Re: Hypothetical suggestions for planner, indexing improvement - Mailing list pgsql-hackers

Josh Berkus <josh@agliodbs.com> writes:
> The reason that I mention EXISTS is because that's where the lack of
> cross-column corellation is most dramatic; the planner seems to estimate a
> flat 50% for EXISTS clauses regardless of the content.

No "seems to" about that one: see src/backend/optimizer/path/clausesel.c

    else if (is_subplan(clause))
    {
        /*
         * Just for the moment! FIX ME! - vadim 02/04/98
         */
        s1 = (Selectivity) 0.5;
    }

Patches to improve this are welcome ;-).  But I'm not at all sure how to
write something that would extract a reliable selectivity estimate from
a subplan.

            regards, tom lane