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Don Burleson Blog 







Oracle autostats_target Tips

Oracle Database Tips by Donald BurlesonUpdated January 8, 2015

Question: I want to change my automatic optimizer statistics collection.  I execute this command to see the automatic statistics collection mode:

select dbms_stats.get_param ('AUTOSTATS_TARGET') from dual;  

It gives value as ORACLE but I want to change it AUTO.  

Answer:  By default, Oracle collects statistics for the SQL optimizer via the default of autostats_target = auto.  Sadly, this is confusing because this command appears to be similar in function to this command to disable statistics collection:

SQL> exec dbms_scheduler.disable('SYS.GATHER_STATS_JOB');

It astonishes me how many shops prohibit any un-approved production changes and yet re-analyze schema stats weekly. Evidently, they do not understand that the purpose of schema re-analysis is to change their production SQL execution plans, and they act surprised when performance changes!

See these important notes on Oracle automatic statistics refreshing with dbms_stats

Me, I don't like surprises, and when we automatically re-analyze schema statistics, the optimizer can force thousands of SQL statements to change execution plans!  If we are satisfied that our SQL workload is already optimal, we can set autostats_target to "oracle" to turn off automatic statistics collection in 10g and beyond. 

exec dbms_stats.set_param('autostats_target','oracle');

This is a good move if you already have optimal CBO statistics and don't want to risk changing your SQL execution plans.

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