RE: Optimizer change from 8i-9i
- From: "Christian Antognini" <Christian.Antognini@xxxxxxxxxxxx>
- To: "zhu chao" <zhuchao@xxxxxxxxx>
- Date: Wed, 11 May 2005 15:56:37 +0200
Chao
>As Wolfgang said, I think although your autotrace shows RULE
>optimizer, oracle maybe actually using CBO.
>Can you enable SQL Trace and check whether it is actually=20
>parsed by RBO or CBO?
When the CBO is used statistics (Cost, Card and Bytes) are shown...
>By the way, team, anyone aware of some common situlation that
>autotrace can lie to us printing the execution plan and the optimizer
>used?
Yes. When there's an automatic switch from RBO to CBO.=20
E.g. when you want to use partitioned tables with the RBO:
SQL> alter session set optimizer_mode =3D rule;
SQL> select * from sales;
Execution Plan
----------------------------------------------------------
SELECT STATEMENT Optimizer=3DRULE (Cost=3D405 Card=3D929300 =
Bytes=3D26949700)
PARTITION RANGE (ALL) (Cost=3D405 Card=3D929300 Bytes=3D26949700)
TABLE ACCESS (FULL) OF 'SALES' (TABLE) (Cost=3D405 Card=3D929300 =
Bytes
>Can I ask one more question , about the
>"the index tie-break rule being the only reasonable exception, all
>the other possibilities are a bit far fetched IMO"
What do you mean???
HTH
Chris
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