pgsql: Fix partial aggregation for the case of a degenerate GROUP BY cl

Fix partial aggregation for the case of a degenerate GROUP BY clause.

The plan generated for sorted partial aggregation with "GROUP BY constant"
included a Sort node with no sort keys, which the executor does not like.

Per report from Steve Randall.  I'd add a regression test case if I could
think of a compact one, but it doesn't seem worth expending lots of cycles
on.

Report: <CABVd52UAdGXpg_rCk46egpNKYdXOzCjuJ1zG26E2xBe_8bj+Fg@mail.gmail.com>

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/7defc3b97a31537547053946808a83e7234d1b61

Modified Files
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src/backend/optimizer/plan/planner.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)