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.
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.
