Create a third option named "partition" for constraint_exclusion, and make it

the default.  This setting enables constraint exclusion checks only for
appendrel members (ie, inheritance children and UNION ALL arms), which are
the cases in which constraint exclusion is most likely to be useful.  Avoiding
the overhead for simple queries that are unlikely to benefit should bring
the cost down to the point where this is a reasonable default setting.
Per today's discussion.

4 files changed, 49 insertions, 19 deletions

diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index 948c0602136..f4f8ac8a763 100644
--- a/src/backend/optimizer/util/plancat.c
+++ b/src/backend/optimizer/util/plancat.c
@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/util/plancat.c,v 1.153 2009/01/01 17:23:45 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/util/plancat.c,v 1.154 2009/01/07 22:40:49 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -27,6 +27,7 @@
 #include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
 #include "optimizer/clauses.h"
+#include "optimizer/cost.h"
 #include "optimizer/plancat.h"
 #include "optimizer/predtest.h"
 #include "optimizer/prep.h"
@@ -43,7 +44,7 @@
 
 
 /* GUC parameter */
-bool		constraint_exclusion = false;
+int			constraint_exclusion = CONSTRAINT_EXCLUSION_PARTITION;
 
 /* Hook for plugins to get control in get_relation_info() */
 get_relation_info_hook_type get_relation_info_hook = NULL;
@@ -561,8 +562,9 @@ get_relation_constraints(PlannerInfo *root,
  * self-inconsistent restrictions, or restrictions inconsistent with the
  * relation's CHECK constraints.
  *
- * Note: this examines only rel->relid and rel->baserestrictinfo; therefore
- * it can be called before filling in other fields of the RelOptInfo.
+ * Note: this examines only rel->relid, rel->reloptkind, and
+ * rel->baserestrictinfo; therefore it can be called before filling in
+ * other fields of the RelOptInfo.
  */
 bool
 relation_excluded_by_constraints(PlannerInfo *root,
@@ -573,8 +575,10 @@ relation_excluded_by_constraints(PlannerInfo *root,
 	List	   *safe_constraints;
 	ListCell   *lc;
 
-	/* Skip the test if constraint exclusion is disabled */
-	if (!constraint_exclusion)
+	/* Skip the test if constraint exclusion is disabled for the rel */
+	if (constraint_exclusion == CONSTRAINT_EXCLUSION_OFF ||
+		(constraint_exclusion == CONSTRAINT_EXCLUSION_PARTITION &&
+		 rel->reloptkind != RELOPT_OTHER_MEMBER_REL))
 		return false;
 
 	/*
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 479279cdc43..22cfbff6105 100644
--- a/src/backend/utils/misc/guc.c
+++ b/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 1.490 2009/01/06 16:39:52 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.491 2009/01/07 22:40:49 tgl Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -312,6 +312,23 @@ static const struct config_enum_entry backslash_quote_options[] = {
 };
 
 /*
+ * Although only "on", "off", and "partition" are documented, we
+ * accept all the likely variants of "on" and "off".
+ */
+static const struct config_enum_entry constraint_exclusion_options[] = {
+	{"partition", CONSTRAINT_EXCLUSION_PARTITION, false},
+	{"on", CONSTRAINT_EXCLUSION_ON, false},
+	{"off", CONSTRAINT_EXCLUSION_OFF, false},
+	{"true", CONSTRAINT_EXCLUSION_ON, true},
+	{"false", CONSTRAINT_EXCLUSION_OFF, true},
+	{"yes", CONSTRAINT_EXCLUSION_ON, true},
+	{"no", CONSTRAINT_EXCLUSION_OFF, true},
+	{"1", CONSTRAINT_EXCLUSION_ON, true},
+	{"0", CONSTRAINT_EXCLUSION_OFF, true},
+	{NULL, 0, false}
+};
+
+/*
  * Options for enum values stored in other modules
  */
 extern const struct config_enum_entry sync_method_options[];
@@ -636,15 +653,6 @@ static struct config_bool ConfigureNamesBool[] =
 		true, NULL, NULL
 	},
 	{
-		{"constraint_exclusion", PGC_USERSET, QUERY_TUNING_OTHER,
-			gettext_noop("Enables the planner to use constraints to optimize queries."),
-			gettext_noop("Child table scans will be skipped if their "
-					   "constraints guarantee that no rows match the query.")
-		},
-		&constraint_exclusion,
-		false, NULL, NULL
-	},
-	{
 		{"geqo", PGC_USERSET, QUERY_TUNING_GEQO,
 			gettext_noop("Enables genetic query optimization."),
 			gettext_noop("This algorithm attempts to do planning without "
@@ -2522,6 +2530,17 @@ static struct config_enum ConfigureNamesEnum[] =
 	},
 
 	{
+		{"constraint_exclusion", PGC_USERSET, QUERY_TUNING_OTHER,
+			gettext_noop("Enables the planner to use constraints to optimize queries."),
+			gettext_noop("Table scans will be skipped if their constraints"
+						 " guarantee that no rows match the query.")
+		},
+		&constraint_exclusion,
+		CONSTRAINT_EXCLUSION_PARTITION, constraint_exclusion_options,
+		NULL, NULL
+	},
+
+	{
 		{"default_transaction_isolation", PGC_USERSET, CLIENT_CONN_STATEMENT,
 			gettext_noop("Sets the transaction isolation level of each new transaction."),
 			NULL
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index e28feb8e428..d53861511ae 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -211,7 +211,7 @@
 # - Other Planner Options -
 
 #default_statistics_target = 100	# range 1-10000
-#constraint_exclusion = off
+#constraint_exclusion = partition	# on, off, or partition
 #cursor_tuple_fraction = 0.1		# range 0.0-1.0
 #from_collapse_limit = 8
 #join_collapse_limit = 8		# 1 disables collapsing of explicit 
diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h
index 39ece8f4ff7..27600fa1acc 100644
--- a/src/include/optimizer/cost.h
+++ b/src/include/optimizer/cost.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/optimizer/cost.h,v 1.95 2009/01/01 17:24:00 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/optimizer/cost.h,v 1.96 2009/01/07 22:40:49 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -29,6 +29,13 @@
 
 #define DEFAULT_EFFECTIVE_CACHE_SIZE  16384		/* measured in pages */
 
+typedef enum
+{
+	CONSTRAINT_EXCLUSION_OFF,			/* do not use c_e */
+	CONSTRAINT_EXCLUSION_ON,			/* apply c_e to all rels */
+	CONSTRAINT_EXCLUSION_PARTITION		/* apply c_e to otherrels only */
+} ConstraintExclusionType;
+
 
 /*
  * prototypes for costsize.c
@@ -52,7 +59,7 @@ extern bool enable_hashagg;
 extern bool enable_nestloop;
 extern bool enable_mergejoin;
 extern bool enable_hashjoin;
-extern bool constraint_exclusion;
+extern int	constraint_exclusion;
 
 extern double clamp_row_est(double nrows);
 extern double index_pages_fetched(double tuples_fetched, BlockNumber pages,