Re: Retire has_multiple_baserels()

I thought this test wasn't too complete, because has_multiple_baserels
isn't reached at all in many cases thanks to the way the calling if()
is coded.  I tried testing like this instead:

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index eea49cca7b..3f6fc51fb4 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -2649,6 +2649,8 @@ set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
      */
     remove_unused_subquery_outputs(subquery, rel, run_cond_attrs);

+    Assert(has_multiple_baserels(root) == (bms_membership(root->all_baserels) == BMS_MULTIPLE));
+
     /*
      * We can safely pass the outer tuple_fraction down to the subquery if the
      * outer level has no joining, aggregation, or sorting to do. Otherwise

and came to the same conclusion: check-world finds no cases where
the assertion fails.  So it LGTM too.  Pushed.