Marginal performance hack: avoid unnecessary work in expression_tree_mutator.

We can just palloc, instead of using makeNode, when we are going to
overwrite the whole node anyway in the FLATCOPY macro.  Also, use
FLATCOPY instead of copyObject for common node types Var and Const.

src/backend/optimizer/util/clauses.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/util/clauses.c,v 1.242 2007/04/27 22:05:48 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/util/clauses.c,v 1.243 2007/04/30 00:14:54 tgl Exp $
  *
  * HISTORY
  *	  AUTHOR			DATE			MAJOR EVENT
@@ -3770,12 +3770,12 @@ expression_tree_mutator(Node *node,
 	 */
 
 #define FLATCOPY(newnode, node, nodetype)  \
-	( (newnode) = makeNode(nodetype), \
+	( (newnode) = (nodetype *) palloc(sizeof(nodetype)), \
 	  memcpy((newnode), (node), sizeof(nodetype)) )
 
 #define CHECKFLATCOPY(newnode, node, nodetype)	\
 	( AssertMacro(IsA((node), nodetype)), \
-	  (newnode) = makeNode(nodetype), \
+	  (newnode) = (nodetype *) palloc(sizeof(nodetype)), \
 	  memcpy((newnode), (node), sizeof(nodetype)) )
 
 #define MUTATE(newfield, oldfield, fieldtype)  \
@@ -3789,15 +3789,36 @@ expression_tree_mutator(Node *node,
 
 	switch (nodeTag(node))
 	{
+		/*
+		 * Primitive node types with no expression subnodes.  Var and Const
+		 * are frequent enough to deserve special cases, the others we just
+		 * use copyObject for.
+		 */
 		case T_Var:
+			{
+				Var	   *var = (Var *) node;
+				Var	   *newnode;
+
+				FLATCOPY(newnode, var, Var);
+				return (Node *) newnode;
+			}
+			break;
 		case T_Const:
+			{
+				Const	   *oldnode = (Const *) node;
+				Const	   *newnode;
+
+				FLATCOPY(newnode, oldnode, Const);
+				/* XXX we don't bother with datumCopy; should we? */
+				return (Node *) newnode;
+			}
+			break;
 		case T_Param:
 		case T_CoerceToDomainValue:
 		case T_CaseTestExpr:
 		case T_SetToDefault:
 		case T_RangeTblRef:
 		case T_OuterJoinInfo:
-			/* primitive node types with no expression subnodes */
 			return (Node *) copyObject(node);
 		case T_Aggref:
 			{