1 files changed, 63 insertions, 0 deletions
diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index da9431108a2..f5197779684 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -1185,6 +1185,69 @@ generate_nonunion_paths(SetOperationStmt *op, PlannerInfo *root,
 		result_rel->reltarget = create_setop_pathtarget(root, tlist,
 														list_make2(lpath, rpath));
 
+	/* Check for provably empty setop inputs and add short-circuit paths. */
+	if (op->op == SETOP_EXCEPT)
+	{
+		/*
+		 * For EXCEPTs, if the left side is dummy then there's no need to
+		 * inspect the right-hand side as scanning the right to find tuples to
+		 * remove won't make the left-hand input any more empty.
+		 */
+		if (is_dummy_rel(lrel))
+		{
+			mark_dummy_rel(result_rel);
+
+			return result_rel;
+		}
+
+		/* Handle EXCEPTs with dummy right input */
+		if (is_dummy_rel(rrel))
+		{
+			if (op->all)
+			{
+				Path	   *apath;
+
+				/*
+				 * EXCEPT ALL: If the right-hand input is dummy then we can
+				 * simply scan the left-hand input.  To keep createplan.c
+				 * happy, use a single child Append to handle the translation
+				 * between the set op targetlist and the targetlist of the
+				 * left input.  The Append will be removed in setrefs.c.
+				 */
+				apath = (Path *) create_append_path(root, result_rel, list_make1(lpath),
+													NIL, NIL, NULL, 0, false, -1);
+
+				add_path(result_rel, apath);
+
+				return result_rel;
+			}
+			else
+			{
+				/*
+				 * To make EXCEPT with a dummy RHS work means having to
+				 * deduplicate the left input.  That could be done with
+				 * AggPaths, but it doesn't seem worth the effort.  Let the
+				 * normal path generation code below handle this one.
+				 */
+			}
+		}
+	}
+	else
+	{
+		/*
+		 * For INTERSECT, if either input is a dummy rel then we can mark the
+		 * result_rel as dummy since intersecting with an empty relation can
+		 * never yield any results.  This is true regardless of INTERSECT or
+		 * INTERSECT ALL.
+		 */
+		if (is_dummy_rel(lrel) || is_dummy_rel(rrel))
+		{
+			mark_dummy_rel(result_rel);
+
+			return result_rel;
+		}
+	}
+
 	/*
 	 * Estimate number of distinct groups that we'll need hashtable entries
 	 * for; this is the size of the left-hand input for EXCEPT, or the smaller