Tweak trivial_subqueryscan() to consider a SubqueryScan's targetlist

trivial if it contains either Vars referencing the corresponding subplan
columns, or Consts equaling the corresponding subplan columns.  This
lets the planner eliminate the SubqueryScan in some cases generated by
generate_setop_tlist().

src/backend/optimizer/plan/setrefs.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/setrefs.c,v 1.124 2006/08/12 02:52:05 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/setrefs.c,v 1.125 2006/08/28 14:32:41 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -448,16 +448,32 @@ trivial_subqueryscan(SubqueryScan *plan)
 	{
 		TargetEntry *ptle = (TargetEntry *) lfirst(lp);
 		TargetEntry *ctle = (TargetEntry *) lfirst(lc);
-		Var		   *var = (Var *) ptle->expr;
 
 		if (ptle->resjunk != ctle->resjunk)
 			return false;		/* tlist doesn't match junk status */
-		if (!var || !IsA(var, Var))
-			return false;		/* tlist item not a Var */
-		Assert(var->varno == plan->scan.scanrelid);
-		Assert(var->varlevelsup == 0);
-		if (var->varattno != attrno)
-			return false;		/* out of order */
+
+		/*
+		 * We accept either a Var referencing the corresponding element of
+		 * the subplan tlist, or a Const equaling the subplan element.
+		 * See generate_setop_tlist() for motivation.
+		 */
+		if (ptle->expr && IsA(ptle->expr, Var))
+		{
+			Var	   *var = (Var *) ptle->expr;
+
+			Assert(var->varno == plan->scan.scanrelid);
+			Assert(var->varlevelsup == 0);
+			if (var->varattno != attrno)
+				return false;	/* out of order */
+		}
+		else if (ptle->expr && IsA(ptle->expr, Const))
+		{
+			if (!equal(ptle->expr, ctle->expr))
+				return false;
+		}
+		else
+			return false;
+
 		attrno++;
 	}