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
Date: Wed, 16 Oct 2013 10:28:44 +0200
Message-ID: <7176.1381912124@sss.pgh.pa.us> (raw)
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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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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, gavinwahl@gmail.com
Subject: Re: Planner Conceptual Error when Joining a Subquery -- Outer Query Condition not Pulled Into Subquery
In-Reply-To: <7176.1381912124@sss.pgh.pa.us>
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