Re: Fluctuating Cost with Range Scan

dobrien_at_amcad.com (Dan) wrote in message news:<bdc62384.0406121751.5bf79ee7_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

Cost won't necessarily be influenced by relocating tablespaces. The foremost factor in cost is

- selectivity
- selectivity
- selectivity

and adequate optimizer_cost_adj and optimizer_index_cache parameters set.
You probably don't have a histogram on the affected column. Analyze table compute statistics for all indexed columns is probably in order.
Did you try to use hints in your statement (forcing the index)? If so, what was the result?

Please also note : cost is a meaningless number. The only things that counts is the number of consistent gets.

Sybrand Bakker
Senior Oracle DBA Received on Mon Jun 14 2004 - 09:54:59 CEST