From: Tom Lane <tgl@sss.pgh.pa.us>
To: Gavin Wahl <gavinwahl@gmail.com>
cc: pgsql-performance@postgresql.org
Subject: Re: Planner Conceptual Error when Joining a Subquery -- Outer Query
 Condition not Pulled Into Subquery
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Comments: In-reply-to Gavin Wahl <gavinwahl@gmail.com>
 message dated "Wed, 16 Oct 2013 00:33:56 -0600"
Date: Wed, 16 Oct 2013 10:28:44 +0200
Message-ID: <7176.1381912124@sss.pgh.pa.us>
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Sender: pgsql-performance-owner@postgresql.org

Gavin Wahl <gavinwahl@gmail.com> writes:
> It'd be nice if the planner could optimize the query (1) by turning it
> into (2). I understand that it might not be able to, but if it can pull
> the condition up in (3), why can't it in (4)?

(3) is an instance of seeing "a = b and b = c" and deducing "a = c" from
that.  (4) does not follow that pattern, so it's outside the realm of
what the planner can deduce.

In principle we could take "a = b and (b = c or b = d)" and deduce
"a = c or a = d" from that, but it'd be a lot more complication for a
lot less benefit than what we get from the existing logic.

			regards, tom lane


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