Avoid uselessly looking up old LOCK_ONLY multixacts

Commit 0ac5ad5 removed an optimization in multixact.c that skipped
fetching members of MultiXactId that were older than our
OldestVisibleMXactId value.  The reason this was removed is that it is
possible for multixacts that contain updates to be older than that
value.  However, if the caller is certain that the multi does not
contain an update (because the infomask bits say so), it can pass this
info down to GetMultiXactIdMembers, enabling it to use the old
optimization.

Pointed out by Andres Freund in 20131121200517.GM7240@alap2.anarazel.de

Author: alvherre

contrib/pgrowlocks/pgrowlocks.c

@@ -165,7 +165,8 @@ pgrowlocks(PG_FUNCTION_ARGS)
 
 				allow_old = !(infomask & HEAP_LOCK_MASK) &&
 					(infomask & HEAP_XMAX_LOCK_ONLY);
-				nmembers = GetMultiXactIdMembers(xmax, &members, allow_old);
+				nmembers = GetMultiXactIdMembers(xmax, &members, allow_old,
+												 false);
 				if (nmembers == -1)
 				{
 					values[Atnum_xids] = "{0}";

src/backend/access/heap/heapam.c

@@ -4183,7 +4183,9 @@ heap_lock_tuple(Relation relation, HeapTuple tuple,
 			 * the case, HeapTupleSatisfiesUpdate would have returned
 			 * MayBeUpdated and we wouldn't be here.
 			 */
-			nmembers = GetMultiXactIdMembers(xwait, &members, false);
+			nmembers =
+				GetMultiXactIdMembers(xwait, &members, false,
+									  HEAP_XMAX_IS_LOCKED_ONLY(infomask));
 
 			for (i = 0; i < nmembers; i++)
 			{
@@ -4204,7 +4206,8 @@ heap_lock_tuple(Relation relation, HeapTuple tuple,
 				}
 			}
 
-			pfree(members);
+			if (members)
+				pfree(members);
 		}
 
 		/*
@@ -4353,7 +4356,9 @@ heap_lock_tuple(Relation relation, HeapTuple tuple,
 				 * been the case, HeapTupleSatisfiesUpdate would have returned
 				 * MayBeUpdated and we wouldn't be here.
 				 */
-				nmembers = GetMultiXactIdMembers(xwait, &members, false);
+				nmembers =
+					GetMultiXactIdMembers(xwait, &members, false,
+										  HEAP_XMAX_IS_LOCKED_ONLY(infomask));
 
 				if (nmembers <= 0)
 				{
@@ -4834,7 +4839,7 @@ compute_new_xmax_infomask(TransactionId xmax, uint16 old_infomask,
 		 * MultiXactIdExpand if we weren't to do this, so this check is not
 		 * incurring extra work anyhow.
 		 */
-		if (!MultiXactIdIsRunning(xmax))
+		if (!MultiXactIdIsRunning(xmax, HEAP_XMAX_IS_LOCKED_ONLY(old_infomask)))
 		{
 			if (HEAP_XMAX_IS_LOCKED_ONLY(old_infomask) ||
 				TransactionIdDidAbort(MultiXactIdGetUpdateXid(xmax,
@@ -5175,7 +5180,8 @@ heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
 				int			i;
 				MultiXactMember *members;
 
-				nmembers = GetMultiXactIdMembers(rawxmax, &members, false);
+				nmembers = GetMultiXactIdMembers(rawxmax, &members, false,
+									 HEAP_XMAX_IS_LOCKED_ONLY(old_infomask));
 				for (i = 0; i < nmembers; i++)
 				{
 					HTSU_Result res;
@@ -5533,7 +5539,8 @@ FreezeMultiXactId(MultiXactId multi, uint16 t_infomask,
 		 */
 		Assert((!(t_infomask & HEAP_LOCK_MASK) &&
 				HEAP_XMAX_IS_LOCKED_ONLY(t_infomask)) ||
-			   !MultiXactIdIsRunning(multi));
+			   !MultiXactIdIsRunning(multi,
+									 HEAP_XMAX_IS_LOCKED_ONLY(t_infomask)));
 		if (HEAP_XMAX_IS_LOCKED_ONLY(t_infomask))
 		{
 			*flags |= FRM_INVALIDATE_XMAX;
@@ -5576,7 +5583,9 @@ FreezeMultiXactId(MultiXactId multi, uint16 t_infomask,
 
 	allow_old = !(t_infomask & HEAP_LOCK_MASK) &&
 		HEAP_XMAX_IS_LOCKED_ONLY(t_infomask);
-	nmembers = GetMultiXactIdMembers(multi, &members, allow_old);
+	nmembers =
+		GetMultiXactIdMembers(multi, &members, allow_old,
+							  HEAP_XMAX_IS_LOCKED_ONLY(t_infomask));
 	if (nmembers <= 0)
 	{
 		/* Nothing worth keeping */
@@ -5983,7 +5992,7 @@ GetMultiXactIdHintBits(MultiXactId multi, uint16 *new_infomask,
 	 * We only use this in multis we just created, so they cannot be values
 	 * pre-pg_upgrade.
 	 */
-	nmembers = GetMultiXactIdMembers(multi, &members, false);
+	nmembers = GetMultiXactIdMembers(multi, &members, false, false);
 
