From: Tom Lane <tgl@sss.pgh.pa.us>
To: Craig Ringer <craig@2ndquadrant.com>
cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Interesting case of IMMUTABLE significantly hurting performance
In-reply-to: <520AD240.9060508@2ndquadrant.com>
References: <520AD240.9060508@2ndquadrant.com>
Comments: In-reply-to Craig Ringer <craig@2ndquadrant.com>
 message dated "Wed, 14 Aug 2013 08:41:36 +0800"
Date: Wed, 14 Aug 2013 00:17:40 -0400
Message-ID: <22812.1376453860@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

Craig Ringer <craig@2ndquadrant.com> writes:
> I've run into an interesting Stack Overflow post where the user shows
> that marking a particular function as IMMUTABLE significantly hurts the
> performance of a query.

> http://stackoverflow.com/q/18220761/398670

> CREATE OR REPLACE FUNCTION
>   to_datestamp_immutable(time_int double precision) RETURNS date AS $$
>   SELECT date_trunc('day', to_timestamp($1))::date;
> $$ LANGUAGE SQL IMMUTABLE;

[ shrug... ]  Using IMMUTABLE to lie about the mutability of a function
(in this case, date_trunc) is a bad idea.  It's likely to lead to wrong
answers, never mind performance issues.  In this particular case, I
imagine the performance problem comes from having suppressed the option
to inline the function body ... but you should be more worried about
whether you aren't getting flat-out bogus answers in other cases.

			regards, tom lane


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