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
Date: Thu, 29 Oct 2015 14:24:07 -0400
Message-ID: <2141.1446143047@sss.pgh.pa.us> (raw)
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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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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, matthewb@labkey.com
Subject: Re: Query optimizer plans with very small selectivity estimates
In-Reply-To: <2141.1446143047@sss.pgh.pa.us>
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