RE: dba_tables.num_rows is less than dba_indexes.num_rows
- To: "Christian Antognini" <Christian.Antognini@xxxxxxxxxxxx>
- Date: Thu, 11 Aug 2005 18:23:27 +1000
Ok, thanks again.
If only there was a simple answer!
-----Original Message-----
From: Christian Antognini [mailto:Christian.Antognini@xxxxxxxxxxxx]
Sent: Thursday, 11 August 2005 6:21 PM
To: Leng Kaing
Cc: oracle-l@xxxxxxxxxxxxx; Wolfgang Breitling
Subject: RE: dba_tables.num_rows is less than dba_indexes.num_rows
Leng
>Still don't understand part 1 :-( Are you saying that
>num_rows is important on a table, but not on an index?
num_rows on a table is definitively used by the CBO to estimate costs.
Neither Wolfgang nor I know a situation where the num_rows of an index
is used by the CBO. If you, or somebody else, know such a situation,
please, share it! (Of course we would like to see a reproducible test
case...)
>If I manually updated the table's num_rows to be
>bigger than then the indexes' num_rows, and saw
>that the CBO is now favouring an index look up
>rather a full table scan, surely it would prove that
>num_rows is important for both tables and indexes?
No. It only proves that table's num_rows is important.
>So short of doing a compute, how do we give the CBO
>good stats to work with?
As I wrote it depends... and if you need histograms in some situation
even a compute is not enough!
HTH,
Chris
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