With Oracle 11g and 12c, auto optimizer stats collection is enabled on the default instance configuration. This causes the Oracle database to gather optimizer statistics on all database objects automatically and maintain those statistics as an automated maintenance task. The automated maintenance tasks infrastructure (known as AutoTask) schedules tasks to run automatically in Oracle Scheduler windows known as maintenance windows.
Tip:
Disable the AutoTask for auto optimizer stats collection. Guidance on disabling this setting can be found in Oracle's documentation at https://docs.oracle.com/database/121/TGSQL/tgsql_stats.htm#TGSQL398.