I found someone used the following procedure call to disable the automatic statistics gathering job to gather the user (not sys/system) tables by accident.
exec DBMS_STATS.SET_PARAM('AUTOSTATS_TARGET','ORACLE');
For the reason that we may need to disable this job, please refer to this article for details.
Briefly speaking, the automatic statistics gathering job may gather the table statistics at the wrong time which hence most likely result in the wrong SQL execution plans. Keep an eye on this.
