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Re: Query plan degradation 8.2 --> 8.3

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
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 23:28:18
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Josh Berkus <josh(at)agliodbs(dot)com> writes:
> 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.

It does the right thing if t_s_symb is declared as text instead of
varchar.  When it's varchar, even setting enable_sort off won't make
it pick the right plan, which suggests that it fails to recognize that
the index can match the query's ORDER BY.  I'm guessing I overlooked
a binary-compatibility case when I rejiggered the handling of PathKeys
in connection with the NULLS FIRST/LAST stuff.  No time to look deeper
right now.

			regards, tom lane

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Subject: Re: Query plan degradation 8.2 --> 8.3
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