DO understand the implications of gathering system statistics. Once you
have system statistics, the cbo adds a cpu cost element to the plan costs.
In my experience it is largely marginal, especially if you have fast cpus.
The bulk cost is still coming from the IO cost. But with system statistics
gethered, the cbo also starts distinguishing between single block IO
(sequential_reads) and multiblock IO (scattered_reads) costs. Pay attention
the the MBRC statistic and the ratio of mreadtim:sreadtim. Those two
together now determine the cost of scans (FTS and FFS) and that can have
quite an effect on the access path choice.
I am preparing a presentation for IOUG Live 2003 on CBO related changes in 9i.
At 07:03 AM 3/19/2003 -0800, you wrote:
>Hi everyone
>
>I was trying to migrate one of our applications to 9.2 and put in cost at
>the same time
>
>I came across Gather_system_stats and was wondering if anyone had used it?
>Did it improve the executions plan choice at all?
>
>Thanks
>David Hill
>DBA
Wolfgang Breitling
Centrex Consulting Corporation
http://www.centrexcc.com
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