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Re: Cost Based Optimizer

From: Mladen Gogala <mgogala_at_allegientsystems.com>
Date: Thu, 19 May 2005 16:22:55 -0400
Message-ID: <428CF59F.6050301@allegientsystems.com>


Terry Barnett wrote:

>I am having issues with execution plans changing even though the
>statistics on tables are kept static. Can anyone give me any information
>on the dynamic factors which are taken into account by the CBO when
>determining an execution plan and how one could detect that a plan may
>be about to change.
>
>Regards,
>Terry
>
>
>

Terry, if EVERYTHING is the same, then execution plans will not change. If you, on the other hand, are running 9.2 database, have monitoring on for the tables involved, and have OTPIMIZER_DYNAMIC_SAMPLING>0 (default is 1) your optimizer will be smart enough to detect that your statisics is stale (based on V$SEGMENT_STATISTICS) and gather new statistics, based on 32 blocks (if ODS=1).

--
Mladen Gogala
Oracle DBA
Ext. 121


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Received on Thu May 19 2005 - 16:25:52 CDT

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