Allow locking updated tuples in tuple_update() and tuple_delete()

Currently, in read committed transaction isolation mode (default), we have the
following sequence of actions when tuple_update()/tuple_delete() finds
the tuple updated by concurrent transaction.

1. Attempt to update/delete tuple with tuple_update()/tuple_delete(), which
   returns TM_Updated.
2. Lock tuple with tuple_lock().
3. Re-evaluate plan qual (recheck if we still need to update/delete and
   calculate the new tuple for update).
4. Second attempt to update/delete tuple with tuple_update()/tuple_delete().
   This attempt should be successful, since the tuple was previously locked.

This patch eliminates step 2 by taking the lock during first
tuple_update()/tuple_delete() call.  Heap table access method saves some
efforts by checking the updated tuple once instead of twice.  Future
undo-based table access methods, which will start from the latest row version,
can immediately place a lock there.

The code in nodeModifyTable.c is simplified by removing the nested switch/case.

Discussion: https://postgr.es/m/CAPpHfdua-YFw3XTprfutzGp28xXLigFtzNbuFY8yPhqeq6X5kg%40mail.gmail.com
Reviewed-by: Aleksander Alekseev, Pavel Borisov, Vignesh C, Mason Sharp
Reviewed-by: Andres Freund, Chris Travers

src/backend/access/heap/heapam_handler.c

@@ -45,6 +45,12 @@
 #include "utils/builtins.h"
 #include "utils/rel.h"
 
+static TM_Result heapam_tuple_lock_internal(Relation relation, ItemPointer tid,
+											Snapshot snapshot, TupleTableSlot *slot,
+											CommandId cid, LockTupleMode mode,
+											LockWaitPolicy wait_policy, uint8 flags,
+											TM_FailureData *tmfd, bool updated);
+
 static void reform_and_rewrite_tuple(HeapTuple tuple,
 									 Relation OldHeap, Relation NewHeap,
 									 Datum *values, bool *isnull, RewriteState rwstate);
@@ -299,14 +305,46 @@ heapam_tuple_complete_speculative(Relation relation, TupleTableSlot *slot,
 static TM_Result
 heapam_tuple_delete(Relation relation, ItemPointer tid, CommandId cid,
 					Snapshot snapshot, Snapshot crosscheck, bool wait,
-					TM_FailureData *tmfd, bool changingPart)
+					TM_FailureData *tmfd, bool changingPart,
+					LazyTupleTableSlot *lockedSlot)
 {
+	TM_Result	result;
+
 	/*
 	 * Currently Deleting of index tuples are handled at vacuum, in case if
 	 * the storage itself is cleaning the dead tuples by itself, it is the
 	 * time to call the index tuple deletion also.
 	 */
-	return heap_delete(relation, tid, cid, crosscheck, wait, tmfd, changingPart);
+	result = heap_delete(relation, tid, cid, crosscheck, wait,
+						 tmfd, changingPart);
+
+	/*
+	 * If the tuple has been concurrently updated, then get the lock on it.
+	 * (Do this if caller asked for tat by providing a 'lockedSlot'.) With the
+	 * lock held retry of delete should succeed even if there are more
+	 * concurrent update attempts.
+	 */
+	if (result == TM_Updated && lockedSlot)
+	{
+		TupleTableSlot *evalSlot;
+
+		Assert(wait);
+
+		evalSlot = LAZY_TTS_EVAL(lockedSlot);
+		result = heapam_tuple_lock_internal(relation, tid, snapshot,
+											evalSlot, cid, LockTupleExclusive,
+											LockWaitBlock,
+											TUPLE_LOCK_FLAG_FIND_LAST_VERSION,
+											tmfd, true);
+
+		if (result == TM_Ok)
+		{
+			tmfd->traversed = true;
+			return TM_Updated;
+		}
+	}
+
+	return result;
 }
 
