RE: Index skip scan
- From: "Goulet, Dick" <DGoulet@xxxxxxxx>
- To: <breitliw@xxxxxxxxxxxxx>, <Amir.Hameed@xxxxxxxxx>
- Date: Tue, 20 Sep 2005 13:27:17 -0400
Provided your using CBO.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Wolfgang Breitling
Sent: Tuesday, September 20, 2005 1:19 PM
To: Amir.Hameed@xxxxxxxxx
Cc: oracle-l@xxxxxxxxxxxxx
Subject: Re: Index skip scan
The answer is similar to the one for the question "What is the % of rows
where
the optimizer switched from an index access to a full scan, or vice
versa".
There is not fixed threshold. The optimizer with cost the various
possible
access paths and choose the one the comes out with the lowest cost.
BTW, skip scan is not limited to just skipping one leading column. You
can get
an index skip-scan with predicates missing more than one leading column
of the
index.
Quoting "Hameed, Amir" <Amir.Hameed@xxxxxxxxx>:
> Folks,
> Does anyone know that in order for the optimizer to choose "Index skip
> scan" on a concatenated index where the first column is not very
> selective but the second one is, approximately what type of
selectivity
> on the leading column the Optimizer look for before deciding to take
> this route?
--
regards
Wolfgang Breitling
Oracle 7,8,8i,9i OCP DBA
Centrex Consulting Corporation
www.centrexcc.com
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