEXT' values by specifying the column position or by using an alias: mysql> select id,substring(blob_col,1,100) from tbl_name GROUP BY 2; mysql> select id,substring(blob_col,1,100) as b from tbl_name GROUP BY b; * The maximum size of a `BLOB' or `TEXT' object is determined by its type, but the largest value you can actually transmit between the client and server is determined by the amount of available memory and the size of the communications buffers. You can change the message buffer size, but you must do so on both the server and client ends. *Note Server parameters::. Note that each `BLOB' or `TEXT' value is represented internally by a separately allocated object. This is in contrast to all other column types, for which storage is allocated once per column when the table is opened. The `ENUM' Type ............... An `ENUM' is a string object whose value normally is chosen from a list of allowed values that are enumerated explicitly in the column specification at table creation time. The value may also be the empty string (`""') or `NULL' under certain circumstances: * If you insert an invalid value into an `ENUM' (that is, a string not present in the list of allowed values), the empty string is inserted instead as a special error value. * If an `ENUM' is declared `NULL', `NULL' is also a legal value for the column, and the default value is `NULL'. If an `ENUM' is declared `NOT NULL', the default value is the first element of the list of allowed values. Each enumeration value has an index: * Values from the list of allowable elements in the column specification are numbered beginning with 1. * The index value of the empty string error value is 0. This means that you can use the following `SELECT' statement to find rows into which invalid `ENUM' values were assigned: mysql> SELECT * FROM tbl_name WHERE enum_col=0; * The index of the `NULL' value is `NULL'. For example, a column specified as `ENUM("one", "two", "three")' can have any of the values shown below. The index of each value is also shown: *Value* *Index* `NULL' `NULL' `""' 0 `"one"' 1 `"two"' 2 `"three"' 3 An enumeration can have a maximum of 65535 elements. Lettercase is irrelevant when you assign values to an `ENUM' column. However, values retrieved from the column later have lettercase matching the values that were used to specify the allowable values at table creation time. If you retrieve an `ENUM' in a numeric context, the column value's index is returned. For example, you can retrieve numeric values from an `ENUM' column like this: mysql> SELECT enum_col+0 FROM tbl_name; If you store a number into an `ENUM', the number is treated as an index, and the value stored is the enumeration member with that index. (However, this will not work with `LOAD DATA', which treats all input as strings.) `ENUM' values are sorted according to the order in which the enumeration members were listed in the column specification. (In other words, `ENUM' values are sorted according to their index numbers.) For example, `"a"' sorts before `"b"' for `ENUM("a", "b")', but `"b"' sorts before `"a"' for `ENUM("b", "a")'. The empty string sorts before non-empty strings, and `NULL' values sort before all other enumeration values. If you want to get all possible values for an `ENUM' column, you should use: `SHOW COLUMNS FROM table_name LIKE enum_column_name' and parse the `ENUM' definition in the second column. The `SET' Type .............. A `SET' is a string object that can have zero or more values, each of which must be chosen from a list of allowed values specified when the table is created. `SET' column values that consist of multiple set members are specified with members separated by commas (`,'). A consequence of this is that `SET' member values cannot themselves contain commas. For example, a column specified as `SET("one", "two") NOT NULL' can have any of these values: "" "one" "two" "one,two" A `SET' can have a maximum of 64 different members. MySQL stores `SET' values numerically, with the low-order bit of the stored value corresponding to the first set member. If you retrieve a `SET' value in a numeric context, the value retrieved has bits set corresponding to the set members that make up the column value. For example, you can retrieve numeric values from a `SET' column like this: mysql> SELECT set_col+0 FROM tbl_name; If a number is stored into a `SET' column, the bits that are set in the binary representation of the number determine the set members in the column value. Suppose a column is specified as `SET("a","b","c","d")'. Then the members have the following bit values: `SET' *Decimal *Binary value* *member* value* `a' `1' `0001' `b' `2' `0010' `c' `4' `0100' `d' `8' `1000' If you assign a value of `9' to this column, that is `1001' in binary, so the first and fourth `SET' value members `"a"' and `"d"' are selected and the resulting value is `"a,d"'. For a value containing more than one `SET' element, it does not matter what order the elements are listed in when you insert the value. It also does not matter how many times a given element is listed in the value. When the value is retrieved later, each element in the value will appear once, with elements listed according to the order in which they were specified at table creation time. For example, if a column is specified as `SET("a","b","c","d")', then `"a,d"', `"d,a"', and `"d,a,a,d,d"' will all appear as `"a,d"' when retrieved. `SET' values are sorted numerically. `NULL' values sort before non-`NULL' `SET' values. Normally, you perform a `SELECT' on a `SET' column using the `LIKE' operator or the `FIND_IN_SET()' function: mysql> SELECT * FROM tbl_name WHERE set_col LIKE '%value%'; mysql> SELECT * FROM tbl_name WHERE FIND_IN_SET('value',set_col)>0; But the following will also work: mysql> SELECT * FROM tbl_name WHERE set_col = 'val1,val2'; mysql> SELECT * FROM tbl_name WHERE set_col & 1; The first of these statements looks for an exact match. The second looks for values containing the first set member. If you want to get all possible values for a `SET' column, you should use: `SHOW COLUMNS FROM table_name LIKE set_column_name' and parse the `SET' definition in the second column. Choosing the Right Type for a Column ------------------------------------ For the most efficient use of storage, try to use the most precise type in all cases. For example, if an integer column will be used for values in the range between `1' and `99999', `MEDIUMINT UNSIGNED' is the best type. Accurate representation of monetary values is a common problem. In MySQL, you should use the `DECIMAL' type. This is stored as a string, so no loss of accuracy should occur. If accuracy is not too important, the `DOUBLE' type may also be good enough. For high precision, you can always convert to a fixed-point type stored in a `BIGINT'. This allows you to do all calculations with integers and convert results back to floating-point values only when necessary. Using Column Types from Other Database Engines ---------------------------------------------- To make it easier to use code written for SQL implementations from other vendors, MySQL maps column types as shown in the table below. These mappings make it easier to move table definitions from other database engines to MySQL: *Other vendor type* *MySQL type* `BINARY(NUM)' `CHAR(NUM) BINARY' `CHAR VARYING(NUM)' `VARCHAR(NUM)' `FLOAT4' `FLOAT' `FLOAT8' `DOUBLE' `INT1' `TINYINT' `INT2' `SMALLINT' `INT3' `MEDIUMINT' `INT4' `INT' `INT8' `BIGINT' `LONG VARBINARY' `MEDIUMBLOB' `LONG VARCHAR' `MEDIUMTEXT' `MIDDLEINT' `MEDIUMINT' `VARBINARY(NUM)' `VARCHAR(NUM) BINARY' Column type mapping occurs at table creation time. If you create a table with types used by other vendors and then issue a `DESCRIBE tbl_name' statement, MySQL reports