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Re: Fwd: 8.2 Query 10 times slower than 8.1 (view-heavy)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Mark Lewis <mark(dot)lewis(at)mir3(dot)com>
Cc: Evan Carroll <lists(at)evancarroll(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Fwd: 8.2 Query 10 times slower than 8.1 (view-heavy)
Date: 2007-08-28 18:48:00
Message-ID: 1368.1188326880@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Mark Lewis <mark(dot)lewis(at)mir3(dot)com> writes:
> Unfortunately there has been a planner regression in 8.2 in some cases
> with these forms of queries.  This was discussed a few weeks (months?)
> ago on this forum.  I haven't looked closely enough to confirm that this
> is the problem in your case, but it seems likely.

Yeah, the EXPLAIN ANALYZE output clearly shows a drastic underestimate
of the number of rows out of a join like this, and a consequent choice
of a nestloop above it that performs terribly.

> We've been holding back from upgrading to 8.2 because this one is a
> show-stopper for us.

Well, you could always make your own version with this patch reverted:
http://archives.postgresql.org/pgsql-committers/2006-11/msg00066.php

I might end up doing that in the 8.2 branch if a better solution
seems too large to back-patch.

			regards, tom lane

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Subject: Performance problem with table containing a lot of text (blog)
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Subject: Re: 8.2.4 Chooses Bad Query Plan

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