Re: Range Scan Cost Fluctuations

dobrien_at_amcad.com (Dan) wrote in message news:<bdc62384.0406121559.2126beb1_at_posting.google.com>...
> I am a relatively new user on Oracle 9.2.0.1 and I am having trouble
> performance tuning this production database.
>
> I am running a large query that joins two tables, document(3 mil) and
> entity(9 mil). I have reorganized my tablespaces so that the two
> tables are on different tablespaces, different disks. They both have
> their indexes stored on a third tablespace. Before reconfiguring the
> production database, I was getting a range scan on index ix_document_8
> that had a cost of 25. Now that I have reconfigured the prod
> database, the cost of the range scan has gone through the roof, 5933.
>
> I have tried moving this one particular index to different tablespaces
> on all three disks available to me to no avail. I run "analyze index
> compute statistics;" on the index after every time I recreate it. It
> appears that no matter where I create this index it still has trouble
> accessing it. I have computed the statistics for the table before
> computing the index statistics. It appears to me that there is an I/O
> problem here. Why did it cost so little before and now it costs so
> much to access it?
>
> Should the index be on the same disk/different tablespace?
> Different disk/different tablespace?
>
> I have run out of my testing capabilities here to try and troubleshoot
> why the cost is so high. Could this be a problem with my CBO?
>
> Any help is greatly appreciated,
> -Dan

Dan, if the explan plan show the same access path for the query after you moved the indexes and reanalyzed the table/indexes then the odds are that the statistics were out of data prior to your re-calculating them. In other words you are looking at the true cost. Is there a difference in run time, or just in the cost shown by the explain plans?

Also, are or did you use constants in one plan and bind variables in another? There can be major differences in the plan chosen and cost based on the difference in the CBO generates because of the difference in information the two options provide the optimizer.

HTH -- Mark D Powell -- Received on Mon Jun 14 2004 - 17:13:40 CEST