[HACKERS] Teaching query_planner to handle multiple sort orders?

So in the grouping sets patch post, I said:
>> There is one current weakness which I don't see a good solution for:>> the planner code still has to pick a single
valuefor group_pathkeys>> before planning the input path.  This means that we sometimes can't>> choose a minimal set of
sorts,because we may have chosen a sort>> order for a grouping set that should have been hashed,
 

Of course there is one good solution, which is to have query_planner
take a set of acceptable output sort orders rather than just a single
one.

How wild an idea is this? I guess a possible hazard is an increase in
the number of possible mergejoin paths to consider. What other possible
issues are there?

-- 
Andrew (irc:RhodiumToad)