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

src/backend/optimizer/path/allpaths.c

@@ -911,6 +911,9 @@ set_foreign_size(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
 
 	/* ... but do not let it set the rows estimate to zero */
 	rel->rows = clamp_row_est(rel->rows);
+
+	/* also, make sure rel->tuples is not insane relative to rel->rows */
+	rel->tuples = Max(rel->tuples, rel->rows);
 }
 
 /*