 
@@ -314,7 +352,8 @@ static TM_Result
 heapam_tuple_update(Relation relation, ItemPointer otid, TupleTableSlot *slot,
 					CommandId cid, Snapshot snapshot, Snapshot crosscheck,
 					bool wait, TM_FailureData *tmfd,
-					LockTupleMode *lockmode, TU_UpdateIndexes *update_indexes)
+					LockTupleMode *lockmode, TU_UpdateIndexes *update_indexes,
+					LazyTupleTableSlot *lockedSlot)
 {
 	bool		shouldFree = true;
 	HeapTuple	tuple = ExecFetchSlotHeapTuple(slot, true, &shouldFree);
@@ -352,6 +391,32 @@ heapam_tuple_update(Relation relation, ItemPointer otid, TupleTableSlot *slot,
 	if (shouldFree)
 		pfree(tuple);
 
+	/*
+	 * If the tuple has been concurrently updated, then get the lock on it.
+	 * (Do this if caller asked for tat by providing a 'lockedSlot'.) With the
+	 * lock held retry of update should succeed even if there are more
+	 * concurrent update attempts.
+	 */
+	if (result == TM_Updated && lockedSlot)
+	{
+		TupleTableSlot *evalSlot;
+
+		Assert(wait);
+
+		evalSlot = LAZY_TTS_EVAL(lockedSlot);
+		result = heapam_tuple_lock_internal(relation, otid, snapshot,
+											evalSlot, cid, *lockmode,
+											LockWaitBlock,
+											TUPLE_LOCK_FLAG_FIND_LAST_VERSION,
+											tmfd, true);
+
+		if (result == TM_Ok)
+		{
+			tmfd->traversed = true;
+			return TM_Updated;
+		}
+	}
+
 	return result;
 }
 
@@ -360,10 +425,26 @@ heapam_tuple_lock(Relation relation, ItemPointer tid, Snapshot snapshot,
 				  TupleTableSlot *slot, CommandId cid, LockTupleMode mode,
 				  LockWaitPolicy wait_policy, uint8 flags,
 				  TM_FailureData *tmfd)
+{
+	return heapam_tuple_lock_internal(relation, tid, snapshot, slot, cid,
+									  mode, wait_policy, flags, tmfd, false);
+}
+
+/*
+ * This routine does the work for heapam_tuple_lock(), but also support
+ * `updated` argument to re-use the work done by heapam_tuple_update() or
+ * heapam_tuple_delete() on figuring out that tuple was concurrently updated.
+ */
+static TM_Result
+heapam_tuple_lock_internal(Relation relation, ItemPointer tid,
+						   Snapshot snapshot, TupleTableSlot *slot,
+						   CommandId cid, LockTupleMode mode,
+						   LockWaitPolicy wait_policy, uint8 flags,
+						   TM_FailureData *tmfd, bool updated)
 {
 	BufferHeapTupleTableSlot *bslot = (BufferHeapTupleTableSlot *) slot;
 	TM_Result	result;
-	Buffer		buffer;
+	Buffer		buffer = InvalidBuffer;
 	HeapTuple	tuple = &bslot->base.tupdata;
 	bool		follow_updates;
 
@@ -374,16 +455,26 @@ heapam_tuple_lock(Relation relation, ItemPointer tid, Snapshot snapshot,
 
 tuple_lock_retry:
 	tuple->t_self = *tid;
-	result = heap_lock_tuple(relation, tuple, cid, mode, wait_policy,
+	if (!updated)
-							 follow_updates, &buffer, tmfd);
+		result = heap_lock_tuple(relation, tuple, cid, mode, wait_policy,
+								 follow_updates, &buffer, tmfd);
+	else
+		result = TM_Updated;
 
 	if (result == TM_Updated &&
 		(flags & TUPLE_LOCK_FLAG_FIND_LAST_VERSION))
 	{
-		/* Should not encounter speculative tuple on recheck */
+		if (!updated)
-		Assert(!HeapTupleHeaderIsSpeculative(tuple->t_data));
+		{
+			/* Should not encounter speculative tuple on recheck */
+			Assert(!HeapTupleHeaderIsSpeculative(tuple->t_data));
 