the table structure using the equivalent MySQL types. Column Type Storage Requirements -------------------------------- The storage requirements for each of the column types supported by MySQL are listed below by category. Storage requirements for numeric types ...................................... *Column type* *Storage required* `TINYINT' 1 byte `SMALLINT' 2 bytes `MEDIUMINT' 3 bytes `INT' 4 bytes `INTEGER' 4 bytes `BIGINT' 8 bytes `FLOAT(X)' 4 if X <= 24 or 8 if 25 <= X <= 53 `FLOAT' 4 bytes `DOUBLE' 8 bytes `DOUBLE PRECISION' 8 bytes `REAL' 8 bytes `DECIMAL(M,D)' `M+2' bytes if D > 0, `M+1' bytes if D = 0 (`D'+2, if `M < D') `NUMERIC(M,D)' `M+2' bytes if D > 0, `M+1' bytes if D = 0 (`D'+2, if `M < D') Storage requirements for date and time types ............................................ *Column type* *Storage required* `DATE' 3 bytes `DATETIME' 8 bytes `TIMESTAMP' 4 bytes `TIME' 3 bytes `YEAR' 1 byte Storage requirements for string types ..................................... *Column type* *Storage required* `CHAR(M)' `M' bytes, `1 <= M <= 255' `VARCHAR(M)' `L'+1 bytes, where `L <= M' and `1 <= M <= 255' `TINYBLOB', `TINYTEXT' `L'+1 bytes, where `L' < 2^8 `BLOB', `TEXT' `L'+2 bytes, where `L' < 2^16 `MEDIUMBLOB', `MEDIUMTEXT' `L'+3 bytes, where `L' < 2^24 `LONGBLOB', `LONGTEXT' `L'+4 bytes, where `L' < 2^32 `ENUM('value1','value2',...)' 1 or 2 bytes, depending on the number of enumeration values (65535 values maximum) `SET('value1','value2',...)' 1, 2, 3, 4 or 8 bytes, depending on the number of set members (64 members maximum) `VARCHAR' and the `BLOB' and `TEXT' types are variable-length types, for which the storage requirements depend on the actual length of column values (represented by `L' in the preceding table), rather than on the type's maximum possible size. For example, a `VARCHAR(10)' column can hold a string with a maximum length of 10 characters. The actual storage required is the length of the string (`L'), plus 1 byte to record the length of the string. For the string `'abcd'', `L' is 4 and the storage requirement is 5 bytes. The `BLOB' and `TEXT' types require 1, 2, 3, or 4 bytes to record the length of the column value, depending on the maximum possible length of the type. *Note BLOB::. If a table includes any variable-length column types, the record format will also be variable-length. Note that when a table is created, MySQL may, under certain conditions, change a column from a variable-length type to a fixed-length type, or vice-versa. *Note Silent column changes::. The size of an `ENUM' object is determined by the number of different enumeration values. One byte is used for enumerations with up to 255 possible values. Two bytes are used for enumerations with up to 65535 values. *Note ENUM::. The size of a `SET' object is determined by the number of different set members. If the set size is `N', the object occupies `(N+7)/8' bytes, rounded up to 1, 2, 3, 4, or 8 bytes. A `SET' can have a maximum of 64 members. *Note SET::. Functions for Use in `SELECT' and `WHERE' Clauses ================================================= A `select_expression' or `where_definition' in a SQL statement can consist of any expression using the functions described below. An expression that contains `NULL' always produces a `NULL' value unless otherwise indicated in the documentation for the operators and functions involved in the expression. *NOTE:* There must be no whitespace between a function name and the parenthesis following it. This helps the MySQL parser distinguish between function calls and references to tables or columns that happen to have the same name as a function. Spaces around arguments are permitted, though. You can force MySQL to accept spaces after the function name by starting `mysqld' with `--ansi' or using the `CLIENT_IGNORE_SPACE' to `mysql_connect()', but in this case all function names