Basic Terminology
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     The RDD architecture introduces several new terms and concepts that are
     key to the design and usage of RDDs.  You should familiarize yourself
     with these concepts and terms as you begin to use the RDD functionality.
     The meaning of some earlier terminology is also further defined.  The
     following RDD functional glossary defines the terminology for all RDDs.

     .  Key Expression:  A valid CA-Clipper expression that creates a
        key value from a single record.

     .  Key Value:  A value that is based on value(s) contained within
        database fields, associated with a particular record in a database.

     .  Identity:  A unique value guaranteed by the structure of the
        data file to reference a specific record in a database even if the
        record is empty.  In the Xbase file (.dbf), the identity is the
        record number; but it could be the value of a unique primary key or
        even the offset of an array in memory.

     .  Keyed-Pair:  A pair consisting of a key value and an identity.

     .  Identity Order:  Describes a database arranged by identity.
        In Xbase, this refers to the physical arrangement of the records in
        the database in the order in which they were entered (natural order).

     .  Tag:  A set of keyed-pairs that provides ordered access to the
        table based on a key value.  Usually, an Order in a multiple-Order
        index (Order).  An Order.

     .  Order:  A named mechanism (index) that provides logical access
        to a database according to the keyed-pairs.  This term encompasses
        both single indexes and the Tags in multiple-Tag indexes.

        Orders are not, themselves, data files.  They provide access to data
        that gives the appearance of an ordering of the data in a specific
        way.  This ordering is defined by the relationships between keyed-
        pairs.  An Order does not change the physical (the natural or entry)
        order of data in a database.

     .  Controlling Order:  The active Order (index) for a particular
        work area.  Only one Order may control a work area at any time, and
        it controls the order in which the database is accessed during paging
        and searching.

     .  Order List:  A list of all the Orders available to the
        database in the specified work area.

     .  Order Bag:  A container that holds zero or more Orders.
        Normally a disk or memory file.  A traditional index like (.ntx) is
        an Order Bag that holds only one Order.  A multiple-Tag index (.mdx
        or .cdx) is an Order Bag that holds zero or more Orders.  Though
        Order Bags may be a memory or disk file, CA-Clipper 5.2 only supports
        Order Bags as disk files.

     .  Record:  A record in the traditional database paradigm is a
        row of one or more related columns (fields) of data.  In the expanded
        architecture of CA-Clipper, a record could be data that does not
        exactly fit this definition.

        A record is, in this expanded context, data associated with a single
        identity.  In an Xbase data structure, this corresponds to a row
        (fields associated with a record number); in other data structures,
        this may not be the case.

        In this document we use "record" in the traditional sense, but you
        should be aware that CA-Clipper permits expansion of the meaning of
        record.

     .  single-Order Bag:  An Order Bag that can contain only one
        Order.  The (.ntx) and (.ndx) files are examples of single-Order
        Bags.

     .  multiple-Order Bag:  An Order Bag that can contain any number
        of Orders; a multiple-Tag index.  The (.cdx) and (.mdx) files are
        examples of multiple-Order Bags.

     .  maintainable scoped Orders:  Scoped (filtered) Orders created
        using the FOR clause.  The FOR condition is stored in the index
        header.  Orders of this type are correctly updated using the
        expression to reflect record updates, deletions and additions.

     .  non-maintainable/temporary Orders:  Orders created using the
        WHILE or NEXT clauses.  These Orders are useful because they can be
        created quickly.  However, the conditions in these clauses are not
        stored in the index header.  Therefore, Orders of this type are not
        correctly updated to reflect record updates, deletions and additions.
        They are only for temporary use.

     .  Lock List:  A list of the records that are currently locked in
        the work area.