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Re: Join selectivity is 0 causing bad cardinality estimates.

From: <andrew.markiewicz_at_gmail.com>
Date: 12 Dec 2006 14:30:49 -0800
Message-ID: <1165962649.719151.162500@16g2000cwy.googlegroups.com>


We also have histograms calculated using a similar method as you have, FOR ALL COLUMNS SIZE AUTO. On Dec 12, 4:16 pm, "EscVector" <J..._at_webthere.com> wrote:
> Jonathan,
> Would it make sense to histogram the other table as I suggested, even
> if there is little skew, possibly removing the halved cardinality
> estimate?
>
> Jonathan Lewis wrote:
> > <andrew.markiew..._at_gmail.com> wrote in message
> >news:1165960064.046563.211800_at_n67g2000cwd.googlegroups.com...
> > > Hello.
> > > I have a tuning issue which has brought me a situation that is
> > > confusing. I have reduced the issue down to a simple join which should
> > > return all rows in the larger table (7792 rows). But the CBO join
> > > cardinality estimate is 1 for this join. I created a 10053 trace and
> > > found the join cardinality calculation uses a selectivity of zero
> > > causing the CBO to estimate 1 row. What would cause this?
>
> > > There are no implicit conversion issues I can find. There are no
> > > histogram lookups since there are not even any filtering predicates on
> > > the sql....
>
> > You have a histogram on one end of the join.
>
> > Column: VEND_MODEL Col#: 3 Table: EQUIPMENT Alias: E
> > NDV: 172 NULLS: 0 DENS: 3.3694e-03
> > HEIGHT BALANCED HISTOGRAM: #BKT: 200 #VAL: 65
>
> > Wolfgang Breitling and Alberto Dell'Era were exchanging email
> > with me a little while ago about an issue where the optimizer would
> > halve the join cardinality estimate in circumstances similar to yours.
> > Possibly you've been caught by the same "anomaly".
>
> > Try dropping the histogram temporarily to see if the cardinality changes.
>
> > --
> > Regards
>
> > Jonathan Lewis
> >http://jonathanlewis.wordpress.com
>
> > Author: Cost Based Oracle: Fundamentals
> >http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html
>
> > The Co-operative Oracle Users' FAQ
> >http://www.jlcomp.demon.co.uk/faq/ind_faq.html
Received on Tue Dec 12 2006 - 16:30:49 CST

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