参考资料 http://docs.oracle.com/cd/E11882_01/server.112/e41573/stats.htm#PFGRF95153

You must regularly gather statistics on database objects as these database objects are modified over time. To determine whether a given database object needs new database statistics, Oracle Database provides a table monitoring facility. This monitoring is enabled by default when STATISTICS_LEVEL is set to TYPICAL or ALL.

Monitoring tracks the approximate number of INSERTs, UPDATEs, and DELETEs for that table and whether the table has been truncated since the last time statistics were gathered. You can access information about changes of tables in the USER_TAB_MODIFICATIONS view. Following a data-modification, there may be a few minutes delay while Oracle Database propagates the information to this view. Use the DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO procedure to immediately reflect the outstanding monitored information kept in the memory.

The GATHER_DATABASE_STATS or GATHER_SCHEMA_STATS procedures gather new statistics for tables with stale statistics when the OPTIONS parameter is set to GATHER STALE or GATHER AUTO. If a monitored table has been modified more than 10%, then these statistics are considered stale and gathered again.