-		ReleaseBuffer(buffer);
+			ReleaseBuffer(buffer);
+		}
+		else
+		{
+			updated = false;
+		}
 
 		if (!ItemPointerEquals(&tmfd->ctid, &tuple->t_self))
 		{

src/backend/access/table/tableam.c

@@ -306,7 +306,8 @@ simple_table_tuple_delete(Relation rel, ItemPointer tid, Snapshot snapshot)
 								GetCurrentCommandId(true),
 								snapshot, InvalidSnapshot,
 								true /* wait for commit */ ,
-								&tmfd, false /* changingPart */ );
+								&tmfd, false /* changingPart */ ,
+								NULL);
 
 	switch (result)
 	{
@@ -355,7 +356,8 @@ simple_table_tuple_update(Relation rel, ItemPointer otid,
 								GetCurrentCommandId(true),
 								snapshot, InvalidSnapshot,
 								true /* wait for commit */ ,
-								&tmfd, &lockmode, update_indexes);
+								&tmfd, &lockmode, update_indexes,
+								NULL);
 
 	switch (result)
 	{

src/backend/executor/nodeModifyTable.c

@@ -1324,26 +1324,62 @@ ExecDeletePrologue(ModifyTableContext *context, ResultRelInfo *resultRelInfo,
 	return true;
 }
 
+/*
+ * The implementation for LazyTupleTableSlot wrapper for EPQ slot to be passed
+ * to table_tuple_update()/table_tuple_delete().
+ */
+typedef struct
+{
+	EPQState   *epqstate;
+	ResultRelInfo *resultRelInfo;
+} GetEPQSlotArg;
+
+static TupleTableSlot *
+GetEPQSlot(void *arg)
+{
+	GetEPQSlotArg *slotArg = (GetEPQSlotArg *) arg;
+
+	return EvalPlanQualSlot(slotArg->epqstate,
+							slotArg->resultRelInfo->ri_RelationDesc,
+							slotArg->resultRelInfo->ri_RangeTableIndex);
+}
+
 /*
  * ExecDeleteAct -- subroutine for ExecDelete
  *
  * Actually delete the tuple from a plain table.
  *
+ * If the 'lockUpdated' flag is set and the target tuple is updated, then
+ * the latest version gets locked and fetched into the EPQ slot.
+ *
  * Caller is in charge of doing EvalPlanQual as necessary
  */
 static TM_Result
 ExecDeleteAct(ModifyTableContext *context, ResultRelInfo *resultRelInfo,
-			  ItemPointer tupleid, bool changingPart)
+			  ItemPointer tupleid, bool changingPart, bool lockUpdated)
 {
 	EState	   *estate = context->estate;
+	GetEPQSlotArg slotArg = {context->epqstate, resultRelInfo};
+	LazyTupleTableSlot lazyEPQSlot,
+			   *lazyEPQSlotPtr;
 
+	if (lockUpdated)
+	{
+		MAKE_LAZY_TTS(&lazyEPQSlot, GetEPQSlot, &slotArg);
+		lazyEPQSlotPtr = &lazyEPQSlot;
+	}
+	else
+	{
+		lazyEPQSlotPtr = NULL;
+	}
 	return table_tuple_delete(resultRelInfo->ri_RelationDesc, tupleid,
 							  estate->es_output_cid,
 							  estate->es_snapshot,
 							  estate->es_crosscheck_snapshot,
 							  true /* wait for commit */ ,
 							  &context->tmfd,
-							  changingPart);
+							  changingPart,
+							  lazyEPQSlotPtr);
 }
 
 /*
@@ -1488,7 +1524,8 @@ ExecDelete(ModifyTableContext *context,
 		 * transaction-snapshot mode transactions.
 		 */
 ldelete:
-		result = ExecDeleteAct(context, resultRelInfo, tupleid, changingPart);
+		result = ExecDeleteAct(context, resultRelInfo, tupleid, changingPart,
+							   !IsolationUsesXactSnapshot());
 
 		switch (result)
 		{
@@ -1541,103 +1578,49 @@ ldelete:
 								 errmsg("could not serialize access due to concurrent update")));
 
