On Tue, 2003-05-06 at 00:45, Tom Lane wrote:
> 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.
>
given that we have so few GUC variables...
would there be any merit in adding one that would allow folks to change
this assumption?
Robert Treat