From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Wheeler <david@kineticode.com>
Cc: pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: IMMUTABLE?
Date: Tue, 16 May 2006 00:31:41 -0400
Message-ID: <5894.1147753901@sss.pgh.pa.us> (raw)
In-Reply-To: <3F0CF576-8C6F-433C-9942-BB36F4E1A194@kineticode.com>
References: <2D4C0FA5-DB75-45EA-91D2-7B4F6DD55083@kineticode.com>
<5211.1147749711@sss.pgh.pa.us>
<3F0CF576-8C6F-433C-9942-BB36F4E1A194@kineticode.com>
David Wheeler <david@kineticode.com> writes:
> But seriously, the documentation says (as if I need to tell you, but
> I was reading it again to make sure that I'm not insane):
>> IMMUTABLE indicates that the function always returns the same
>> result when given the same argument values; that is, it does not do
>> database lookups or otherwise use information not directly present
>> in its argument list. If this option is given, any call of the
>> function with all-constant arguments can be immediately replaced
>> with the function value.
Sure. As I read it, that's talking about a static transformation:
planner sees 2 + 2 (or if you prefer, int4pl(2,2)), planner runs the
function and replaces the expression with 4. Nothing there about
memoization.
It's true that the system *could* memoize (or in our more usual
parlance, cache function values) given the assumptions embodied in
IMMUTABLE. But we don't, and I don't see any statement in the docs
that promises that we do. For 99% of the functions that the planner
deals with, memoization would be seriously counterproductive because
the function evaluation cost is comparable to if not less than the
lookup cost in a memo table. (int4pl is a good case in point.)
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, david@kineticode.com
Subject: Re: IMMUTABLE?
In-Reply-To: <5894.1147753901@sss.pgh.pa.us>
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