Re: db_file_multiblock_read_count and performance
- From: Wolfgang Breitling <breitliw@xxxxxxxxxxxxx>
- To: "Post, Ethan" <Ethan.Post@xxxxxx>
- Date: Mon, 06 Dec 2004 14:08:55 -0700
Post, Ethan wrote:
> I think what he is trying to prove is that given two full table scans
> one should perform faster by adjusting the size of the multiblock reads.
> The right questions might be....
>
> 1) Is DBMRC informative in nature only (tells Oracle what the actual
> read count size is for OS)?
The db_file_multiblock_read count is an operational directive telling
Oracle how many blocks it should try to read with a single IO when it
does full scans. Unfortunately, prior to Oracle 9 and system statistics,
the CBO also used this value to determine the cost of a full scan.
When gathering system statistics from an actual workload, Oracle will
observe how many blocks it typically / on average actually reads with a
single IO during full scans. That number is then used by the CBO in full
scan costing. That frees dfmrc to be a purely opreational parameter
without the sideffect on access plan costing and choice.
>
> or
>
> 2) Does DBMRC actually influence the size of read count at IO level?
>
Yes. It sets the maximum # of blocks that Oracle is "allowed" to read
with a single IO. The actual # of blocks it reads with each multiblock
IO can be lower due to several factors.
--
Regards
Wolfgang Breitling
Centrex Consulting Corporation
www.centrexcc.com
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