query_planner() API change

I've been looking at what it would take to do proper cost estimation
for the recently-discussed patch to suppress calculation of unnecessary
ORDER BY expressions. It turns out that knowledge of that would have
to propagate into query_planner(), because the place where we do the cost
comparison between unsorted and presorted paths is in there (planmain.c
lines 390ff in HEAD). As it stands, query_planner() will actually refuse
to return the presorted path to grouping_planner() at all if it thinks
it's a loser cost-wise, meaning grouping_planner() would have no
opportunity to override the decision. So there's no way to fix this
without some API change for query_planner().

While we could complicate query_planner()'s API even more to add some
understanding of unnecessary resjunk items, I think this is probably
the straw that breaks the camel's back for the current approach here.
There is already a comment like this in query_planner():

* This introduces some undesirable coupling between this code and
* grouping_planner, but the alternatives seem even uglier; we couldn't
* pass back completed paths without making these decisions here.

I think it's time to bite the bullet and *not* pass back completed paths.
What's looking more attractive now is to just pass back the top-level
RelOptInfo ("final_rel" in query_planner()). We could remove all three
output parameters of query_planner(), and essentially just move lines
265-420 (pretty much everything after the make_one_rel() call) into
planner.c. Since that code is almost all about grouping-related choices,
this seems like it'll be a net improvement modularity-wise, even though
it'll make grouping_planner() even bigger. We could probably ameliorate
the latter problem by putting the calculation of num_groups and adjustment
of tuple_fraction into a subroutine.

Objections, better ideas?

regards, tom lane