Re: WIP: Upper planner pathification

David Rowley <david.rowley@2ndquadrant.com> writes:
> My gripe is that I've added the required code to build the parallel
> group aggregate to create_agg_path() already, but since Group
> Aggregate uses the RollupPath I'm forced to add code in
> create_rollup_plan() which manually stacks up Plan nodes rather than
> just dealing with Paths and create_plan() and its recursive call
> magic.

Yeah, RollupPath was something I did to be expeditious rather than
something I'm particularly in love with.  It's OK for a first version,
I think, but we'd need to refactor it if we were to consider more than
one implementation strategy for a rollup.  Also it's pretty ugly that
the code makes a RollupPath even when a basic AggPath is what is meant;
that's a leftover from the fact that current HEAD goes through
build_grouping_chain() even for simple aggregation without grouping sets.

One point to consider is that we don't want the create_foo_path stage
to do much more than what's necessary to get a cost/rows estimate.
So in general, postponing as much work as possible to createplan.c
is a good thing.  But we don't want the Path representation to leave
any interesting planning choices implicit.

My general feeling about this is that I don't want it to be a blocker
for getting the basic patch in, but I'll happily consider further
refactoring of individual path types once we're over that hump.
If you wanted to start on a follow-on patch to deal with this particular
issue, be my guest.
        regards, tom lane