Re: "could not find pathkey item to sort" for TPC-DS queries 94-96

I wrote:
> I think it's time for some refactoring of this code so that we can
> actually share the logic. Accordingly, I propose the attached.

After sleeping on it, here's an improved version that gets rid of
an unnecessary assumption about ECs usually not containing both
parallel-safe and parallel-unsafe members. I'd tried to do this
yesterday but didn't like the amount of side-effects on createplan.c
(caused by the direct call sites not being passed the "root" pointer).
However, we can avoid refactoring createplan.c APIs by saying that it's
okay to pass root = NULL to find_computable_ec_member if you're not
asking it to check parallel safety. And there's not really a need to
put a parallel-safety check into find_ec_member_matching_expr at all;
that task can be left with the one caller that cares.

regards, tom lane