Matteo Beccati <php@beccati.com> writes:
> This also made me think: is there a reason why geometric selectivity
> functions return constant values rather than checking statistics for a
> better result?
Because no one's ever bothered to work on them. You should talk to
the PostGIS guys, however, because they *have* been working on real
statistics and real estimation functions for geometric types. It'd
be nice to get some of that work back-ported into the core database.
http://www.postgis.org/
regards, tom lane