Re: System statistics
- From: Christian Antognini <Christian.Antognini@xxxxxxxxxxxx>
- To: "Tony Hasler (tony@xxxxxxxxxxxxxxxxxxxx)" <tony@xxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 19 Aug 2013 06:15:15 +0000
Hi Tony
Even though you probably know what I think about it, here is a short feedback...
> Given our tolerance for cardinality errors why are we so concerned about
> accurately quantifying the size and performance of multi-block reads?
1) I see execution plans flipping for much less than a factor of 2 or 3.
2) You increase the likelihood to see features like cardinality feedback
kicking in.
> Why not just delete system statistics and set DB_FILE_MULTIBLOCK_READ_COUNT
> to 0 (implying the maximum possible I/O size) and be done with it?
1) Even though you delete them they will be automatically re-gathered. As of
10g it is simply not possible to run a database without system statistics.
2) The auto DB_FILE_MULTIBLOCK_READ_COUNT is not good for at least two reasons:
- Suboptimal performance is possible
- When MEMORY_TARGET or SGA_TARGET are in use the value can change at
every instance bounce. As a result, execution plans can change when you bounce
an instance.
Just my two cents...
HTH
Chris Antognini
Troubleshooting Oracle Performance, Apress 2008
http://top.antognini.ch
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