 					/*
-					 * Already know that we're going to need to do EPQ, so
+					 * ExecDeleteAct() has already locked the old tuple for
-					 * fetch tuple directly into the right slot.
+					 * us. Now we need to copy it to the right slot.
 					 */
 					EvalPlanQualBegin(context->epqstate);
 					inputslot = EvalPlanQualSlot(context->epqstate, resultRelationDesc,
 												 resultRelInfo->ri_RangeTableIndex);
 
-					result = table_tuple_lock(resultRelationDesc, tupleid,
+					/*
-											  estate->es_snapshot,
+					 * Save locked table for further processing for RETURNING
-											  inputslot, estate->es_output_cid,
+					 * clause.
-											  LockTupleExclusive, LockWaitBlock,
+					 */
-											  TUPLE_LOCK_FLAG_FIND_LAST_VERSION,
+					if (processReturning &&
-											  &context->tmfd);
+						resultRelInfo->ri_projectReturning &&
-
+						!resultRelInfo->ri_FdwRoutine)
-					switch (result)
 					{
-						case TM_Ok:
+						TupleTableSlot *returningSlot;
-							Assert(context->tmfd.traversed);
 
-							/*
+						returningSlot = ExecGetReturningSlot(estate,
-							 * Save locked tuple for further processing of
+															 resultRelInfo);
-							 * RETURNING clause.
+						ExecCopySlot(returningSlot, inputslot);
-							 */
+						ExecMaterializeSlot(returningSlot);
-							if (processReturning &&
-								resultRelInfo->ri_projectReturning &&
-								!resultRelInfo->ri_FdwRoutine)
-							{
-								TupleTableSlot *returningSlot;
-
-								returningSlot = ExecGetReturningSlot(estate, resultRelInfo);
-								ExecCopySlot(returningSlot, inputslot);
-								ExecMaterializeSlot(returningSlot);
-							}
-
-							epqslot = EvalPlanQual(context->epqstate,
-												   resultRelationDesc,
-												   resultRelInfo->ri_RangeTableIndex,
-												   inputslot);
-							if (TupIsNull(epqslot))
-								/* Tuple not passing quals anymore, exiting... */
-								return NULL;
-
-							/*
-							 * If requested, skip delete and pass back the
-							 * updated row.
-							 */
-							if (epqreturnslot)
-							{
-								*epqreturnslot = epqslot;
-								return NULL;
-							}
-							else
-								goto ldelete;
-
-						case TM_SelfModified:
-
-							/*
-							 * This can be reached when following an update
-							 * chain from a tuple updated by another session,
-							 * reaching a tuple that was already updated in
-							 * this transaction. If previously updated by this
-							 * command, ignore the delete, otherwise error
-							 * out.
-							 *
-							 * See also TM_SelfModified response to
-							 * table_tuple_delete() above.
-							 */
-							if (context->tmfd.cmax != estate->es_output_cid)
-								ereport(ERROR,
-										(errcode(ERRCODE_TRIGGERED_DATA_CHANGE_VIOLATION),
-										 errmsg("tuple to be deleted was already modified by an operation triggered by the current command"),
-										 errhint("Consider using an AFTER trigger instead of a BEFORE trigger to propagate changes to other rows.")));
-							return NULL;
-
-						case TM_Deleted:
-							/* tuple already deleted; nothing to do */
-							return NULL;
-
-						default:
-
-							/*
-							 * TM_Invisible should be impossible because we're
-							 * waiting for updated row versions, and would
-							 * already have errored out if the first version
-							 * is invisible.
-							 *
-							 * TM_Updated should be impossible, because we're
-							 * locking the latest version via
-							 * TUPLE_LOCK_FLAG_FIND_LAST_VERSION.
-							 */
-							elog(ERROR, "unexpected table_tuple_lock status: %u",
-								 result);
-							return NULL;
 					}
 
