Make queries' locking of indexes more consistent.

The assertions added by commit b04aeb0a0 exposed that there are some
code paths wherein the executor will try to open an index without
holding any lock on it.  We do have some lock on the index's table,
so it seems likely that there's no fatal problem with this (for
instance, the index couldn't get dropped from under us).  Still,
it's bad practice and we should fix it.

To do so, remove the optimizations in ExecInitIndexScan and friends
that tried to avoid taking a lock on an index belonging to a target
relation, and just take the lock always.  In non-bug cases, this
will result in no additional shared-memory access, since we'll find
in the local lock table that we already have a lock of the desired
type; hence, no significant performance degradation should occur.

Also, adjust the planner and executor so that the type of lock taken
on an index is always identical to the type of lock taken for its table,
by relying on the recently added RangeTblEntry.rellockmode field.
This avoids some corner cases where that might not have been true
before (possibly resulting in extra locking overhead), and prevents
future maintenance issues from having multiple bits of logic that
all needed to be in sync.  In addition, this change removes all core
calls to ExecRelationIsTargetRelation, which avoids a possible O(N^2)
startup penalty for queries with large numbers of target relations.
(We'd probably remove that function altogether, were it not that we
advertise it as something that FDWs might want to use.)

Also adjust some places in selfuncs.c to not take any lock on indexes
they are transiently opening, since we can assume that plancat.c
did that already.

In passing, change gin_clean_pending_list() to take RowExclusiveLock
not AccessShareLock on its target index.  Although it's not clear that
that's actually a bug, it seemed very strange for a function that's
explicitly going to modify the index to use only AccessShareLock.

David Rowley, reviewed by Julien Rouhaud and Amit Langote,
a bit of further tweaking by me

Discussion: https://postgr.es/m/19465.1541636036@sss.pgh.pa.us

src/backend/access/gin/ginfast.c

@@ -1031,7 +1031,7 @@ Datum
 gin_clean_pending_list(PG_FUNCTION_ARGS)
 {
 	Oid			indexoid = PG_GETARG_OID(0);
-	Relation	indexRel = index_open(indexoid, AccessShareLock);
+	Relation	indexRel = index_open(indexoid, RowExclusiveLock);
 	IndexBulkDeleteResult stats;
 	GinState	ginstate;
 
@@ -1068,7 +1068,7 @@ gin_clean_pending_list(PG_FUNCTION_ARGS)
 	initGinState(&ginstate, indexRel);
 	ginInsertCleanup(&ginstate, true, true, true, &stats);
 
-	index_close(indexRel, AccessShareLock);
+	index_close(indexRel, RowExclusiveLock);
 
 	PG_RETURN_INT64((int64) stats.pages_deleted);
 }

src/backend/executor/execUtils.c

@@ -664,6 +664,10 @@ ExecCreateScanSlotFromOuterPlan(EState *estate,
  *
  *		Detect whether a relation (identified by rangetable index)
  *		is one of the target relations of the query.
+ *
+ * Note: This is currently no longer used in core.  We keep it around
+ * because FDWs may wish to use it to determine if their foreign table
+ * is a target relation.
  * ----------------------------------------------------------------
  */
 bool

src/backend/executor/nodeBitmapIndexscan.c

@@ -211,7 +211,7 @@ BitmapIndexScanState *
 ExecInitBitmapIndexScan(BitmapIndexScan *node, EState *estate, int eflags)
 {
 	BitmapIndexScanState *indexstate;
-	bool		relistarget;
+	LOCKMODE	lockmode;
 
 	/* check for unsupported flags */
 	Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));
@@ -260,16 +260,9 @@ ExecInitBitmapIndexScan(BitmapIndexScan *node, EState *estate, int eflags)
 	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
-	 * InitPlan already opened and write-locked the index, so we can avoid
-	 * taking another lock here.  Otherwise we need a normal reader's lock.
-	 */
-	relistarget = ExecRelationIsTargetRelation(estate, node->scan.scanrelid);
-	indexstate->biss_RelationDesc = index_open(node->indexid,
-											   relistarget ? NoLock : AccessShareLock);
+	/* Open the index relation. */
+	lockmode = exec_rt_fetch(node->scan.scanrelid, estate)->rellockmode;
+	indexstate->biss_RelationDesc = index_open(node->indexid, lockmode);
 
