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.

9 files changed, 159 insertions, 53 deletions

diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index cb95fea771..078b1bbc9e 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -33,6 +33,12 @@
 #include "utils/tuplesort.h"
 
 
+/* Hook for plugins to get control in ExplainOneQuery() */
+ExplainOneQuery_hook_type ExplainOneQuery_hook = NULL;
+/* Hook for plugins to get control in explain_get_index_name() */
+explain_get_index_name_hook_type explain_get_index_name_hook = NULL;
+
+
 typedef struct ExplainState
 {
 	/* options */
@@ -61,6 +67,8 @@ static void show_sort_keys(Plan *sortplan, int nkeys, AttrNumber *keycols,
 			   StringInfo str, int indent, ExplainState *es);
 static void show_sort_info(SortState *sortstate,
 			   StringInfo str, int indent, ExplainState *es);
+static const char *explain_get_index_name(Oid indexId);
+
 
 /*
  * ExplainQuery -
@@ -140,9 +148,6 @@ static void
 ExplainOneQuery(Query *query, ExplainStmt *stmt, const char *queryString,
 				ParamListInfo params, TupOutputState *tstate)
 {
-	PlannedStmt *plan;
-	QueryDesc  *queryDesc;
-
 	/* planner will not cope with utility statements */
 	if (query->commandType == CMD_UTILITY)
 	{
@@ -151,25 +156,19 @@ ExplainOneQuery(Query *query, ExplainStmt *stmt, const char *queryString,
 		return;
 	}
 
-	/* plan the query */
-	plan = planner(query, 0, params);
-
-	/*
-	 * Update snapshot command ID to ensure this query sees results of any
-	 * previously executed queries.  (It's a bit cheesy to modify
-	 * ActiveSnapshot without making a copy, but for the limited ways in which
-	 * EXPLAIN can be invoked, I think it's OK, because the active snapshot
-	 * shouldn't be shared with anything else anyway.)
-	 */
-	ActiveSnapshot->curcid = GetCurrentCommandId();
+	/* if an advisor plugin is present, let it manage things */
+	if (ExplainOneQuery_hook)
+		(*ExplainOneQuery_hook) (query, stmt, queryString, params, tstate);
+	else
+	{
+		PlannedStmt *plan;
 
-	/* Create a QueryDesc requesting no output */
-	queryDesc = CreateQueryDesc(plan,
-								ActiveSnapshot, InvalidSnapshot,
-								None_Receiver, params,
-								stmt->analyze);
+		/* plan the query */
+		plan = planner(query, 0, params);
 
-	ExplainOnePlan(queryDesc, stmt, tstate);
+		/* run it (if needed) and produce output */
+		ExplainOnePlan(plan, params, stmt, tstate);
+	}
 }
 
 /*
@@ -210,20 +209,35 @@ ExplainOneUtility(Node *utilityStmt, ExplainStmt *stmt,
  * not running the query.  No cursor will be created, however.
  *
  * This is exported because it's called back from prepare.c in the
- * EXPLAIN EXECUTE case
- *
- * Note: the passed-in QueryDesc is freed when we're done with it
+ * EXPLAIN EXECUTE case, and because an index advisor plugin would need
+ * to call it.
  */
 void
-ExplainOnePlan(QueryDesc *queryDesc, ExplainStmt *stmt,
-			   TupOutputState *tstate)
+ExplainOnePlan(PlannedStmt *plannedstmt, ParamListInfo params,
+			   ExplainStmt *stmt, TupOutputState *tstate)
 {
+	QueryDesc  *queryDesc;
 	instr_time	starttime;
 	double		totaltime = 0;
 	ExplainState *es;
 	StringInfoData buf;
 	int			eflags;
 
