pgsql: Remove some bogus logic from create_gather_merge_plan.

Remove some bogus logic from create_gather_merge_plan.

This logic was adapated from create_merge_append_plan, but the two
cases aren't really analogous, because create_merge_append_plan is not
projection-capable and must therefore have a tlist identical to that
of the underlying paths. Overwriting the tlist of Gather Merge with
whatever the underlying plan happens to produce is no good at all.

Patch by me, reviewed by Rushabh Lathia, who also reported the issue
and made an initial attempt at a fix.

Discussion: http://postgr.es/m/CA+Tgmob_-oHEOBfT9S25bjqokdqv8e8xEmh9zOY+3MPr_LmuhA@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/bce352fb46196340749cf907f3168b5f63969329

Modified Files
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src/backend/optimizer/plan/createplan.c | 48 ++++++++-------------------------
1 file changed, 11 insertions(+), 37 deletions(-)