 	/*
 	 * Initialize index-specific scan state

src/backend/executor/nodeIndexonlyscan.c

@@ -493,7 +493,7 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 {
 	IndexOnlyScanState *indexstate;
 	Relation	currentRelation;
-	bool		relistarget;
+	LOCKMODE	lockmode;
 	TupleDesc	tupDesc;
 
 	/*
@@ -556,16 +556,9 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 	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
-	 * InitPlan already opened and write-locked the index, so we can avoid
-	 * taking another lock here.  Otherwise we need a normal reader's lock.
-	 */
-	relistarget = ExecRelationIsTargetRelation(estate, node->scan.scanrelid);
-	indexstate->ioss_RelationDesc = index_open(node->indexid,
-											   relistarget ? NoLock : AccessShareLock);
+	/* Open the index relation. */
+	lockmode = exec_rt_fetch(node->scan.scanrelid, estate)->rellockmode;
+	indexstate->ioss_RelationDesc = index_open(node->indexid, lockmode);
 
 	/*
 	 * Initialize index-specific scan state

src/backend/executor/nodeIndexscan.c

@@ -901,7 +901,7 @@ ExecInitIndexScan(IndexScan *node, EState *estate, int eflags)
 {
 	IndexScanState *indexstate;
 	Relation	currentRelation;
-	bool		relistarget;
+	LOCKMODE	lockmode;
 
 	/*
 	 * create state structure
@@ -964,16 +964,9 @@ ExecInitIndexScan(IndexScan *node, EState *estate, int eflags)
 	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
-	 * InitPlan already opened and write-locked the index, so we can avoid
-	 * taking another lock here.  Otherwise we need a normal reader's lock.
-	 */
-	relistarget = ExecRelationIsTargetRelation(estate, node->scan.scanrelid);
-	indexstate->iss_RelationDesc = index_open(node->indexid,
-											  relistarget ? NoLock : AccessShareLock);
+	/* Open the index relation. */
+	lockmode = exec_rt_fetch(node->scan.scanrelid, estate)->rellockmode;
+	indexstate->iss_RelationDesc = index_open(node->indexid, lockmode);
 
 	/*
 	 * Initialize index-specific scan state

src/backend/optimizer/plan/planner.c

@@ -6285,6 +6285,7 @@ plan_create_index_workers(Oid tableOid, Oid indexOid)
 	/* Build RelOptInfo */
 	rel = build_simple_rel(root, 1, NULL);
 
+	/* Rels are assumed already locked by the caller */
 	heap = table_open(tableOid, NoLock);
 	index = index_open(indexOid, NoLock);
 

src/backend/optimizer/util/plancat.c

@@ -162,8 +162,8 @@ get_relation_info(PlannerInfo *root, Oid relationObjectId, bool inhparent,
 	if (hasindex)
 	{
 		List	   *indexoidlist;
-		ListCell   *l;
 		LOCKMODE	lmode;
+		ListCell   *l;
 
 		indexoidlist = RelationGetIndexList(relation);
 
@@ -172,13 +172,10 @@ get_relation_info(PlannerInfo *root, Oid relationObjectId, bool inhparent,
 		 * need, and do not release it.  This saves a couple of trips to the
 		 * shared lock manager while not creating any real loss of
 		 * concurrency, because no schema changes could be happening on the
-		 * index while we hold lock on the parent rel, and neither lock type
-		 * blocks any other kind of index operation.
+		 * index while we hold lock on the parent rel, and no lock type used
+		 * for queries blocks any other kind of index operation.
 		 */
-		if (rel->relid == root->parse->resultRelation)
-			lmode = RowExclusiveLock;
-		else
-			lmode = AccessShareLock;
+		lmode = root->simple_rte_array[varno]->rellockmode;
 
 		foreach(l, indexoidlist)
 		{
@@ -592,8 +589,8 @@ infer_arbiter_indexes(PlannerInfo *root)
 	OnConflictExpr *onconflict = root->parse->onConflict;
 
