RE: Skip scans

 
the danger with index skip scanning is that (afaik) some heuristics are
involved. 
it is very complicated for the CBO to base decisions purely on statistics;
in other words, the decisions are also based upon some "reasonable"
guesswork...
additions/corrections welcome,
 
Lex.
 
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-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Barbara Baker
Sent: Wednesday, December 29, 2004 21:27
To: mgogala@xxxxxxxxxxxxxxxxxxxx
Cc: Oracle-L (E-mail)
Subject: Re: Skip scans

Hmmm
I wasn't really aware that you could turn them off. Thought you "got them"
starting with 9i.

I had a 9.2.0.4 database that had optimizer_features set to 8.0.4 When I
removed the optimizer_features param, I had a query go from 5 minutes to 6
hours.  It started using a skip scan.  The problem was that the developer
went view-crazy.  The query went something like "select * from tab1 minus
select * from tab2".  But tab1 and tab2 were really views, and one of those
contained yet another view.  Under 8.0 everything was date-driven, which
worked.  But in 9i Oracle decided to outsmart itself and use another index.
Oracle could not see the "big picture" because rather than a nice query that
it could optimize, everything was hidden in the views.

Only time I've seen a problem, but it was a biggie.  Others have raved about
skip scans.  I just always seem to get the flaky queries . . .

I'd say go for it, but watch for the sucker-punch.

Barb


On Wed, 29 Dec 2004 13:47:12 -0500, Mladen Gogala
<mgogala@xxxxxxxxxxxxxxxxxxxx> wrote:
> Is anybody using index skip scan feature? What would be a situation in 
> which the feature should be used? I have a SQL statement, part of  ETL 
> process that I have to tune. The initial reuslts are encouraging 
> although not dramatic.  Does anybody else have any type of practical 
> experience with skip scans and index_ss hint?
> 
> --
> Mladen Gogala
> Oracle DBA
> Ext. 121
> 
> --
> http://www.freelists.org/webpage/oracle-l
>
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