-					Assert(false);
+					Assert(context->tmfd.traversed);
-					break;
+					epqslot = EvalPlanQual(context->epqstate,
+										   resultRelationDesc,
+										   resultRelInfo->ri_RangeTableIndex,
+										   inputslot);
+					if (TupIsNull(epqslot))
+						/* Tuple not passing quals anymore, exiting... */
+						return NULL;
+
+					/*
+					 * If requested, skip delete and pass back the updated
+					 * row.
+					 */
+					if (epqreturnslot)
+					{
+						*epqreturnslot = epqslot;
+						return NULL;
+					}
+					else
+						goto ldelete;
 				}
 
 			case TM_Deleted:
@@ -1982,12 +1965,15 @@ ExecUpdatePrepareSlot(ResultRelInfo *resultRelInfo,
 static TM_Result
 ExecUpdateAct(ModifyTableContext *context, ResultRelInfo *resultRelInfo,
 			  ItemPointer tupleid, HeapTuple oldtuple, TupleTableSlot *slot,
-			  bool canSetTag, UpdateContext *updateCxt)
+			  bool canSetTag, bool lockUpdated, UpdateContext *updateCxt)
 {
 	EState	   *estate = context->estate;
 	Relation	resultRelationDesc = resultRelInfo->ri_RelationDesc;
 	bool		partition_constraint_failed;
 	TM_Result	result;
+	GetEPQSlotArg slotArg = {context->epqstate, resultRelInfo};
+	LazyTupleTableSlot lazyEPQSlot,
+			   *lazyEPQSlotPtr;
 
 	updateCxt->crossPartUpdate = false;
 
@@ -2113,13 +2099,23 @@ lreplace:
 	 * for referential integrity updates in transaction-snapshot mode
 	 * transactions.
 	 */
+	if (lockUpdated)
+	{
+		MAKE_LAZY_TTS(&lazyEPQSlot, GetEPQSlot, &slotArg);
+		lazyEPQSlotPtr = &lazyEPQSlot;
+	}
+	else
+	{
+		lazyEPQSlotPtr = NULL;
+	}
 	result = table_tuple_update(resultRelationDesc, tupleid, slot,
 								estate->es_output_cid,
 								estate->es_snapshot,
 								estate->es_crosscheck_snapshot,
 								true /* wait for commit */ ,
 								&context->tmfd, &updateCxt->lockmode,
-								&updateCxt->updateIndexes);
+								&updateCxt->updateIndexes,
+								lazyEPQSlotPtr);
 	if (result == TM_Ok)
 		updateCxt->updated = true;
 
@@ -2273,7 +2269,7 @@ ExecCrossPartitionUpdateForeignKey(ModifyTableContext *context,
 static TupleTableSlot *
 ExecUpdate(ModifyTableContext *context, ResultRelInfo *resultRelInfo,
 		   ItemPointer tupleid, HeapTuple oldtuple, TupleTableSlot *slot,
-		   bool canSetTag)
+		   bool canSetTag, bool locked)
 {
 	EState	   *estate = context->estate;
 	Relation	resultRelationDesc = resultRelInfo->ri_RelationDesc;
@@ -2335,7 +2331,8 @@ ExecUpdate(ModifyTableContext *context, ResultRelInfo *resultRelInfo,
 		 */
 redo_act:
 		result = ExecUpdateAct(context, resultRelInfo, tupleid, oldtuple, slot,
-							   canSetTag, &updateCxt);
+							   canSetTag, !IsolationUsesXactSnapshot(),
+							   &updateCxt);
 
 		/*
 		 * If ExecUpdateAct reports that a cross-partition update was done,
@@ -2394,81 +2391,39 @@ redo_act:
 						ereport(ERROR,
 								(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
 								 errmsg("could not serialize access due to concurrent update")));
+					Assert(!locked);
 
 					/*
-					 * Already know that we're going to need to do EPQ, so
+					 * ExecUpdateAct() has already locked the old tuple for
-					 * fetch tuple directly into the right slot.
+					 * us. Now we need to copy it to the right slot.
 					 */
 					inputslot = EvalPlanQualSlot(context->epqstate, resultRelationDesc,
 												 resultRelInfo->ri_RangeTableIndex);
 
