Re: Better cardinality estimate when dialing optimizer_features_enable back
Date: Wed, 25 Nov 2009 08:27:22 -0800
Did the dbms_stats.gather_* command change there?
What may be happening is that the sample size chosen does not see as many of the values that only have a few occurrences. As a result the NDV is lower which in turn would raise the cardinality, which in turn would likely make the NLJ a HJ as seen from examples you provided. Using a 100% sample guarantees that all the distinct values are seen so the NDV would naturally be higher. This should be fairly easy to validate in a test environment by adjusting the dbms_stats.gather_* parameters and looking at the NDV, cardinality and plans for the queries
I also think it is important to recognize that the goal here is not to have absolutely every query have perfect cardinality estimates, but rather have stats that yield representative stats AND yield the desired execution plans. Do not become micro-focused solely on the cardinality estimates.
On Wed, Nov 25, 2009 at 7:29 AM, Neil Kodner <nkodner_at_gmail.com> wrote:
> Well look what I found while poking around sys.wri$_optstat_histhead_history
> http://bit.ly/75oNst (screenshot)
> Could this have been a function of sample size changing? Could parallel
> have anything to do with it?
-- Regards, Greg Rahn http://structureddata.org -- http://www.freelists.org/webpage/oracle-lReceived on Wed Nov 25 2009 - 10:27:22 CST