Re: Add proper planner support for ORDER BY / DISTINCT aggregates

While doing some random testing, I noticed that the following is broken in HEAD:

SELECT COUNT(DISTINCT random()) FROM generate_series(1,10);

ERROR:  ORDER/GROUP BY expression not found in targetlist

It appears to have been broken by 1349d279, though I haven't looked at
the details.

I'm somewhat surprised that a case as simple as this wasn't covered by
any pre-existing regression tests.

Regards,
Dean