Re: Fire create_upper_paths_hook for UPPERREL_PARTIAL_GROUP_AGG

> It's a good idea to write something like this to convince yourself
> that the hook is usable for something useful, but that doesn't mean
> we need to carry that proof in-tree forevermore.

You're right. The test only checks that the hook gets called. It
doesn't check that anything behaves differently, and that isn't worth
spinning up a session for. Writing it did convince me the hook is
usable, like you say, but that doesn't mean it has to stay in the tree.
Dropping 0002.

I should also correct what I wrote about it. That description was from
an different version which added a path and checked that the planner
gathered it and built Finalize Aggregate on top. Even that is existing
planner behavior, not something 0001 adds.

Review is otherwise unchanged. I only have the Nit commit message and
comment suggestions from my earlier mail.

Given there is no major feedback open, I change the patch to Ready for
Committer.

Thanks,
Shihao