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Re: Query plan degradation 8.2 --> 8.3

From: Gregory Stark <stark(at)enterprisedb(dot)com>
To: <josh(at)agliodbs(dot)com>
Cc: <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Query plan degradation 8.2 --> 8.3
Date: 2007-05-30 23:35:35
Message-ID: 87lkf5j35k.fsf@oxford.xeocode.com (view raw or flat)
Thread:
Lists: pgsql-hackers
"Josh Berkus" <josh(at)agliodbs(dot)com> writes:

> On Wednesday 30 May 2007 15:51, Josh Berkus wrote:
>> I now have a simple test case which shows significant performance
>> degradation on 8.3devel for a specific query, apparenly due to an
>> unnecessary call to Top-N sort.  I've tried to forward the test case to
>> the lists but the package is 3.5m, so I'm putting it on pgFoundry
>> instead:

How recently did you check out your 8.3 tree? 

When I run it I get a bitmap index scan which I think might mean you're
suffering from the same problem Tom found and fixed a few days ago. The
planner is finding the bitmap index scan with the sort is the best possible
plan but then discarding that option later leaving it with a suboptimal
choice.

The exact manifestation is somewhat different from what other people saw. iirc
they saw sequential scans when there was an index scan available. But I
suspect it's the same thing going on.

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com


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