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Don Burleson Blog 








Experts agree on adjusting Oracle tuning parameters

Anjo Kolk, creator of the oraperf tool, notes that changing important Oracle optimizer parameters can have a profound impact on database performance:

So the decision made to include the directio mount option again and at the same time change two init.ora paramters: optimzer_index_cost_adj and optimzer_index_caching. The database was bounced and a simple batch job was run to validate the performance improvement. That batch job ran before with 50-70 calculations per hour and after the changes it ran with 2500+ calculations per hour. There was a factor of 35-50 times improvement.

Of course, broad-brush approaches have a downside too, as Kolk noted when some SQL began to take-on sub-optimal plans:

So this was great news of course, however the next day one of the other databases on the system reported a severe performance problem. And in fact a quick check revealed that it was waiting most of the time in I/O related events. A further analysis showed a couple of things:

1)      The Oracle buffer cache was really small and so the File System buffer cache was heavily used to compensate for that small Oracle buffer cache.

2)      It also showed that a handful of statements were responsible for 80+ percent of the I/O workload.

3)      In fact it was discovered that one statement did around 600K physical I/Os in just over 2 hours.

4)      And there were a couple of other statements just like that.

In my latest book Oracle Tuning: the Definitive Reference, I show a script that can be used to compute the starting values for optimizer_index_cost_adj and optimizer_index_caching, based on historical data.  Here is the listing from this script in the Oracle Tuning Book:





                                         Percent of      Percent of     index

 Average Waits for Average Waits for      I/O Waits       I/O Waits      cost

Full Scan Read I/O    Index Read I/O for Full Scans for Index Scans       adj

------------------ ----------------- -------------- --------------- ---------

             1.473              .289            .02             .98        20


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