Craig Ringer <craig@xxxxxxxxxxxxxxx> 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 -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