will become reserved words. *Note ANSI mode::. For the sake of brevity, examples display the output from the `mysql' program in abbreviated form. So this: mysql> select MOD(29,9); 1 rows in set (0.00 sec) +-----------+ | mod(29,9) | +-----------+ | 2 | +-----------+ is displayed like this: mysql> select MOD(29,9); -> 2 Non-Type-Specific Operators and Functions ----------------------------------------- Parenthesis ........... ( ... ) Use parenthesis to force the order of evaluation in an expression. For example: mysql> select 1+2*3; -> 7 mysql> select (1+2)*3; -> 9 Comparison Operators .................... Comparison operations result in a value of `1' (TRUE), `0' (FALSE), or `NULL'. These functions work for both numbers and strings. Strings are automatically converted to numbers and numbers to strings as needed (as in Perl). MySQL performs comparisons using the following rules: * If one or both arguments are `NULL', the result of the comparison is `NULL', except for the `<=>' operator. * If both arguments in a comparison operation are strings, they are compared as strings. * If both arguments are integers, they are compared as integers. * Hexadecimal values are treated as binary strings if not compared to a number. * If one of the arguments is a `TIMESTAMP' or `DATETIME' column and the other argument is a constant, the constant is converted to a timestamp before the comparison is performed. This is done to be more ODBC-friendly. * In all other cases, the arguments are compared as floating-point (real) numbers. By default, string comparisons are done in case-independent fashion using the current character set (ISO-8859-1 Latin1 by default, which also works excellently for English). The examples below illustrate conversion of strings to numbers for comparison operations: mysql> SELECT 1 > '6x'; -> 0 mysql> SELECT 7 > '6x'; -> 1 mysql> SELECT 0 > 'x6'; -> 0 mysql> SELECT 0 = 'x6'; -> 1 `=' Equal: mysql> select 1 = 0; -> 0 mysql> select '0' = 0; -> 1 mysql> select '0.0' = 0; -> 1 mysql> select '0.01' = 0; -> 0 mysql> select '.01' = 0.01; -> 1 `<>' `!=' Not equal: mysql> select '.01' <> '0.01'; -> 1 mysql> select .01 <> '0.01'; -> 0 mysql> select 'zapp' <> 'zappp'; -> 1 `<=' Less than or equal: mysql> select 0.1 <= 2; -> 1 `<' Less than: mysql> select 2 < 2; -> 0 `>=' Greater than or equal: mysql> select 2 >= 2; -> 1 `>' Greater than: mysql> select 2 > 2; -> 0 `<=>' Null safe equal: mysql> select 1 <=> 1, NULL <=> NULL, 1 <=> NULL; -> 1 1 0 `IS NULL' `IS NOT NULL' Test whether or not a value is or is not `NULL': mysql> select 1 IS NULL, 0 IS NULL, NULL IS NULL; -> 0 0 1 mysql> select 1 IS NOT NULL, 0 IS NOT NULL, NULL IS NOT NULL; -> 1 1 0 `expr BETWEEN min AND max' If `expr' is greater than or equal to `min' and `expr' is less than or equal to `max', `BETWEEN' returns `1', otherwise it returns `0'. This is equivalent to the expression `(min <= expr AND expr <= max)' if all the arguments are of the same type. The first argument (`expr') determines how the comparison is performed as follows: * If `expr' is a `TIMESTAMP', `DATE', or `DATETIME' column, `MIN()' and `MAX()' are formatted to the same format if they are constants. * If `expr' is a case-insensitive string expression, a case-insensitive string comparison is done. * If `expr' is a case-sensitive string expression, a case-sensitive string comparison is done. * If `expr' is an integer expression, an integer comparison is done. * Otherwise, a floating-point (real) comparison is done. mysql> select 1 BETWEEN 2 AND 3; -> 0 mysql> select 'b' BETWEEN 'a' AND 'c'; -> 1 mysql> select 2 BETWEEN 2 AND '3'; -> 1 mysql> select 2 BETWEEN 2 AND 'x-3'; -> 0 `expr IN (value,...)' Returns `1' if `expr' is any of the values in the `IN' list, else returns `0'. If all values are constants, then all values are evaluated according to the type of `expr' and sorted. The search for the item is then done using a binary search. This means `IN' is very quick if the `IN' value list consists entirely of constants. If `expr' is a case-sensitive string expression, the string comparison is performed in case-sensitive fashion: mysql> select 2 IN (0,3,5,'wefwf'); -> 0 mysql> select 'wefwf' IN (0,3,5,'wefwf'); -> 1 `expr NOT IN (value,...)' Same as `NOT (expr IN (value,...))'