pgsql: Avoid generating bogus paths with partitionwise aggregate.

Avoid generating bogus paths with partitionwise aggregate.

Previously, if some or all partitions had no partially aggregated path,
we would still try to generate a partially aggregated path for the
parent, leading to assertion failures or wrong answers.

Report by Rajkumar Raghuwanshi.  Patch by Jeevan Chalke, reviewed
by Ashutosh Bapat.  A few changes by me.

Discussion: http://postgr.es/m/CAKcux6=q4+Mw8gOOX16ef6ZMFp9Cve7KWFstUsrDa4GiFaXGUQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c6f28af5d7af87d7370e5f169251d91437f100a2

Modified Files
--------------
src/backend/optimizer/plan/planner.c              |  21 +++--
src/test/regress/expected/partition_aggregate.out | 105 ++++++++++++++++++++++
src/test/regress/sql/partition_aggregate.sql      |  24 +++++
3 files changed, 142 insertions(+), 8 deletions(-)