Re: [BUG] Remove self joins causes 'variable not found in subplan target lists' error

I wrote:
> Yeah.  I think this is an oversight in create_unique_paths(): it's
> building an ORDER BY list without consideration for the possibility
> that some of the entries are known to be constant.  In fact, because
> make_pathkeys_for_sortclauses will get rid of redundancies, this
> example actually ends up with a unique_rel whose unique_pathkeys
> are shorter than the unique_groupclause, which is pretty bogus.

Here is a patch that fixes it that way.  I like this better than
Sergey's approach because it is making the plans better not worse.

There are a couple loose ends yet to be dealt with:

* The fact that we now avoid duplicate unique-ifications is good,
but I think it breaks the test case added by 44e95b572, since
that's no longer demonstrating what it set out to demonstrate.
(See the delta in aggregates.out.)  I'm not sure that that's a
huge problem, but we probably at least ought to modify the comment
on that test case.

* What happens if we find that *all* the semi_rhs_exprs are known
constant?  We'll compute empty pathkeys and groupClause, but then
what?  It looks like create_final_unique_paths etc then build a
useless Unique step.  Maybe we should just absorb the input
relation's path list as-is?

			regards, tom lane