Re: System Statistics and the CBO
- From: Patty.Charlebois@xxxxxxxxxxxxxx
- To: gogala@xxxxxxxxxxxxx
- Date: Fri, 17 Jun 2005 9:51:28 -0400
I gathered system statistics over during a typical day and stored them in a
table as I do not want them 'turned on' until I have done some testing and can
ensure that they will not negatively impact our production OLTP system.
I then imported the system statistics that I gathered from production into our
test system and compared plans before and after the system stats were imported.
The production and test server are the exact same hardware and database.
Where would I find the default values for our system?
Mladen Gogala <gogala@xxxxxxxxxxxxx> Sent by: oracle-l-bounce@xxxxxxxxxxxxx
2005/06/17 08:52 AM
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gogala@xxxxxxxxxxxxx
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Re: System Statistics and the CBO
On 06/17/2005 08:29:56 AM, Patty.Charlebois@xxxxxxxxxxxxxx wrote:
> I am just now getting around to implementing system statistics in my 9.2.0.6
> database that runs on an IBM P650 under AIX 5L. > > I have found a handful of
> explain plan differences with and without system statistics and am trying to
> understand why the plan changes when I import the system statistics. Can
> someone explain to my why, once I import my system statistics, the CBO
> chooses a index that has a higher cardinality (X01CLAIM_FORM instead of
> X04CLAIM_FORM) for the following query:
The difference in the plans is choice of single index:
INDEX (RANGE SCAN) OF 'X04CLAIM_FORM' (NON-UNIQUE) (Cost=2 Card=1)
vs.
INDEX (RANGE SCAN) OF 'X01CLAIM_FORM' (NON-UNIQUE) (Cost=2 Card=153)
To have any idea why is that, I should know the default values for your system,
which I don't.
Because of larger cardinality, I somehow think that the latter has larger MBRC,
which makes
multiblock reads cheap enough rot the CBO to choose the latter plan. Have you
ever tried gathering system statistics? Importing it doesn't seem like a very
good idea to me, because system stats
are supposed to calibrate your system. If you import system stats, you are
using a calibration
from another system.
-- Mladen Gogala
Oracle DBA
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