. `ISNULL(expr)' If `expr' is `NULL', `ISNULL()' returns `1', otherwise it returns `0': mysql> select ISNULL(1+1); -> 0 mysql> select ISNULL(1/0); -> 1 Note that a comparison of `NULL' values using `=' will always be false! `COALESCE(list)' Returns first non-`NULL' element in list: mysql> select COALESCE(NULL,1); -> 1 mysql> select COALESCE(NULL,NULL,NULL); -> NULL `INTERVAL(N,N1,N2,N3,...)' Returns `0' if `N' < `N1', `1' if `N' < `N2' and so on. All arguments are treated as integers. It is required that `N1' < `N2' < `N3' < `...' < `Nn' for this function to work correctly. This is because a binary search is used (very fast): mysql> select INTERVAL(23, 1, 15, 17, 30, 44, 200); -> 3 mysql> select INTERVAL(10, 1, 10, 100, 1000); -> 2 mysql> select INTERVAL(22, 23, 30, 44, 200); -> 0 If you are comparing case sensitive string with any of the standard operators (`=', `<>'..., but not `LIKE') end space will be ignored. mysql> select "a" ="A "; -> 1 Logical Operators ................. All logical functions return `1' (TRUE), `0' (FALSE) or `NULL' (unknown, which is in most cases the same as FALSE): `NOT' `!' Logical NOT. Returns `1' if the argument is `0', otherwise returns `0'. Exception: `NOT NULL' returns `NULL': mysql> select NOT 1; -> 0 mysql> select NOT NULL; -> NULL mysql> select ! (1+1); -> 0 mysql> select ! 1+1; -> 1 The last example returns `1' because the expression evaluates the same way as `(!1)+1'. `OR' `||' Logical OR. Returns `1' if either argument is not `0' and not `NULL': mysql> select 1 || 0; -> 1 mysql> select 0 || 0; -> 0 mysql> select 1 || NULL; -> 1 `AND' `&&' Logical AND. Returns `0' if either argument is `0' or `NULL', otherwise returns `1': mysql> select 1 && NULL; -> 0 mysql> select 1 && 0; -> 0 Control Flow Functions ...................... `IFNULL(expr1,expr2)' If `expr1' is not `NULL', `IFNULL()' returns `expr1', else it returns `expr2'. `IFNULL()' returns a numeric or string value, depending on the context in which it is used: mysql> select IFNULL(1,0); -> 1 mysql> select IFNULL(NULL,10); -> 10 mysql> select IFNULL(1/0,10); -> 10 mysql> select IFNULL(1/0,'yes'); -> 'yes' `NULLIF(expr1,expr2)' If `expr1 = expr2' is true, return `NULL' else return `expr1'. This is the same as `CASE WHEN x = y THEN NULL ELSE x END': mysql> select NULLIF(1,1); -> NULL mysql> select NULLIF(1,2); -> 1 Note that `expr1' is evaluated twice in MySQL if the arguments are equal. `IF(expr1,expr2,expr3)' If `expr1' is TRUE (`expr1 <> 0' and `expr1 <> NULL') then `IF()' returns `expr2', else it returns `expr3'. `IF()' returns a numeric or string value, depending on the context in which it is used: mysql> select IF(1>2,2,3); -> 3 mysql> select IF(1<2,'yes','no'); -> 'yes' mysql> select IF(strcmp('test','test1'),'no','yes'); -> 'no' `expr1' is evaluated as an integer value, which means that if you are testing floating-point or string values, you should do so using a comparison operation: mysql> select IF(0.1,1,0); -> 0 mysql> select IF(0.1<>0,1,0); -> 1 In the first case above, `IF(0.1)' returns `0' because `0.1' is converted to an integer value, resulting in a test of `IF(0)'. This may not be what you expect. In the second case, the comparison tests the original floating-point value to see whether it is non-zero. The result of the comparison is used as an integer. The default return type of `IF()' (which may matter when it is stored into a temporary table) is calculated in MySQL Version 3.23 as follows: *Expression* *Return value* expr2 or expr3 returns