-					result = table_tuple_lock(resultRelationDesc, tupleid,
+					/* Make sure ri_oldTupleSlot is initialized. */
-											  estate->es_snapshot,
+					if (unlikely(!resultRelInfo->ri_projectNewInfoValid))
-											  inputslot, estate->es_output_cid,
+						ExecInitUpdateProjection(context->mtstate,
-											  updateCxt.lockmode, LockWaitBlock,
+												 resultRelInfo);
-											  TUPLE_LOCK_FLAG_FIND_LAST_VERSION,
-											  &context->tmfd);
 
-					switch (result)
+					/*
-					{
+					 * Save the locked tuple for further calculation of the
-						case TM_Ok:
+					 * new tuple.
-							Assert(context->tmfd.traversed);
+					 */
+					oldSlot = resultRelInfo->ri_oldTupleSlot;
+					ExecCopySlot(oldSlot, inputslot);
+					ExecMaterializeSlot(oldSlot);
+					Assert(context->tmfd.traversed);
 
-							/* Make sure ri_oldTupleSlot is initialized. */
+					epqslot = EvalPlanQual(context->epqstate,
-							if (unlikely(!resultRelInfo->ri_projectNewInfoValid))
+										   resultRelationDesc,
-								ExecInitUpdateProjection(context->mtstate,
+										   resultRelInfo->ri_RangeTableIndex,
-														 resultRelInfo);
+										   inputslot);
-
+					if (TupIsNull(epqslot))
-							/*
+						/* Tuple not passing quals anymore, exiting... */
-							 * Save the locked tuple for further calculation
+						return NULL;
-							 * of the new tuple.
+					slot = ExecGetUpdateNewTuple(resultRelInfo,
-							 */
+												 epqslot, oldSlot);
-							oldSlot = resultRelInfo->ri_oldTupleSlot;
+					goto redo_act;
-							ExecCopySlot(oldSlot, inputslot);
-							ExecMaterializeSlot(oldSlot);
-
-							epqslot = EvalPlanQual(context->epqstate,
-												   resultRelationDesc,
-												   resultRelInfo->ri_RangeTableIndex,
-												   inputslot);
-							if (TupIsNull(epqslot))
-								/* Tuple not passing quals anymore, exiting... */
-								return NULL;
-
-							slot = ExecGetUpdateNewTuple(resultRelInfo,
-														 epqslot, oldSlot);
-							goto redo_act;
-
-						case TM_Deleted:
-							/* tuple already deleted; nothing to do */
-							return NULL;
-
-						case TM_SelfModified:
-
-							/*
-							 * This can be reached when following an update
-							 * chain from a tuple updated by another session,
-							 * reaching a tuple that was already updated in
-							 * this transaction. If previously modified by
-							 * this command, ignore the redundant update,
-							 * otherwise error out.
-							 *
-							 * See also TM_SelfModified response to
-							 * table_tuple_update() above.
-							 */
-							if (context->tmfd.cmax != estate->es_output_cid)
-								ereport(ERROR,
-										(errcode(ERRCODE_TRIGGERED_DATA_CHANGE_VIOLATION),
-										 errmsg("tuple to be updated was already modified by an operation triggered by the current command"),
-										 errhint("Consider using an AFTER trigger instead of a BEFORE trigger to propagate changes to other rows.")));
-							return NULL;
-
-						default:
-							/* see table_tuple_lock call in ExecDelete() */
-							elog(ERROR, "unexpected table_tuple_lock status: %u",
-								 result);
-							return NULL;
-					}
 				}
 
 				break;
@@ -2710,7 +2665,7 @@ ExecOnConflictUpdate(ModifyTableContext *context,
 	*returning = ExecUpdate(context, resultRelInfo,
 							conflictTid, NULL,
 							resultRelInfo->ri_onConflict->oc_ProjSlot,
-							canSetTag);
+							canSetTag, true);
 
