Re: Primary Key seems to be harmful for performance
- From: Mladen Gogala <gogala@xxxxxxxxxxxxx>
- To: oracle-l <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 09 Mar 2006 19:40:52 -0500
On 03/09/2006 09:12:05 AM, genegurevich@xxxxxxxxxxxxxxxxxxxxx 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).
--
Mladen Gogala
http://www.mgogala.com
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