4 files changed, 46 insertions, 17 deletions

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 2aa8837880c..e376c49265c 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -1453,7 +1453,7 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
 	/*
 	 * Consider an append of partial unordered, unparameterized partial paths.
 	 */
-	if (partial_subpaths_valid)
+	if (partial_subpaths_valid && partial_subpaths != NIL)
 	{
 		AppendPath *appendpath;
 		ListCell   *lc;
@@ -1750,9 +1750,11 @@ accumulate_append_subpath(List *subpaths, Path *path)
  *	  Build a dummy path for a relation that's been excluded by constraints
  *
  * Rather than inventing a special "dummy" path type, we represent this as an
- * AppendPath with no members (see also IS_DUMMY_PATH/IS_DUMMY_REL macros).
+ * AppendPath with no members (see also IS_DUMMY_APPEND/IS_DUMMY_REL macros).
  *
- * This is exported because inheritance_planner() has need for it.
+ * (See also mark_dummy_rel, which does basically the same thing, but is
+ * typically used to change a rel into dummy state after we already made
+ * paths for it.)
  */
 void
 set_dummy_rel_pathlist(RelOptInfo *rel)
@@ -1765,13 +1767,14 @@ set_dummy_rel_pathlist(RelOptInfo *rel)
 	rel->pathlist = NIL;
 	rel->partial_pathlist = NIL;
 
+	/* Set up the dummy path */
 	add_path(rel, (Path *) create_append_path(rel, NIL, NULL, 0, NIL));
 
 	/*
-	 * We set the cheapest path immediately, to ensure that IS_DUMMY_REL()
-	 * will recognize the relation as dummy if anyone asks.  This is redundant
-	 * when we're called from set_rel_size(), but not when called from
-	 * elsewhere, and doing it twice is harmless anyway.
+	 * We set the cheapest-path fields immediately, just in case they were
+	 * pointing at some discarded path.  This is redundant when we're called
+	 * from set_rel_size(), but not when called from elsewhere, and doing it
+	 * twice is harmless anyway.
 	 */
 	set_cheapest(rel);
 }
diff --git a/src/backend/optimizer/path/joinrels.c b/src/backend/optimizer/path/joinrels.c
index 4bbad04c055..ae3c487e08a 100644
--- a/src/backend/optimizer/path/joinrels.c
+++ b/src/backend/optimizer/path/joinrels.c
@@ -28,7 +28,6 @@ static void make_rels_by_clauseless_joins(PlannerInfo *root,
 							  ListCell *other_rels);
 static bool has_join_restriction(PlannerInfo *root, RelOptInfo *rel);
 static bool has_legal_joinclause(PlannerInfo *root, RelOptInfo *rel);
-static bool is_dummy_rel(RelOptInfo *rel);
 static void mark_dummy_rel(RelOptInfo *rel);
 static bool restriction_is_constant_false(List *restrictlist,
 							  RelOptInfo *joinrel,
@@ -1177,10 +1176,38 @@ have_dangerous_phv(PlannerInfo *root,
 /*
  * is_dummy_rel --- has relation been proven empty?
  */
-static bool
+bool
 is_dummy_rel(RelOptInfo *rel)
 {
-	return IS_DUMMY_REL(rel);
+	Path	   *path;
+
+	/*
+	 * A rel that is known dummy will have just one path that is a childless
+	 * Append.  (Even if somehow it has more paths, a childless Append will
+	 * have cost zero and hence should be at the front of the pathlist.)
+	 */
+	if (rel->pathlist == NIL)
+		return false;
+	path = (Path *) linitial(rel->pathlist);
+
+	/*
+	 * Initially, a dummy path will just be a childless Append.  But in later
+	 * planning stages we might stick a ProjectSetPath and/or ProjectionPath
+	 * on top, since Append can't project.  Rather than make assumptions about
+	 * which combinations can occur, just descend through whatever we find.
+	 */
+	for (;;)
+	{
+		if (IsA(path, ProjectionPath))
+			path = ((ProjectionPath *) path)->subpath;
+		else if (IsA(path, ProjectSetPath))
+			path = ((ProjectSetPath *) path)->subpath;
+		else
+			break;
+	}
+	if (IS_DUMMY_APPEND(path))
+		return true;
+	return false;
 }
 
 /*
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 486157209ff..c446f7cd207 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -1013,7 +1013,7 @@ create_append_plan(PlannerInfo *root, AppendPath *best_path)
 	 *
 	 * Note that an AppendPath with no members is also generated in certain
 	 * cases where there was no appending construct at all, but we know the
-	 * relation is empty (see set_dummy_rel_pathlist).
+	 * relation is empty (see set_dummy_rel_pathlist and mark_dummy_rel).
 	 */
 	if (best_path->subpaths == NIL)
 	{
@@ -6428,12 +6428,11 @@ is_projection_capable_path(Path *path)
 		case T_Append:
 
 			/*
-			 * Append can't project, but if it's being used to represent a
-			 * dummy path, claim that it can project.  This prevents us from
-			 * converting a rel from dummy to non-dummy status by applying a
-			 * projection to its dummy path.
+			 * Append can't project, but if an AppendPath is being used to
+			 * represent a dummy path, what will actually be generated is a
+			 * Result which can project.
 			 */
-			return IS_DUMMY_PATH(path);
+			return IS_DUMMY_APPEND(path);
 		case T_ProjectSet:
 
 			/*
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 2da7678a798..2eeaf8539df 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -1326,7 +1326,7 @@ inheritance_planner(PlannerInfo *root)
 		 * If this child rel was excluded by constraint exclusion, exclude it
 		 * from the result plan.
 		 */
-		if (IS_DUMMY_PATH(subpath))
+		if (IS_DUMMY_REL(sub_final_rel))
 			continue;
 
 		/*