Database keys (dbkeys) are internal pointers to specific table
    rows in a database. Application programs can use the DBKEY or
    ROWID keyword in SQL statements to refer to the database key for
    a table row. The ROWID keyword is a synonym to the DBKEY keyword.

    Database keys are considered value expressions. As such, they can
    be specified as part of a select expression.

    SQL statements that retrieve rows by specifying their database
    keys have the following advantages:

    o  Fast access: Retrieval through database keys is direct and
       bypasses any indexed or sequential searches.

    o  Reduced locking of data: Because access is direct, the
       database system locks only the row retrieved or updated.

    o  Uniqueness: Within the database key scope specified in the
       CREATE DATABASE or DECLARE ALIAS statements, database keys
       are guaranteed to be unique. This means singleton SELECT
       statements based on database keys will never return more than
       a single row, and that they will return the same row, or an
       error if the row was deleted.

    The scope of a database key refers to how long the database
    system guarantees that a particular row's database key will
    point only to that row and not be used again even if the row
    is deleted. In ATTACH, CREATE DATABASE, DECLARE ALIAS, and IMPORT
    statements, you can specify that the database key scope be for
    the duration of a transaction (the default) or for the duration
    of an attachment to the database.

    Applications that plan to use database keys across transaction
    boundaries should declare databases with the DBKEY SCOPE IS
    ATTACH clause, which allows the program to use a database key for
    a particular table row over the course of many transactions. If
    another user deletes the row, the database key will not be used
    again for a newly inserted row, ensuring that the database key is
    still valid. Any further reference to that DBKEY wukk generate an
    error.

    When you use the DBKEY, some space on the page is not reclaimed
    until all users are using DBKEY SCOPE IS TRANSACTION and the page
    is updated. Also, see the RMU RECLAIM command which allows online
    reclaiming of this space.

                                   NOTE

       Oracle Rdb recommends using DBKEY SCOPE IS TRANSACTION to
       reclaim space on a database page faster than if you use
       DBKEY SCOPE IS ATTACH.