pgsql: Allow LIMIT pushdown in postgres_fdw with empty ORDER BY pathkey

Allow LIMIT pushdown in postgres_fdw with empty ORDER BY pathkeys

When all of the ORDER BY clause's PathKeys are redundant due to consts in
all of the pathkeys' EquivalenceClasses, sort_pathkeys is NIL, but
query_pathkeys may then get set to setop_pathkeys, which may not be
shippable. add_foreign_ordered_paths() previously used the shippability
of query_pathkeys to decide whether the UPPERREL_ORDERED step was safe to
push down, which needlessly prevented the subsequent UPPERREL_FINAL step
from pushing down the LIMIT. No sort is needed in this case, so just mark
the ordered step as pushdown-safe.

Author: David Rowley <dgrowleyml(at)gmail(dot)com>
Reviewed-by: Robins Tharakan <tharakan(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAEP4nAwnSefmi4w+89XbHQ0f-0OixCYUJo9kVcbXSYL8L4rEPg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b6178c942ea7a85f72beb6a775d2793062a7ff8e

Modified Files
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contrib/postgres_fdw/expected/postgres_fdw.out | 10 ++++++++++
contrib/postgres_fdw/postgres_fdw.c | 14 ++++++++++----
contrib/postgres_fdw/sql/postgres_fdw.sql | 4 ++++
3 files changed, 24 insertions(+), 4 deletions(-)