Replace the switching function ExecEvalExpr() with a macro that jumps

directly to the appropriate per-node execution function, using a function
pointer stored by ExecInitExpr.  This speeds things up by eliminating one
level of function call.  The function-pointer technique also enables further
small improvements such as only making one-time tests once (and then
changing the function pointer).  Overall this seems to gain about 10%
on evaluation of simple expressions, which isn't earthshaking but seems
a worthwhile gain for a relatively small hack.  Per recent discussion
on pghackers.

src/include/nodes/execnodes.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.112 2004/02/28 19:46:06 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.113 2004/03/17 01:02:24 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -390,13 +390,25 @@ typedef HASH_SEQ_STATUS TupleHashIterator;
  *
  * It can also be instantiated directly for leaf Expr nodes that need no
  * local run-time state (such as Var, Const, or Param).
+ *
+ * To save on dispatch overhead, each ExprState node contains a function
+ * pointer to the routine to execute to evaluate the node.
  * ----------------
  */
-typedef struct ExprState
+
+typedef struct ExprState ExprState;
+
+typedef Datum (*ExprStateEvalFunc) (ExprState *expression,
+									ExprContext *econtext,
+									bool *isNull,
+									ExprDoneCond *isDone);
+
+struct ExprState
 {
 	NodeTag		type;
 	Expr	   *expr;			/* associated Expr node */
-} ExprState;
+	ExprStateEvalFunc evalfunc;	/* routine to run to execute node */
+};
 
 /* ----------------
  *		GenericExprState node