From: Tom Lane <tgl@sss.pgh.pa.us>
To: Richard Huxton <dev@archonet.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: Expression transformation curiosity
Date: Sun, 17 Aug 2003 11:23:01 -0400
Message-ID: <11422.1061133781@sss.pgh.pa.us> (raw)
In-Reply-To: <200308171132.58073.dev@archonet.com>
References: <200308171132.58073.dev@archonet.com>
Richard Huxton <dev@archonet.com> writes:
> Boiling the problem down, I've looked at a clause of the form:
> a OR (b AND c)
> which PG converts to:
> (a OR b) AND (a OR c)
> Is it an artefact of displaying the EXPLAIN, or is it actually
> processed that way? You could see how testing "a" twice could be
> expensive in some situations.
It's actually done that way --- see the comments near the head of
src/backend/optimizer/prep/prepqual.c. There are some heuristics
to not do it if the expression expands "a lot", though.
regards, tom lane
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To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, dev@archonet.com
Subject: Re: Expression transformation curiosity
In-Reply-To: <11422.1061133781@sss.pgh.pa.us>
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