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Re: improper estimates even with high statistic values

From: Robert Treat <xzilla(at)users(dot)sourceforge(dot)net>
To: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
Cc: pgsql-bugs(at)postgresql(dot)org
Subject: Re: improper estimates even with high statistic values
Date: 2006-01-27 15:35:35
Message-ID: 200601271035.35473.xzilla@users.sourceforge.net (view raw or flat)
Thread:
Lists: pgsql-bugs
To my knowledge it hasn't, at least no one has asked me for the sample 
database. 

Robert Treat

On Thursday 26 January 2006 17:42, Bruce Momjian wrote:
> Has this been researched.  Josh posted he thought it was an optimizer
> bug, but I haven't seen anyone investigate it:
>
>  http://archives.postgresql.org/pgsql-performance/2006-01/msg00248.php
>
>  http://archives.postgresql.org/pgsql-performance/2006-01/msg00265.php
>
> ---------------------------------------------------------------------------
>
> Robert Treat wrote:
> > Magnus's case seemed like a beast of a different animal to me, given it
> > was a direct index scan using a wildcard based search on a primary key
> > column; I'd agree I don't know exactly how it would determine a value
> > different that 1.
> >
> > But in my example, this misestimation comes between columns that are not
> > primary keys, contain duplicate values (so they are aren't unique), and
> > involves left joining subqueries. It doesn't seem to follow that it
> > would always reduce to 1 row quite so easily.
> >
> >
> > Robert Treat
> >
> > On Wed, 2006-01-18 at 16:11, Bruce Momjian wrote:
> > > Magnus reported a similar problem with path names.  I looked at his
> > > statistics and found that even at 100 buckets, his LIKE 'f:/.../%"
> > > query would never span more than one bucket, and because all the path
> > > names were unique, there were no most common values.
> > >
> > > In the case where the LIKE hits only one bucket, and there are no most
> > > common values, how is the optimzier supposed to estimate the number of
> > > rows, especially for cases where the values in the buckets are unevenly
> > > distributed.
> > >
> > > -----------------------------------------------------------------------
> > >----
> > >
> > > Robert Treat wrote:
> > > > After some extensive discussion on irc, berkus, myself and a few
> > > > others think we have uncovered a possible bug, or at the least some
> > > > odd behavior in > 8.1.1. It centers around my recent post to
> > > > performance
> > > > http://archives.postgresql.org/pgsql-performance/2006-01/msg00248.php
> > > > and how I could not seem to get some of the estimates to become
> > > > reasonable even after bumping up my stats target to 400 which caused
> > > > every row to be analyzed. If you look at the left join and hash join
> > > > estimates of the third query you'll note they seem to always get
> > > > estimated to 1 for no reason that we could come up with.
> > > >
> > > > Someone else on irc seemed to have a similar problem to this, so we
> > > > are wondering if there is some problem here. So the question really
> > > > is if someone can deduce the behavior from looking at what was
> > > > provided in the email? If not and you have questions let me know,
> > > > otherwise I can send a chopped up test database which can reproduce
> > > > the query issues off list should someone want to walk through the pg
> > > > code to investigate. TIA
> > > >
> > > >
> > > > Robert Treat
> > > > --
> > > > Build A Brighter Lamp :: Linux Apache {middleware} PostgreSQL
> > > >
> > > >
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> > > --
> > >   Bruce Momjian                        |  http://candle.pha.pa.us
> > >   pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 359-1001
> > >   +  If your life is a hard drive,     |  13 Roberts Road
> > >   +  Christ can be your backup.        |  Newtown Square, Pennsylvania
> > > 19073
> > >
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> >
> > --
> > Build A Brighter Lamp :: Linux Apache {middleware} PostgreSQL

-- 
Robert Treat
Build A Brighter Lamp :: Linux Apache {middleware} PostgreSQL

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