3 files changed, 53 insertions, 9 deletions

diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index d668128ccb1..65a2e38fc33 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -937,10 +937,12 @@ create_unique_plan(PlannerInfo *root, UniquePath *best_path)
 	if (newitems || best_path->umethod == UNIQUE_PATH_SORT)
 	{
 		/*
-		 * If the top plan node can't do projections, we need to add a Result
-		 * node to help it along.
+		 * If the top plan node can't do projections and its existing target
+		 * list isn't already what we need, we need to add a Result node to
+		 * help it along.
 		 */
-		if (!is_projection_capable_plan(subplan))
+		if (!is_projection_capable_plan(subplan) &&
+			!tlist_same_exprs(newtlist, subplan->targetlist))
 			subplan = (Plan *) make_result(root, newtlist, NULL, subplan);
 		else
 			subplan->targetlist = newtlist;
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 0847e787c39..670776b032b 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -1379,10 +1379,12 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 			{
 				/*
 				 * If the top-level plan node is one that cannot do expression
-				 * evaluation, we must insert a Result node to project the
+				 * evaluation and its existing target list isn't already what
+				 * we need, we must insert a Result node to project the
 				 * desired tlist.
 				 */
-				if (!is_projection_capable_plan(result_plan))
+				if (!is_projection_capable_plan(result_plan) &&
+					!tlist_same_exprs(sub_tlist, result_plan->targetlist))
 				{
 					result_plan = (Plan *) make_result(root,
 													   sub_tlist,
@@ -1542,10 +1544,13 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 			 * If the top-level plan node is one that cannot do expression
 			 * evaluation, we must insert a Result node to project the desired
 			 * tlist.  (In some cases this might not really be required, but
-			 * it's not worth trying to avoid it.)  Note that on second and
-			 * subsequent passes through the following loop, the top-level
-			 * node will be a WindowAgg which we know can project; so we only
-			 * need to check once.
+			 * it's not worth trying to avoid it.  In particular, think not to
+			 * skip adding the Result if the initial window_tlist matches the
+			 * top-level plan node's output, because we might change the tlist
+			 * inside the following loop.)  Note that on second and subsequent
+			 * passes through the following loop, the top-level node will be a
+			 * WindowAgg which we know can project; so we only need to check
+			 * once.
 			 */
 			if (!is_projection_capable_plan(result_plan))
 			{
diff --git a/src/backend/optimizer/util/tlist.c b/src/backend/optimizer/util/tlist.c
index 9728ac8333b..5dc4b835fa3 100644
--- a/src/backend/optimizer/util/tlist.c
+++ b/src/backend/optimizer/util/tlist.c
@@ -159,6 +159,43 @@ get_tlist_exprs(List *tlist, bool includeJunk)
 
 
 /*
+ * tlist_same_exprs
+ *		Check whether two target lists contain the same expressions
+ *
+ * Note: this function is used to decide whether it's safe to jam a new tlist
+ * into a non-projection-capable plan node.  Obviously we can't do that unless
+ * the node's tlist shows it already returns the column values we want.
+ * However, we can ignore the TargetEntry attributes resname, ressortgroupref,
+ * resorigtbl, resorigcol, and resjunk, because those are only labelings that
+ * don't affect the row values computed by the node.  (Moreover, if we didn't
+ * ignore them, we'd frequently fail to make the desired optimization, since
+ * the planner tends to not bother to make resname etc. valid in intermediate
+ * plan nodes.)  Note that on success, the caller must still jam the desired
+ * tlist into the plan node, else it won't have the desired labeling fields.
+ */
+bool
+tlist_same_exprs(List *tlist1, List *tlist2)
+{
+	ListCell   *lc1,
+			   *lc2;
+
+	if (list_length(tlist1) != list_length(tlist2))
+		return false;			/* not same length, so can't match */
+
+	forboth(lc1, tlist1, lc2, tlist2)
+	{
+		TargetEntry *tle1 = (TargetEntry *) lfirst(lc1);
+		TargetEntry *tle2 = (TargetEntry *) lfirst(lc2);
+
+		if (!equal(tle1->expr, tle2->expr))
+			return false;
+	}
+
+	return true;
+}
+
+
+/*
  * Does tlist have same output datatypes as listed in colTypes?
  *
  * Resjunk columns are ignored if junkOK is true; otherwise presence of