pgsql: Remove redundant code in create_gather_merge_path

Remove redundant code in create_gather_merge_path

In create_gather_merge_path, we should always guarantee that the
subpath is adequately ordered, and we do not add a Sort node in
createplan.c for a Gather Merge node.  Therefore, the 'else' branch in
create_gather_merge_path, which computes the cost for a Sort node, is
redundant.

This patch removes the redundant code and emits an error if the
subpath is not sufficiently ordered.  Meanwhile, this patch changes
the check for the subpath's pathkeys in create_gather_merge_plan to an
Assert.

Author: Richard Guo
Discussion: https://postgr.es/m/CAMbWs48u=0bWf3epVtULjJ-=M9Hbkz+ieZQAOS=BfbXZFqbDCg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8b2e9fd26afd9f379ea8bedeb4b11a354c09a3e1

Modified Files
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src/backend/optimizer/plan/createplan.c |  9 ++-------
src/backend/optimizer/util/pathnode.c   | 35 +++++++++++----------------------
2 files changed, 14 insertions(+), 30 deletions(-)