Re: *****SPAM***** Re: I/O and db_file_multiblock_read_count

At 01:44 AM 12/9/2006, Stefan Knecht wrote:
>The thing is, when you collect system statistics (aka make use of
>cpu costing) - Oracle gathers stats about your systems' average
>multiblock read count. It then uses that information instead of
>whatever you might have set in your init.ora.
>
>Therefore, his test may be wrong - IF he's got sysstats.

That is not correct. Kevin's test is independent of whether he has system statistics or not. System statistics come into play only during the CBO's parse phase. If system statistics are in place then the CBO during parsing will use the MBRC value of the system statistics to calculate the cost of full scans, but Oracle's execution engine will use the db_file_multiblock_read_count value when doing full scans. That is one of the beauties of system statistics - you can use the OS' full IO bandwidth without messing up the CBO's cost calculation for full scans.

Regards

Wolfgang Breitling
Centrex Consulting Corporation
www.centrexcc.com

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