2013/8/14 Craig Ringer <craig@xxxxxxxxxxxxxxx>
Hi folks
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;
With IMMUTABLE: 33060.918
With STABLE: 6063.498
The plans are the same for both, though the cost estimate for the
IMMUTABLE variant is (surprisingly) massively higher.
The question contains detailed instructions to reproduce the issue, and
I can confirm the same results on my machine.
It looks like the difference is created by to_timestamp , in that if
to_timestamp is replaced with interval maths the difference goes away.
I'm very curious and am doing a quick profile now, but I wanted to raise
this on the list for comment/opinions, since it's very
counter-intuitive. IIRC docs don't suggest that IMMUTABLE can ever be
more expensive.
If I understand, a used IMMUTABLE flag disables inlining. What you see, is SQL eval overflow.
My rule is - don't use flags in SQL functions, when it is possible.
Pavel
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Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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