pgsql: Fix bogus handling of JOIN_UNIQUE_OUTER/INNER cases for parallel

Fix bogus handling of JOIN_UNIQUE_OUTER/INNER cases for parallel joins.

consider_parallel_nestloop passed the wrong jointype down to its
subroutines for JOIN_UNIQUE_INNER cases (it should pass JOIN_INNER), and it
thought that it could pass paths other than innerrel->cheapest_total_path
to create_unique_path, which create_unique_path is not on board with.
These bugs would lead to assertion failures or other errors, suggesting
that this code path hasn't been tested much.

hash_inner_and_outer's code for parallel join effectively treated both
JOIN_UNIQUE_OUTER and JOIN_UNIQUE_INNER the same as JOIN_INNER (for
different reasons :-(), leading to incorrect plans that treated a semijoin
as if it were a plain join.

Michael Day submitted a test case demonstrating that hash_inner_and_outer
failed for JOIN_UNIQUE_OUTER, and I found the other cases through code
review.

Report: https://postgr.es/m/D0E8A029-D1AC-42E8-979A-5DE4A77E4413@rcmail.com

Branch
------
REL9_6_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/e5b8aa636a63a9446e244fcc8d3a262e3e14049a

Modified Files
--------------
src/backend/optimizer/path/joinpath.c | 33 ++++++++++++++++++++-------------
1 file changed, 20 insertions(+), 13 deletions(-)