From: Tom Lane <tgl@sss.pgh.pa.us>
To: Matthew Bellew <matthewb@labkey.com>
cc: pgsql-performance@postgresql.org
Subject: Re: Query optimizer plans with very small selectivity estimates
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Comments: In-reply-to Matthew Bellew <matthewb@labkey.com>
 message dated "Thu, 29 Oct 2015 09:52:13 -0700"
Date: Thu, 29 Oct 2015 14:24:07 -0400
Message-ID: <2141.1446143047@sss.pgh.pa.us>
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Matthew Bellew <matthewb@labkey.com> writes:
> I made have several users encounter performance problems, which all
> seem to come down to this problem: multiplying selectivity estimates can
> cause tuple estimates to grow very small very quickly, once the estimator
> gets to 1 row, the planner may choose plans that are very good ONLY WHEN
> there is exactly 1 row (maybe even O(N^large)).  Unfortunately, these may
> be the worst plans if the estimate is even slightly off (even just
> returning 2 or 3 rows versus 1).

Yeah, this is a well-known problem.  There has been prior discussion along
the same lines as you mention (only believe 1-row estimates when it's
provably true that there's at most one row), but it hasn't looked like an
easy change.  See the pgsql-hackers archives for previous threads.

			regards, tom lane


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