Common Services

The following sections describe several services of the package dbms_stats that are used for both system statistics and object statistics.

Statistics History

As of Oracle Database 10g, whenever system statistics or object statistics are gathered through the package dbms_stats, instead of simply overwriting current statistics with the new statistics, the current statistics are saved in other data dictionary tables that keep a history of all changes occurring within a retention period. The purpose is to be able to restore old statistics in case that new statistics lead to inefficient execution plans.

Up to Oracle9i, you can also implement such a history by taking advantage of a backup table. Nevertheless, it is a good thing that the package dbms_stats automatically takes care of it.