 	/*
 	 * Clear out existing tuple, as there might not be another conflict among
@@ -2913,7 +2868,7 @@ lmerge_matched:
 					break;		/* concurrent update/delete */
 				}
 				result = ExecUpdateAct(context, resultRelInfo, tupleid, NULL,
-									   newslot, false, &updateCxt);
+									   newslot, false, false, &updateCxt);
 				if (result == TM_Ok && updateCxt.updated)
 				{
 					ExecUpdateEpilogue(context, &updateCxt, resultRelInfo,
@@ -2931,7 +2886,8 @@ lmerge_matched:
 						return true;	/* "do nothing" */
 					break;		/* concurrent update/delete */
 				}
-				result = ExecDeleteAct(context, resultRelInfo, tupleid, false);
+				result = ExecDeleteAct(context, resultRelInfo, tupleid,
+									   false, false);
 				if (result == TM_Ok)
 				{
 					ExecDeleteEpilogue(context, resultRelInfo, tupleid, NULL,
@@ -3837,7 +3793,7 @@ ExecModifyTable(PlanState *pstate)
 
 				/* Now apply the update. */
 				slot = ExecUpdate(&context, resultRelInfo, tupleid, oldtuple,
-								  slot, node->canSetTag);
+								  slot, node->canSetTag, false);
 				break;
 
 			case CMD_DELETE:

src/include/access/tableam.h

@@ -530,7 +530,8 @@ typedef struct TableAmRoutine
 								 Snapshot crosscheck,
 								 bool wait,
 								 TM_FailureData *tmfd,
-								 bool changingPart);
+								 bool changingPart,
+								 LazyTupleTableSlot *lockedSlot);
 
 	/* see table_tuple_update() for reference about parameters */
 	TM_Result	(*tuple_update) (Relation rel,
@@ -542,7 +543,8 @@ typedef struct TableAmRoutine
 								 bool wait,
 								 TM_FailureData *tmfd,
 								 LockTupleMode *lockmode,
-								 TU_UpdateIndexes *update_indexes);
+								 TU_UpdateIndexes *update_indexes,
+								 LazyTupleTableSlot *lockedSlot);
 
 	/* see table_tuple_lock() for reference about parameters */
 	TM_Result	(*tuple_lock) (Relation rel,
@@ -1457,7 +1459,7 @@ table_multi_insert(Relation rel, TupleTableSlot **slots, int nslots,
 }
 
 /*
- * Delete a tuple.
+ * Delete a tuple (or lock last tuple version if lockedSlot is given).
  *
  * NB: do not call this directly unless prepared to deal with
  * concurrent-update conditions.  Use simple_table_tuple_delete instead.
@@ -1473,6 +1475,8 @@ table_multi_insert(Relation rel, TupleTableSlot **slots, int nslots,
  *	tmfd - filled in failure cases (see below)
  *	changingPart - true iff the tuple is being moved to another partition
  *		table due to an update of the partition key. Otherwise, false.
+ *	lockedSlot - lazy slot to save the locked tuple if should lock the last
+ *		row version during the concurrent update. NULL if not needed.
  *
  * Normal, successful return value is TM_Ok, which means we did actually
  * delete it.  Failure return codes are TM_SelfModified, TM_Updated, and
@@ -1485,15 +1489,17 @@ table_multi_insert(Relation rel, TupleTableSlot **slots, int nslots,
 static inline TM_Result
 table_tuple_delete(Relation rel, ItemPointer tid, CommandId cid,
 				   Snapshot snapshot, Snapshot crosscheck, bool wait,
-				   TM_FailureData *tmfd, bool changingPart)
+				   TM_FailureData *tmfd, bool changingPart,
+				   LazyTupleTableSlot *lockedSlot)
 {
 	return rel->rd_tableam->tuple_delete(rel, tid, cid,
 										 snapshot, crosscheck,
-										 wait, tmfd, changingPart);
+										 wait, tmfd, changingPart,
+										 lockedSlot);
 }
 
