From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Christoph Berg <christoph.berg@credativ.de>
Cc: PostgreSQL Performance <pgsql-performance@postgresql.org>
Subject: Re: RT3.4 query needed a lot more tuning with 9.2 than it did with 8.1
Date: Mon, 13 May 2013 16:33:19 -0400
Message-ID: <11894.1368477199@sss.pgh.pa.us> (raw)
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Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, May 13, 2013 at 4:14 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> You know, of course, that the join size estimate isn't arrived at that
>> way. Still, this point does make it seem more like a planner bug and
>> less like bad input stats. It would be nice to see a self-contained
>> example ...
> Yeah, I remember there have been examples like this that have come up
> before. Unfortunately, I haven't fully grokked what's actually going
> on here that allows this kind of thing to happen. Refresh my memory
> on where the relevant code is?
The point is that we estimate the size of a joinrel independently of
any particular input paths for it, and indeed before we've built any
such paths. So this seems like a bug somewhere in selectivity
estimation, but I'm not prepared to speculate as to just where.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, robertmhaas@gmail.com, christoph.berg@credativ.de
Subject: Re: RT3.4 query needed a lot more tuning with 9.2 than it did with 8.1
In-Reply-To: <11894.1368477199@sss.pgh.pa.us>
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