Date: Fri, 13 Jan 2006 18:06:40 -0600
From: "Jim C. Nasby" <jnasby@pervasive.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Mark Liberman <mliberman@mixedsignals.com>,
	pgsql-performance@postgresql.org
Subject: Re: Stable function being evaluated more than once in a single query
Message-ID: <20060114000640.GN9017@pervasive.com>
References: <200601111641.20627.mliberman@mixedsignals.com>
	<935.1137040403@sss.pgh.pa.us>
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On Wed, Jan 11, 2006 at 11:33:23PM -0500, Tom Lane wrote:
> Mark Liberman <mliberman@mixedsignals.com> writes:
> > I've got a set-returning function,  defined as STABLE, that I reference twice
> > within a single query, yet appears to be evaluated via two seperate function 
> > scans.
> 
> There is no guarantee, express or implied, that this won't be the case.
> 
> (Seems like we just discussed this a couple days ago...)

Well, from 32.6:

"This category allows the optimizer to optimize away multiple calls of
the function within a single query."

That could certainly be read as indicating that if the function is used
twice in one query it could be optimized to one call.

Is the issue that the optimizer won't combine two function calls (ie:
SELECT foo(..) ... WHERE foo(..)), or is it that sometimes it won't make
the optimization (maybe depending on the query plan, for example)?
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461