+	/*
+	 * Update snapshot command ID to ensure this query sees results of any
+	 * previously executed queries.  (It's a bit cheesy to modify
+	 * ActiveSnapshot without making a copy, but for the limited ways in which
+	 * EXPLAIN can be invoked, I think it's OK, because the active snapshot
+	 * shouldn't be shared with anything else anyway.)
+	 */
+	ActiveSnapshot->curcid = GetCurrentCommandId();
+
+	/* Create a QueryDesc requesting no output */
+	queryDesc = CreateQueryDesc(plannedstmt,
+								ActiveSnapshot, InvalidSnapshot,
+								None_Receiver, params,
+								stmt->analyze);
+
 	INSTR_TIME_SET_CURRENT(starttime);
 
 	/* If analyzing, we need to cope with queued triggers */
@@ -592,7 +606,7 @@ explain_outNode(StringInfo str,
 			if (ScanDirectionIsBackward(((IndexScan *) plan)->indexorderdir))
 				appendStringInfoString(str, " Backward");
 			appendStringInfo(str, " using %s",
-			  quote_identifier(get_rel_name(((IndexScan *) plan)->indexid)));
+					explain_get_index_name(((IndexScan *) plan)->indexid));
 			/* FALL THRU */
 		case T_SeqScan:
 		case T_BitmapHeapScan:
@@ -618,7 +632,7 @@ explain_outNode(StringInfo str,
 			break;
 		case T_BitmapIndexScan:
 			appendStringInfo(str, " on %s",
-							 quote_identifier(get_rel_name(((BitmapIndexScan *) plan)->indexid)));
+				explain_get_index_name(((BitmapIndexScan *) plan)->indexid));
 			break;
 		case T_SubqueryScan:
 			if (((Scan *) plan)->scanrelid > 0)
@@ -1150,3 +1164,29 @@ show_sort_info(SortState *sortstate,
 		pfree(sortinfo);
 	}
 }
+
+/*
+ * Fetch the name of an index in an EXPLAIN
+ *
+ * We allow plugins to get control here so that plans involving hypothetical
+ * indexes can be explained.
+ */
+static const char *
+explain_get_index_name(Oid indexId)
+{
+	const char   *result;
+
+	if (explain_get_index_name_hook)
+		result = (*explain_get_index_name_hook) (indexId);
+	else
+		result = NULL;
+	if (result == NULL)
+	{
+		/* default behavior: look in the catalogs and quote it */
+		result = get_rel_name(indexId);
+		if (result == NULL)
+			elog(ERROR, "cache lookup failed for index %u", indexId);
+		result = quote_identifier(result);
+	}
+	return result;
+}
diff --git a/src/backend/commands/prepare.c b/src/backend/commands/prepare.c
index f29e1a1859..aad8dc05c9 100644
--- a/src/backend/commands/prepare.c
+++ b/src/backend/commands/prepare.c
@@ -678,8 +678,6 @@ ExplainExecuteQuery(ExecuteStmt *execstmt, ExplainStmt *stmt,
 
 		if (IsA(pstmt, PlannedStmt))
 		{
-			QueryDesc  *qdesc;
-
 			if (execstmt->into)
 			{
 				if (pstmt->commandType != CMD_SELECT ||
@@ -694,22 +692,7 @@ ExplainExecuteQuery(ExecuteStmt *execstmt, ExplainStmt *stmt,
 				pstmt->intoClause = execstmt->into;
 			}
 
-			/*
-			 * Update snapshot command ID to ensure this query sees results of
-			 * any previously executed queries.  (It's a bit cheesy to modify
-			 * ActiveSnapshot without making a copy, but for the limited ways
-			 * in which EXPLAIN can be invoked, I think it's OK, because the
-			 * active snapshot shouldn't be shared with anything else anyway.)
-			 */
-			ActiveSnapshot->curcid = GetCurrentCommandId();
-
-			/* Create a QueryDesc requesting no output */
-			qdesc = CreateQueryDesc(pstmt,
-									ActiveSnapshot, InvalidSnapshot,
-									None_Receiver,
-									paramLI, stmt->analyze);
-
-			ExplainOnePlan(qdesc, stmt, tstate);
+			ExplainOnePlan(pstmt, paramLI, stmt, tstate);
 		}
 		else
 		{
diff --git a/src/backend/executor/nodeBitmapIndexscan.c b/src/backend/executor/nodeBitmapIndexscan.c
index d0ac90e819..87667f72fc 100644
--- a/src/backend/executor/nodeBitmapIndexscan.c
+++ b/src/backend/executor/nodeBitmapIndexscan.c
@@ -198,10 +198,12 @@ ExecEndBitmapIndexScan(BitmapIndexScanState *node)
 #endif
 
