Make entirely-dummy appendrels get marked as such in set_append_rel_size.

The planner generally expects that the estimated rowcount of any relation
is at least one row, *unless* it has been proven empty by constraint
exclusion or similar mechanisms, which is marked by installing a dummy path
as the rel's cheapest path (cf. IS_DUMMY_REL).  When I split up
allpaths.c's processing of base rels into separate set_base_rel_sizes and
set_base_rel_pathlists steps, the intention was that dummy rels would get
marked as such during the "set size" step; this is what justifies an Assert
in indxpath.c's get_loop_count that other relations should either be dummy
or have positive rowcount.  Unfortunately I didn't get that quite right
for append relations: if all the child rels have been proven empty then
set_append_rel_size would come up with a rowcount of zero, which is
correct, but it didn't then do set_dummy_rel_pathlist.  (We would have
ended up with the right state after set_append_rel_pathlist, but that's
too late, if we generate indexpaths for some other rel first.)

In addition to fixing the actual bug, I installed an Assert enforcing this
convention in set_rel_size; that then allows simplification of a couple
of now-redundant tests for zero rowcount in set_append_rel_size.

Also, to cover the possibility that third-party FDWs have been careless
about not returning a zero rowcount estimate, apply clamp_row_est to
whatever an FDW comes up with as the rows estimate.

Per report from Andreas Seltenreich.  Back-patch to 9.2.  Earlier branches
did not have the separation between set_base_rel_sizes and
set_base_rel_pathlists steps, so there was no intermediate state where an
appendrel would have had inconsistent rowcount and pathlist.  It's possible
that adding the Assert to set_rel_size would be a good idea in older
branches too; but since they're not under development any more, it's likely
not worth the trouble.

src/backend/optimizer/path/allpaths.c

@@ -360,6 +360,11 @@ set_rel_size(PlannerInfo *root, RelOptInfo *rel,
 				break;
 		}
 	}
+
+	/*
+	 * We insist that all non-dummy rels have a nonzero rowcount estimate.
+	 */
+	Assert(rel->rows > 0 || IS_DUMMY_REL(rel));
 }
 
 /*
@@ -579,6 +584,9 @@ set_foreign_size(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
 
 	/* Let FDW adjust the size estimates, if it can */
 	rel->fdwroutine->GetForeignRelSize(root, rel, rte->relid);
+
+	/* ... but do not let it set the rows estimate to zero */
+	rel->rows = clamp_row_est(rel->rows);
 }
 
 /*
@@ -608,6 +616,7 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
 					Index rti, RangeTblEntry *rte)
 {
 	int			parentRTindex = rti;
+	bool		has_live_children;
 	double		parent_rows;
 	double		parent_size;
 	double	   *parent_attrsizes;
@@ -628,6 +637,7 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
 	 * Note: if you consider changing this logic, beware that child rels could
 	 * have zero rows and/or width, if they were excluded by constraints.
 	 */
+	has_live_children = false;
 	parent_rows = 0;
 	parent_size = 0;
 	nattrs = rel->max_attr - rel->min_attr + 1;
@@ -755,19 +765,22 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
 		if (IS_DUMMY_REL(childrel))
 			continue;
 
+		/* We have at least one live child. */
+		has_live_children = true;
+
 		/*
 		 * Accumulate size information from each live child.
 		 */
-		if (childrel->rows > 0)
-		{
+		Assert(childrel->rows > 0);
+
 		parent_rows += childrel->rows;
 		parent_size += childrel->width * childrel->rows;
 
 		/*
-			 * Accumulate per-column estimates too.  We need not do anything
-			 * for PlaceHolderVars in the parent list.  If child expression
-			 * isn't a Var, or we didn't record a width estimate for it, we
-			 * have to fall back on a datatype-based estimate.
+		 * Accumulate per-column estimates too.  We need not do anything for
+		 * PlaceHolderVars in the parent list.  If child expression isn't a
+		 * Var, or we didn't record a width estimate for it, we have to fall
+		 * back on a datatype-based estimate.
 		 *
 		 * By construction, child's reltargetlist is 1-to-1 with parent's.
 		 */
@@ -797,28 +810,35 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
 			}
 		}
 	}
-	}
 
+	if (has_live_children)
+	{
 		/*
 		 * Save the finished size estimates.
 		 */
-	rel->rows = parent_rows;
-	if (parent_rows > 0)
-	{
 		int			i;
 
+		Assert(parent_rows > 0);
+		rel->rows = parent_rows;
 		rel->width = rint(parent_size / parent_rows);
 		for (i = 0; i < nattrs; i++)
 			rel->attr_widths[i] = rint(parent_attrsizes[i] / parent_rows);
-	}
-	else
-		rel->width = 0;			/* attr_widths should be zero already */
 
 		/*
 		 * Set "raw tuples" count equal to "rows" for the appendrel; needed
 		 * because some places assume rel->tuples is valid for any baserel.
 		 */
 		rel->tuples = parent_rows;
+	}
+	else
+	{
+		/*
+		 * All children were excluded by constraints, so mark the whole
+		 * appendrel dummy.  We must do this in this phase so that the rel's
+		 * dummy-ness is visible when we generate paths for other rels.
+		 */
+		set_dummy_rel_pathlist(rel);
+	}
 
 	pfree(parent_attrsizes);
 }

src/test/regress/expected/join.out

@@ -2164,6 +2164,26 @@ select count(*) from tenk1 x where
 (1 row)
 
 rollback;
+--
+-- regression test: be sure we cope with proven-dummy append rels
+--
+explain (costs off)
+select aa, bb, unique1, unique1
+  from tenk1 right join b on aa = unique1
+  where bb < bb and bb is null;
+        QUERY PLAN        
+--------------------------
+ Result
+   One-Time Filter: false
+(2 rows)
+
+select aa, bb, unique1, unique1
+  from tenk1 right join b on aa = unique1
+  where bb < bb and bb is null;
+ aa | bb | unique1 | unique1 
+----+----+---------+---------
+(0 rows)
+
 --
 -- Clean up
 --

src/test/regress/sql/join.sql

@@ -353,6 +353,17 @@ select count(*) from tenk1 x where
   x.unique1 in (select aa.f1 from int4_tbl aa,float8_tbl bb where aa.f1=bb.f1);
 rollback;
 
+--
+-- regression test: be sure we cope with proven-dummy append rels
+--
+explain (costs off)
+select aa, bb, unique1, unique1
+  from tenk1 right join b on aa = unique1
+  where bb < bb and bb is null;
+
+select aa, bb, unique1, unique1
+  from tenk1 right join b on aa = unique1
+  where bb < bb and bb is null;
 
 --
 -- Clean up
@@ -1120,6 +1131,7 @@ select atts.relid::regclass, s.* from pg_stats s join
     a.attrelid::regclass::text join (select unnest(indkey) attnum,
     indexrelid from pg_index i) atts on atts.attnum = a.attnum where
     schemaname != 'pg_catalog';
+
 --
 -- Test LATERAL
 --