string string expr2 or expr3 returns a floating-point floating-point value expr2 or expr3 returns an integer integer `CASE value WHEN [compare-value] THEN result [WHEN [compare-value] THEN result ...] [ELSE result] END' `CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END' The first version returns the `result' where `value=compare-value'. The second version returns the result for the first condition, which is true. If there was no matching result value, then the result after `ELSE' is returned. If there is no `ELSE' part then `NULL' is returned: mysql> SELECT CASE 1 WHEN 1 THEN "one" WHEN 2 THEN "two" ELSE "more" END; -> "one" mysql> SELECT CASE WHEN 1>0 THEN "true" ELSE "false" END; -> "true" mysql> SELECT CASE BINARY "B" when "a" then 1 when "b" then 2 END; -> NULL The type of the return value (`INTEGER', `DOUBLE' or `STRING') is the same as the type of the first returned value (the expression after the first `THEN'). String Functions ---------------- String-valued functions return `NULL' if the length of the result would be greater than the `max_allowed_packet' server parameter. *Note Server parameters::. For functions that operate on string positions, the first position is numbered 1. `ASCII(str)' Returns the ASCII code value of the leftmost character of the string `str'. Returns `0' if `str' is the empty string. Returns `NULL' if `str' is `NULL': mysql> select ASCII('2'); -> 50 mysql> select ASCII(2); -> 50 mysql> select ASCII('dx'); -> 100 See also the `ORD()' function. `ORD(str)' If the leftmost character of the string str is a multi-byte character, returns the code of multi-byte character by returning the ASCII code value of the character in the format of: `((first byte ASCII code)*256+(second byte ASCII code))[*256+third byte ASCII code...]'. If the leftmost character is not a multi-byte character, returns the same value as the like `ASCII()' function does: mysql> select ORD('2'); -> 50 `CONV(N,from_base,to_base)' Converts numbers between different number bases. Returns a string representation of the number `N', converted from base `from_base' to base `to_base'. Returns `NULL' if any argument is `NULL'. The argument `N' is interpreted as an integer, but may be specified as an integer or a string. The minimum base is `2' and the maximum base is `36'. If `to_base' is a negative number, `N' is regarded as a signed number. Otherwise, `N' is treated as unsigned. `CONV' works with 64-bit precision: mysql> select CONV("a",16,2); -> '1010' mysql> select CONV("6E",18,8); -> '172' mysql> select CONV(-17,10,-18); -> '-H' mysql> select CONV(10+"10"+'10'+0xa,10,10); -> '40' `BIN(N)' Returns a string representation of the binary value of `N', where `N' is a longlong (`BIGINT') number. This is equivalent to `CONV(N,10,2)'. Returns `NULL' if `N' is `NULL': mysql> select BIN(12); -> '1100' `OCT(N)' Returns a string representation of the octal value of `N', where `N' is a longlong number. This is equivalent to `CONV(N,10,8)'. Returns `NULL' if `N' is `NULL': mysql> select OCT(12); -> '14' `HEX(N)' Returns a string representation of the hexadecimal value of `N', where `N' is a longlong (`BIGINT') number. This is equivalent to `CONV(N,10,16)'. Returns `NULL' if `N' is `NULL': mysql> select HEX(255); -> 'FF' `CHAR(N,...)' `CHAR()' interprets the arguments as integers and returns a string consisting of the characters given by the ASCII code values of those integers. `NULL' values are skipped: mysql> select CHAR(77,121,83,81,'76'); -> 'MySQL' mysql> select CHAR(77,77.3,'77.3'); -> 'MMM' `CONCAT(str1,str2,...)' Returns the string that results from concatenating the arguments. Returns `NULL' if any argument is `NULL'. May have more than 2 arguments. A numeric argument is converted to the equivalent string form: mysql> select CONCAT('My', 'S', 'QL'); -> 'MySQL' mysql> select CONCAT('My', NULL, 'QL'); -> NULL mysql> select CONCAT(14.