Add an 'enable_material' GUC.

The logic for determining whether to materialize has been significantly
overhauled for 9.0.  In case there should be any doubt about whether
materialization is a win in any particular case, this should provide a
convenient way of seeing what happens without it; but even with enable_material
turned off, we still materialize in cases where it is required for
correctness.

Thanks to Tom Lane for the review.

src/backend/utils/misc/guc.c

@@ -10,7 +10,7 @@
  * Written by Peter Eisentraut <peter_e@gmx.net>.
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.547 2010/04/12 09:52:29 heikki Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.548 2010/04/19 00:55:25 rhaas Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -643,6 +643,14 @@ static struct config_bool ConfigureNamesBool[] =
 		&enable_hashagg,
 		true, NULL, NULL
 	},
+	{
+		{"enable_material", PGC_USERSET, QUERY_TUNING_METHOD,
+			gettext_noop("Enables the planner's use of materialization."),
+			NULL
+		},
+		&enable_material,
+		true, NULL, NULL
+	},
 	{
 		{"enable_nestloop", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of nested-loop join plans."),

src/backend/utils/misc/postgresql.conf.sample

@@ -206,6 +206,7 @@
 #enable_hashagg = on
 #enable_hashjoin = on
 #enable_indexscan = on
+#enable_material = on
 #enable_mergejoin = on
 #enable_nestloop = on
 #enable_seqscan = on