Perf tuning OPTIMIZER_INDEX_COST

From: Jesse, Rich <Rich.Jesse_at_qtiworld.com>
Date: Thu, 09 Aug 2001 16:37:31 -0700
Message-ID: <F001.00365D24.20010809085531@fatcity.com>

After just migrating from 8.0.6 to 8.1.7 and RBO to CBO, I've been researching about perf tuning and have come across the init.ora parameter OPTIMIZER_INDEX_COST_ADJ. The guide, by Tim Gorman of evdbt.com, explained that it's default value is 100 (percent), but should probably be in the 10-50 range for OLTP systems. It recommended using the following query as a guideline to set this:

        SELECT event, average_wait
        FROM v$system_event
        WHERE event LIKE 'db file s%read';

...and to take the ratio of "db file sequential read" to "db file scattered read" as a value for OPTIMIZER_INDEX_COST_ADJ. That may be fine for most systems, but our ratio is around .08%. No, it's not 8% -- and it's not a decimal problem. I'm getting this:

        db file sequential read 1.83687542789106
        db file scattered read  2151.12743289383

I'm reasonably certain that the seemingly outrageous "2151.127" is due to our use of HP's AutoRAID (SLOWWWWWW!) -- we won't be getting one in our next box.

So, I'm a little skiddish about changing the default value from 100 to 1 or even 10, based on the above query alone. The scattered read average wait seems to be dropping steadily (it's now down to ~1900 over a period of 14 hours), so I'm trying to provide a "good" value for this parm without adversely affecting the optimizer.

Anyone have any ideas?

TIA!

Rich Jesse                          System/Database Administrator
Rich.Jesse_at_qtiworld.com             Quad/Tech International, Sussex, WI USA

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Author: Jesse, Rich
INET: Rich.Jesse_at_qtiworld.com

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Perf tuning OPTIMIZER_INDEX_COST_ADJ