pgsql: Clamp total-tuples estimates for foreign tables to ensure planne

Clamp total-tuples estimates for foreign tables to ensure planner sanity.

After running GetForeignRelSize for a foreign table, adjust rel->tuples
to be at least as large as rel->rows.  This prevents bizarre behavior
in estimate_num_groups() and perhaps other places, especially in the
scenario where rel->tuples is zero because pg_class.reltuples is
(suggesting that ANALYZE has never been run for the table).  As things
stood, we'd end up estimating one group out of any GROUP BY on such a
table, whereas the default group-count estimate is more likely to result
in a sane plan.

Also, clarify in the documentation that GetForeignRelSize has the option
to override the rel->tuples value if it has a better idea of what to use
than what is in pg_class.reltuples.

Per report from Jeff Janes.  Back-patch to all supported branches.

Patch by me; thanks to Etsuro Fujita for review

Discussion: https://postgr.es/m/CAMkU=1xNo9cnan+Npxgz0eK7394xmjmKg-QEm8wYG9P5-CcaqQ@mail.gmail.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/69a3fe6e639ad57f65536ca7b1873b9a3f1814e6

Modified Files
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doc/src/sgml/fdwhandler.sgml          | 8 ++++++++
src/backend/optimizer/path/allpaths.c | 3 +++
2 files changed, 11 insertions(+)