When you enable and collect workload statistics, the system
    table RDB$WORKLOAD is created and populated. (See Collect_
    Optimizer_Statistics for details.) You can update or delete these
    statistics using the RMU Collect Optimizer_Statistics command or
    the RMU Delete Optimizer_Statistics command, respectively.

    You might delete entries in the RDB$WORKLOAD table by accident
    or you might delete them to test how effective it is to maintain
    those particular workload statistics. If you decide that you
    want to maintain those deleted statistics, you can insert them
    with the RMU Insert Optimizer_Statistics command. To ensure that
    you insert accurate values, always issue an RMU Show Optimizer_
    Statistics command with the Log qualifier before you issue an
    RMU Delete Optimizer_Statistics command. Refer to your generated
    log file for the values you should specify with the RMU Insert
    Optimizer_Statistics command.

    In addition you can use the RMU Insert Optimizer_Statistics
    command to create workload statistics in a copy of your master
    database.

    If you issue an RMU Collect Optimizer_Statistics command after
    having issued an RMU Insert Optimizer_Statistics command,
    statistics for the specified column groups are updated.