pgsql: Don't allow LIMIT/OFFSET clause within sub-selects to bepushed

Don't allow LIMIT/OFFSET clause within sub-selects to be pushed to workers.

Allowing sub-select containing LIMIT/OFFSET in workers can lead to
inconsistent results at the top-level as there is no guarantee that the
row order will be fully deterministic.  The fix is to prohibit pushing
LIMIT/OFFSET within sub-selects to workers.

Reported-by: Andrew Fletcher
Bug: 15324
Author: Amit Kapila
Reviewed-by: Dilip Kumar
Backpatch-through: 9.6
Discussion: https://postgr.es/m/153417684333.10284.11356259990921828616@wrigleys.postgresql.org

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1ceb103e7d1e794c0b171b0594fc6936003eb4ab

Modified Files
--------------
src/backend/optimizer/path/allpaths.c         | 13 +++++++++++++
src/backend/optimizer/plan/planner.c          |  3 +--
src/include/optimizer/planner.h               |  2 ++
src/test/regress/expected/select_parallel.out | 19 +++++++++++++++++++
src/test/regress/sql/select_parallel.sql      |  5 +++++
5 files changed, 40 insertions(+), 2 deletions(-)