 /*
- * Update a tuple.
+ * Update a tuple (or lock last tuple version if lockedSlot is given).
  *
  * NB: do not call this directly unless you are prepared to deal with
  * concurrent-update conditions.  Use simple_table_tuple_update instead.
@@ -1511,7 +1517,9 @@ table_tuple_delete(Relation rel, ItemPointer tid, CommandId cid,
  *	lockmode - filled with lock mode acquired on tuple
  *  update_indexes - in success cases this is set to true if new index entries
  *		are required for this tuple
- *
+ * 	lockedSlot - lazy slot to save the locked tuple if should lock the last
+ *		row version during the concurrent update. NULL if not needed.
+
  * Normal, successful return value is TM_Ok, which means we did actually
  * update it.  Failure return codes are TM_SelfModified, TM_Updated, and
  * TM_BeingModified (the last only possible if wait == false).
@@ -1530,12 +1538,14 @@ static inline TM_Result
 table_tuple_update(Relation rel, ItemPointer otid, TupleTableSlot *slot,
 				   CommandId cid, Snapshot snapshot, Snapshot crosscheck,
 				   bool wait, TM_FailureData *tmfd, LockTupleMode *lockmode,
-				   TU_UpdateIndexes *update_indexes)
+				   TU_UpdateIndexes *update_indexes,
+				   LazyTupleTableSlot *lockedSlot)
 {
 	return rel->rd_tableam->tuple_update(rel, otid, slot,
 										 cid, snapshot, crosscheck,
 										 wait, tmfd,
-										 lockmode, update_indexes);
+										 lockmode, update_indexes,
+										 lockedSlot);
 }
 
 /*

src/include/executor/tuptable.h

@@ -300,6 +300,44 @@ typedef struct MinimalTupleTableSlot
 #define TupIsNull(slot) \
 	((slot) == NULL || TTS_EMPTY(slot))
 
+/*----------
+ * LazyTupleTableSlot -- a lazy version of TupleTableSlot.
+ *
+ * Sometimes caller might need to pass to the function a slot, which most
+ * likely will reain undemanded.  Preallocating such slot would be a waste of
+ * resources in the  majority of cases.  Lazy slot is aimed to resolve this
+ * problem.  It is basically a promise to allocate the slot once it's needed.
+ * Once callee needs the slot, it could get it using LAZY_TTS_EVAL(lazySlot)
+ * macro.
+ */
+typedef struct
+{
+	TupleTableSlot *slot;		/* cached slot or NULL if not yet allocated */
+	TupleTableSlot *(*getSlot) (void *arg); /* callback for slot allocation */
+	void	   *getSlotArg;		/* argument for the callback above */
+} LazyTupleTableSlot;
+
+/*
+ * A constructor for the lazy slot.
+ */
+#define MAKE_LAZY_TTS(lazySlot, callback, arg) \
+	do { \
+		(lazySlot)->slot = NULL; \
+		(lazySlot)->getSlot = callback; \
+		(lazySlot)->getSlotArg = arg; \
+	} while (false)
+
+/*
+ * Macro for lazy slot evaluation.  NULL lazy slot evaluates to NULL slot.
+ * Cached version is used if present.  Use the callback otherwise.
+ */
+#define LAZY_TTS_EVAL(lazySlot) \
+	((lazySlot) ? \
+		((lazySlot)->slot ? \
+			(lazySlot)->slot : \
+			((lazySlot)->slot = (lazySlot)->getSlot((lazySlot)->getSlotArg))) : \
+		NULL)
+
 /* in executor/execTuples.c */
 extern TupleTableSlot *MakeTupleTableSlot(TupleDesc tupleDesc,
 										  const TupleTableSlotOps *tts_ops);

src/tools/pgindent/typedefs.list

@@ -955,6 +955,7 @@ GenerationPointer
 GenericCosts
 GenericXLogState
 GeqoPrivateData
+GetEPQSlotArg
 GetForeignJoinPaths_function
 GetForeignModifyBatchSize_function
 GetForeignPaths_function
@@ -1399,6 +1400,7 @@ LagTracker
 LargeObjectDesc
 LastAttnumInfo
 Latch
+LazyTupleTableSlot
 LerpFunc
 LexDescr
 LexemeEntry