 	/*
-	 * close the index relation
+	 * close the index relation (no-op if we didn't open it)
 	 */
-	index_endscan(indexScanDesc);
-	index_close(indexRelationDesc, NoLock);
+	if (indexScanDesc)
+		index_endscan(indexScanDesc);
+	if (indexRelationDesc)
+		index_close(indexRelationDesc, NoLock);
 }
 
 /* ----------------------------------------------------------------
@@ -257,6 +259,14 @@ ExecInitBitmapIndexScan(BitmapIndexScan *node, EState *estate, int eflags)
 	indexstate->ss.ss_currentScanDesc = NULL;
 
 	/*
+	 * If we are just doing EXPLAIN (ie, aren't going to run the plan),
+	 * stop here.  This allows an index-advisor plugin to EXPLAIN a plan
+	 * containing references to nonexistent indexes.
+	 */
+	if (eflags & EXEC_FLAG_EXPLAIN_ONLY)
+		return indexstate;
+
+	/*
 	 * Open the index relation.
 	 *
 	 * If the parent table is one of the target relations of the query, then
diff --git a/src/backend/executor/nodeIndexscan.c b/src/backend/executor/nodeIndexscan.c
index d4d9aef793..e6412a8fec 100644
--- a/src/backend/executor/nodeIndexscan.c
+++ b/src/backend/executor/nodeIndexscan.c
@@ -415,10 +415,12 @@ ExecEndIndexScan(IndexScanState *node)
 	ExecClearTuple(node->ss.ss_ScanTupleSlot);
 
 	/*
-	 * close the index relation
+	 * close the index relation (no-op if we didn't open it)
 	 */
-	index_endscan(indexScanDesc);
-	index_close(indexRelationDesc, NoLock);
+	if (indexScanDesc)
+		index_endscan(indexScanDesc);
+	if (indexRelationDesc)
+		index_close(indexRelationDesc, NoLock);
 
 	/*
 	 * close the heap relation.
@@ -521,6 +523,14 @@ ExecInitIndexScan(IndexScan *node, EState *estate, int eflags)
 	ExecAssignScanType(&indexstate->ss, RelationGetDescr(currentRelation));
 
 	/*
+	 * If we are just doing EXPLAIN (ie, aren't going to run the plan),
+	 * stop here.  This allows an index-advisor plugin to EXPLAIN a plan
+	 * containing references to nonexistent indexes.
+	 */
+	if (eflags & EXEC_FLAG_EXPLAIN_ONLY)
+		return indexstate;
+
+	/*
 	 * Open the index relation.
 	 *
 	 * If the parent table is one of the target relations of the query, then
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index bb9f468f2b..987c113e77 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -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;
diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index fd29e53720..a15b9393f9 100644
--- a/src/backend/optimizer/util/plancat.c
+++ b/src/backend/optimizer/util/plancat.c
@@ -40,6 +40,9 @@
 /* GUC parameter */
 bool		constraint_exclusion = false;
 
