RE: Primary Key seems to be harmful for performance

Gene,

Before playing with optimizer_index_cost_adj and optimizer_index_caching, try to collect system statistics using DBMS_STATS.GATHER_SYSTEM_STATS. Igor

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[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of genegurevich_at_discoverfinancial.com
Sent: Friday, March 10, 2006 6:04 PM
To: oracle-l
Subject: Re: Primary Key seems to be harmful for performance

Mladen:

Thanks for your help. I have posted the SQL and the explain plans a day or two ago. My optimizer parameteres are set to optimizer_index_cost_adj 100 and optimizer_index_caching to 0 ( I think these are default parms). I'll need to read up on tuning them. I have tried using ALL_ROWS hint but that did not change the performance. Can't throw out Cognos, I'm afraid :(

thank you

Gene Gurevich
Oracle Engineering  

             Mladen Gogala

             <gogala_at_sbcglobal

             .net>
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                                       Re: Primary Key seems to be
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             03/09/2006 06:40          for performance

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On 03/09/2006 09:12:05 AM, genegurevich_at_discoverfinancial.com wrote:
> I was refering to the fact that i had multiple tables in my query and
they
> all have PKs. I should have been clearer though and say that I have
> disabled a PK on one of the tables. I can't modify the SQL in any way
> because it is generated by a reporting tool (Cognos) it won't add
> hints or +0 etc.

Gene, execution plan would be helpful. What you wrote sounds like a full table scan, murdered by using index. That usually happens if parameters with the goal of forcing index use and emulating RBO are in effect (OPTIMIZER_INDEX_CACHING and cost adjust). What happens is that Oracle reads every index key and then reads the corresponding table block, performing, in effect, several times as many I/O requests as without using an index. Two other things you can do are to make sure that users that use Cognos have optimizer goal set to ALL_ROWS and you can fake statistics and make the PK index look bad to CBO (look at my home page, I describe the way of doing that there). Third option is to try throwing Cognos out and then getting something that always generates an optimal execution plan, like Business Objects or Crystal Reports (duck).

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Mladen Gogala
http://www.mgogala.com

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