Skip eval_const_expressions when the query is such that the expression

would be evaluated only once anyway (ie, it's just a SELECT with no
FROM or an INSERT ... VALUES).  The planner can't do it any faster than
the executor, so no point in an extra copying of the expression tree.

src/backend/optimizer/plan/planner.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.186 2005/05/22 22:30:19 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.187 2005/05/30 01:04:44 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -385,6 +385,14 @@ subquery_planner(Query *parse, double tuple_fraction)
 static Node *
 preprocess_expression(Query *parse, Node *expr, int kind)
 {
+	/*
+	 * Fall out quickly if expression is empty.  This occurs often enough
+	 * to be worth checking.  Note that null->null is the correct conversion
+	 * for implicit-AND result format, too.
+	 */
+	if (expr == NULL)
+		return NULL;
+
 	/*
 	 * If the query has any join RTEs, replace join alias variables with
 	 * base-relation variables. We must do this before sublink processing,
@@ -401,8 +409,19 @@ preprocess_expression(Query *parse, Node *expr, int kind)
 	 * form.  All processing of a qual expression after this point must be
 	 * careful to maintain AND/OR flatness --- that is, do not generate a tree
 	 * with AND directly under AND, nor OR directly under OR.
+	 *
+	 * Because this is a relatively expensive process, we skip it when the
+	 * query is trivial, such as "SELECT 2+2;" or "INSERT ... VALUES()".
+	 * The expression will only be evaluated once anyway, so no point in
+	 * pre-simplifying; we can't execute it any faster than the executor can,
+	 * and we will waste cycles copying the tree.  Notice however that we
+	 * still must do it for quals (to get AND/OR flatness); and if we are
+	 * in a subquery we should not assume it will be done only once.
 	 */
-	expr = eval_const_expressions(expr);
+	if (parse->jointree->fromlist != NIL ||
+		kind == EXPRKIND_QUAL ||
+		PlannerQueryLevel > 1)
+		expr = eval_const_expressions(expr);
 
 	/*
 	 * If it's a qual or havingQual, canonicalize it.