+/* Hook for plugins to get control in get_relation_info() */
+get_relation_info_hook_type get_relation_info_hook = NULL;
+
 
 static void estimate_rel_size(Relation rel, int32 *attr_widths,
 				  BlockNumber *pages, double *tuples);
@@ -279,6 +282,14 @@ get_relation_info(PlannerInfo *root, Oid relationObjectId, bool inhparent,
 	rel->indexlist = indexinfos;
 
 	heap_close(relation, NoLock);
+
+	/*
+	 * Allow a plugin to editorialize on the info we obtained from the
+	 * catalogs.  Actions might include altering the assumed relation size,
+	 * removing an index, or adding a hypothetical index to the indexlist.
+	 */
+	if (get_relation_info_hook)
+		(*get_relation_info_hook) (root, relationObjectId, inhparent, rel);
 }
 
 /*
diff --git a/src/include/commands/explain.h b/src/include/commands/explain.h
index 7d882e5b14..5565380e9b 100644
--- a/src/include/commands/explain.h
+++ b/src/include/commands/explain.h
@@ -15,6 +15,18 @@
 
 #include "executor/executor.h"
 
+/* Hook for plugins to get control in ExplainOneQuery() */
+typedef void (*ExplainOneQuery_hook_type) (Query *query,
+										   ExplainStmt *stmt,
+										   const char *queryString,
+										   ParamListInfo params,
+										   TupOutputState *tstate);
+extern DLLIMPORT ExplainOneQuery_hook_type ExplainOneQuery_hook;
+
+/* Hook for plugins to get control in explain_get_index_name() */
+typedef const char * (*explain_get_index_name_hook_type) (Oid indexId);
+extern DLLIMPORT explain_get_index_name_hook_type explain_get_index_name_hook;
+
 
 extern void ExplainQuery(ExplainStmt *stmt, const char *queryString,
 						 ParamListInfo params, DestReceiver *dest);
@@ -26,7 +38,7 @@ extern void ExplainOneUtility(Node *utilityStmt, ExplainStmt *stmt,
 							  ParamListInfo params,
 							  TupOutputState *tstate);
 
-extern void ExplainOnePlan(QueryDesc *queryDesc, ExplainStmt *stmt,
-			   TupOutputState *tstate);
+extern void ExplainOnePlan(PlannedStmt *plannedstmt, ParamListInfo params,
+						   ExplainStmt *stmt, TupOutputState *tstate);
 
 #endif   /* EXPLAIN_H */
diff --git a/src/include/optimizer/plancat.h b/src/include/optimizer/plancat.h
index da3a4cd0ca..261f7f5799 100644
--- a/src/include/optimizer/plancat.h
+++ b/src/include/optimizer/plancat.h
@@ -16,6 +16,13 @@
 
 #include "nodes/relation.h"
 
+/* Hook for plugins to get control in get_relation_info() */
+typedef void (*get_relation_info_hook_type) (PlannerInfo *root,
+											 Oid relationObjectId,
+											 bool inhparent,
+											 RelOptInfo *rel);
+extern DLLIMPORT get_relation_info_hook_type get_relation_info_hook;
+
 
 extern void get_relation_info(PlannerInfo *root, Oid relationObjectId,
 				  bool inhparent, RelOptInfo *rel);
diff --git a/src/include/optimizer/planner.h b/src/include/optimizer/planner.h
index c48c596f1b..7cfca18a65 100644
--- a/src/include/optimizer/planner.h
+++ b/src/include/optimizer/planner.h
@@ -18,8 +18,17 @@
 #include "nodes/relation.h"
 
 
+/* Hook for plugins to get control in planner() */
+typedef PlannedStmt * (*planner_hook_type) (Query *parse,
+											int cursorOptions,
+											ParamListInfo boundParams);
+extern DLLIMPORT planner_hook_type planner_hook;
+
+
 extern PlannedStmt *planner(Query *parse, int cursorOptions,
 		ParamListInfo boundParams);
+extern PlannedStmt *standard_planner(Query *parse, int cursorOptions,
+		ParamListInfo boundParams);
 extern Plan *subquery_planner(PlannerGlobal *glob, Query *parse,
 							  Index level, double tuple_fraction,
 							  PlannerInfo **subroot);