Create hooks to let a loadable plugin monitor (or even replace) the planner

and/or create plans for hypothetical situations; in particular, investigate
plans that would be generated using hypothetical indexes.  This is a
heavily-rewritten version of the hooks proposed by Gurjeet Singh for his
Index Advisor project.  In this formulation, the index advisor can be
entirely a loadable module instead of requiring a significant part to be
in the core backend, and plans can be generated for hypothetical indexes
without requiring the creation and rolling-back of system catalog entries.

The index advisor patch as-submitted is not compatible with these hooks,
but it needs significant work anyway due to other 8.2-to-8.3 planner
changes.  With these hooks in the core backend, development of the advisor
can proceed as a pgfoundry project.

1 files changed, 25 insertions, 1 deletions

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 366c50f273..0f9776cdca 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.219 2007/05/04 01:13:44 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.220 2007/05/25 17:54:25 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -42,6 +42,10 @@
 #include "utils/syscache.h"
 
 
+/* Hook for plugins to get control in planner() */
+planner_hook_type planner_hook = NULL;
+
+
 /* Expression kind codes for preprocess_expression */
 #define EXPRKIND_QUAL		0
 #define EXPRKIND_TARGET		1
@@ -79,11 +83,31 @@ static List *postprocess_setop_tlist(List *new_tlist, List *orig_tlist);
  *
  *	   Query optimizer entry point
  *
+ * To support loadable plugins that monitor or modify planner behavior,
+ * we provide a hook variable that lets a plugin get control before and
+ * after the standard planning process.  The plugin would normally call
+ * standard_planner().
+ *
+ * Note to plugin authors: standard_planner() scribbles on its Query input,
+ * so you'd better copy that data structure if you want to plan more than once.
+ *
  *****************************************************************************/
 PlannedStmt *
 planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
 {
 	PlannedStmt *result;
+
+	if (planner_hook)
+		result = (*planner_hook) (parse, cursorOptions, boundParams);
+	else
+		result = standard_planner(parse, cursorOptions, boundParams);
+	return result;
+}
+
+PlannedStmt *
+standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
+{
+	PlannedStmt *result;
 	PlannerGlobal *glob;
 	double		tuple_fraction;
 	PlannerInfo *root;