The actual cardinality values for tables and indexes can be
    different from the stored cardinality values in your database's
    RDB$SYSTEM storage area if RDB$SYSTEM has been set to read-
    only access. When rows are added to or deleted from tables and
    indexes after the RDB$SYSTEM storage area has been set to read-
    only access, the cardinality values for these tables and indexes
    are not updated.

    For indexes, the cardinality value is the number of unique
    entries for an index that allows duplicates. If the index is
    unique, Oracle Rdb stores zero for the cardinality, and uses the
    table cardinality instead. For tables, the cardinality value is
    the number of rows in the table. Oracle Rdb uses the cardinality
    values of indexes and tables to influence decisions made by the
    optimizer. If the actual cardinality values of tables and indexes
    are different from the stored cardinality values, the optimizer's
    performance can be adversely affected.

    When you use the SQL ALTER DATABASE statement to set the
    RDB$SYSTEM storage area to read-only access for your database,
    the Oracle Rdb system tables in the RDB$SYSTEM storage area are
    also set to read-only access. When the Oracle Rdb system tables
    are set to read-only access:

    o  Automatic updates to table and index cardinality are disabled.

    o  Manual changes made to the cardinalities to influence the
       optimizer are not allowed.

    o  The I/O associated with the cardinality update is eliminated.

    With the RMU Analyze Cardinality command, you can:

    o  Display the stored and actual cardinality values for the
       specified tables and indexes.

    o  Update the stored cardinality value for a specified table
       or index with either the actual value or an alternative
       value of your own choosing. Oracle Corporation recommends
       that you update the stored cardinality value with the actual
       cardinality value. Specifying a value other than the actual
       cardinality value can result in poor database performance.