Re: Hypothetical suggestions for planner, indexing improvement

Josh Berkus <josh(at)agliodbs(dot)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