pgsql: Fix logic error in add_paths_to_partial_grouping_rel.

Fix logic error in add_paths_to_partial_grouping_rel.

Commit 3bf05e096b9f8375e640c5d7996aa57efd7f240c sometimes uses the
cheapest_partial_path variable in this function to mean the cheapest
one from the input rel and at other times the cheapest one from the
partially grouped rel, but it never resets it, so we can end up with
bad plans, leading to "ERROR: Aggref found in non-Agg plan node".

Jeevan Chalke, per a report from Andreas Joseph Krogh and a separate
off-list report from Rajkumar Raghuwanshi

Discussion: http://postgr.es/m/CAM2+6=X9kxQoL2ZqZ00E6asBt9z+rfyWbOmhXJ0+8fPAyMZ9Jg@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3bfe957761ace9290072690a6298fc0eb996d857

Modified Files
--------------
src/backend/optimizer/plan/planner.c | 3 +++
1 file changed, 3 insertions(+)