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
Date: Wed, 14 Aug 2013 00:17:40 -0400
Message-ID: <22812.1376453860@sss.pgh.pa.us> (raw)
In-Reply-To: <520AD240.9060508@2ndquadrant.com>
References: <520AD240.9060508@2ndquadrant.com>
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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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Subject: Re: Interesting case of IMMUTABLE significantly hurting performance
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