 	/* Iteration state */
+	RangeTblEntry *rte;
 	Relation	relation;
-	Oid			relationObjectId;
 	Oid			indexOidFromConstraint = InvalidOid;
 	List	   *indexList;
 	ListCell   *l;
@@ -620,10 +617,9 @@ infer_arbiter_indexes(PlannerInfo *root)
 	 * the rewriter or when expand_inherited_rtentry() added it to the query's
 	 * rangetable.
 	 */
-	relationObjectId = rt_fetch(root->parse->resultRelation,
-								root->parse->rtable)->relid;
+	rte = rt_fetch(root->parse->resultRelation, root->parse->rtable);
 
-	relation = table_open(relationObjectId, NoLock);
+	relation = table_open(rte->relid, NoLock);
 
 	/*
 	 * Build normalized/BMS representation of plain indexed attributes, as
@@ -687,15 +683,14 @@ infer_arbiter_indexes(PlannerInfo *root)
 		ListCell   *el;
 
 		/*
-		 * Extract info from the relation descriptor for the index.  We know
-		 * that this is a target, so get lock type it is known will ultimately
-		 * be required by the executor.
+		 * Extract info from the relation descriptor for the index.  Obtain
+		 * the same lock type that the executor will ultimately use.
 		 *
 		 * Let executor complain about !indimmediate case directly, because
 		 * enforcement needs to occur there anyway when an inference clause is
 		 * omitted.
 		 */
-		idxRel = index_open(indexoid, RowExclusiveLock);
+		idxRel = index_open(indexoid, rte->rellockmode);
 		idxForm = idxRel->rd_index;
 
 		if (!idxForm->indisvalid)

src/backend/utils/adt/selfuncs.c

@@ -5187,11 +5187,10 @@ get_actual_variable_range(PlannerInfo *root, VariableStatData *vardata,
 
 			/*
 			 * Open the table and index so we can read from them.  We should
-			 * already have at least AccessShareLock on the table, but not
-			 * necessarily on the index.
+			 * already have some type of lock on each.
 			 */
 			heapRel = table_open(rte->relid, NoLock);
-			indexRel = index_open(index->indexoid, AccessShareLock);
+			indexRel = index_open(index->indexoid, NoLock);
 
 			/* extract index key information from the index's pg_index info */
 			indexInfo = BuildIndexInfo(indexRel);
@@ -5305,7 +5304,7 @@ get_actual_variable_range(PlannerInfo *root, VariableStatData *vardata,
 			/* Clean everything up */
 			ExecDropSingleTupleTableSlot(slot);
 
-			index_close(indexRel, AccessShareLock);
+			index_close(indexRel, NoLock);
 			table_close(heapRel, NoLock);
 
 			MemoryContextSwitchTo(oldcontext);
@@ -6472,9 +6471,10 @@ gincostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 	 */
 	if (!index->hypothetical)
 	{
-		indexRel = index_open(index->indexoid, AccessShareLock);
+		/* Lock should have already been obtained in plancat.c */
+		indexRel = index_open(index->indexoid, NoLock);
 		ginGetStats(indexRel, &ginStats);
-		index_close(indexRel, AccessShareLock);
+		index_close(indexRel, NoLock);
 	}
 	else
 	{
@@ -6781,11 +6781,12 @@ brincostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 							  &spc_seq_page_cost);
 
 	/*
-	 * Obtain some data from the index itself.
+	 * Obtain some data from the index itself.  A lock should have already
+	 * been obtained on the index in plancat.c.
 	 */
-	indexRel = index_open(index->indexoid, AccessShareLock);
+	indexRel = index_open(index->indexoid, NoLock);
 	brinGetStats(indexRel, &statsData);
-	index_close(indexRel, AccessShareLock);
+	index_close(indexRel, NoLock);
 
 	/*
 	 * Compute index correlation