From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200307211523.h6LFNa112572@candle.pha.pa.us>
Subject: Re: avoid select expens_expr(col) like unneccessary calculations
In-Reply-To: <20030707154329.J82767-100000@megazone23.bigpanda.com>
To: Stephan Szabo <sszabo@megazone.bigpanda.com>
Date: Mon, 21 Jul 2003 11:23:36 -0400 (EDT)
Cc: Markus Bertheau <twanger@bluetwanger.de>,
	pgsql-sql@postgresql.org
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Stephan Szabo wrote:
> On 8 Jul 2003, Markus Bertheau wrote:
> 
> > when you have
> > select expensive_expression(column), * from table offset 20 limit 40
> >
> > can you somehow save the cost for the first 20 calculations of
> > expensive_expression?
> 
> Right now the only way I can think of that might work is to push the
> offset/limit into a subselect on table and then do the
> expensive_expression at the top level.

Well, you can do:

	SELECT * FROM (SELECT * FROM pg_class) AS pg_class

so you could do:

	SELECT func(relname), * 
	FROM (SELECT * FROM pg_class OFFSET 20 LIMIT 40) AS pg_class

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