From fcba3b82e2d4fe42a0acfb9251fd1a7b4862ca30 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Mon, 28 Aug 2006 14:32:41 +0000
Subject: [PATCH] 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 | 32 +++++++++++++++++++++-------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/src/backend/optimizer/plan/setrefs.c b/src/backend/optimizer/plan/setrefs.c
index 9669254463..3f3a719941 100644
--- a/src/backend/optimizer/plan/setrefs.c
+++ b/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++;
 	}