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
>
--
http://www.freelists.org/webpage/oracle-l

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