pgsql: postgres_fdw: Push down aggregates to remote servers.

postgres_fdw: Push down aggregates to remote servers.

Now that the upper planner uses paths, and now that we have proper hooks
to inject paths into the upper planning process, it's possible for
foreign data wrappers to arrange to push aggregates to the remote side
instead of fetching all of the rows and aggregating them locally. This
figures to be a massive win for performance, so teach postgres_fdw to
do it.

Jeevan Chalke and Ashutosh Bapat. Reviewed by Ashutosh Bapat with
additional testing by Prabhat Sahu. Various mostly cosmetic changes
by me.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/7012b132d07c2b4ea15b0b3cb1ea9f3278801d98

Modified Files
--------------
contrib/postgres_fdw/deparse.c | 568 +++++++++--
contrib/postgres_fdw/expected/postgres_fdw.out | 1219 +++++++++++++++++++++++-
contrib/postgres_fdw/postgres_fdw.c | 494 +++++++++-
contrib/postgres_fdw/postgres_fdw.h | 5 +-
contrib/postgres_fdw/sql/postgres_fdw.sql | 305 ++++++
src/backend/optimizer/plan/createplan.c | 11 +-
6 files changed, 2452 insertions(+), 150 deletions(-)