 	for (i = 0; i < nmembers; i++)
 	{
@@ -6062,7 +6071,7 @@ MultiXactIdGetUpdateXid(TransactionId xmax, uint16 t_infomask)
 	 * Since we know the LOCK_ONLY bit is not set, this cannot be a multi from
 	 * pre-pg_upgrade.
 	 */
-	nmembers = GetMultiXactIdMembers(xmax, &members, false);
+	nmembers = GetMultiXactIdMembers(xmax, &members, false, false);
 
 	if (nmembers > 0)
 	{
@@ -6148,7 +6157,8 @@ Do_MultiXactIdWait(MultiXactId multi, MultiXactStatus status,
 	int			remain = 0;
 
 	allow_old = !(infomask & HEAP_LOCK_MASK) && HEAP_XMAX_IS_LOCKED_ONLY(infomask);
-	nmembers = GetMultiXactIdMembers(multi, &members, allow_old);
+	nmembers = GetMultiXactIdMembers(multi, &members, allow_old,
+									 HEAP_XMAX_IS_LOCKED_ONLY(infomask));
 
 	if (nmembers >= 0)
 	{
@@ -6294,7 +6304,8 @@ heap_tuple_needs_freeze(HeapTupleHeader tuple, TransactionId cutoff_xid,
 
 			allow_old = !(tuple->t_infomask & HEAP_LOCK_MASK) &&
 				HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask);
-			nmembers = GetMultiXactIdMembers(multi, &members, allow_old);
+			nmembers = GetMultiXactIdMembers(multi, &members, allow_old,
+								HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask));
 
 			for (i = 0; i < nmembers; i++)
 			{

src/backend/access/transam/multixact.c

@@ -428,7 +428,7 @@ MultiXactIdExpand(MultiXactId multi, TransactionId xid, MultiXactStatus status)
 	 * caller of this function does a check that the multixact is no longer
 	 * running.
 	 */
-	nmembers = GetMultiXactIdMembers(multi, &members, false);
+	nmembers = GetMultiXactIdMembers(multi, &members, false, false);
 
 	if (nmembers < 0)
 	{
@@ -517,7 +517,7 @@ MultiXactIdExpand(MultiXactId multi, TransactionId xid, MultiXactStatus status)
  * a pg_upgraded share-locked tuple.
  */
 bool
-MultiXactIdIsRunning(MultiXactId multi)
+MultiXactIdIsRunning(MultiXactId multi, bool isLockOnly)
 {
 	MultiXactMember *members;
 	int			nmembers;
@@ -529,7 +529,7 @@ MultiXactIdIsRunning(MultiXactId multi)
 	 * "false" here means we assume our callers have checked that the given
 	 * multi cannot possibly come from a pg_upgraded database.
 	 */
-	nmembers = GetMultiXactIdMembers(multi, &members, false);
+	nmembers = GetMultiXactIdMembers(multi, &members, false, isLockOnly);
 
 	if (nmembers < 0)
 	{
@@ -1095,10 +1095,15 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
  * the value currently known as the next to assign, raise an error.  Previously
  * these also returned -1, but since this can lead to the wrong visibility
  * results, it is dangerous to do that.
+ *
+ * onlyLock must be set to true if caller is certain that the given multi
+ * is used only to lock tuples; can be false without loss of correctness,
+ * but passing a true means we can return quickly without checking for
+ * old updates.
  */
 int
 GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
-					  bool allow_old)
+					  bool allow_old, bool onlyLock)
 {
 	int			pageno;
 	int			prev_pageno;
@@ -1132,6 +1137,19 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
 	/* Set our OldestVisibleMXactId[] entry if we didn't already */
 	MultiXactIdSetOldestVisible();
 
+	/*
+	 * If we know the multi is used only for locking and not for updates,
+	 * then we can skip checking if the value is older than our oldest
+	 * visible multi.  It cannot possibly still be running.
+	 */
+	if (onlyLock &&
+		MultiXactIdPrecedes(multi, OldestVisibleMXactId[MyBackendId]))
+	{
+		debug_elog2(DEBUG2, "GetMembers: a locker-only multi is too old");
+		*members = NULL;
+		return -1;
+	}
+
 	/*
 	 * We check known limits on MultiXact before resorting to the SLRU area.
 	 *
@@ -1335,7 +1353,7 @@ MultiXactHasRunningRemoteMembers(MultiXactId multi)
 	int			nmembers;
 	int			i;
 
-	nmembers = GetMultiXactIdMembers(multi, &members, true);
+	nmembers = GetMultiXactIdMembers(multi, &members, true, false);
 	if (nmembers <= 0)
 		return false;
 
@@ -2807,7 +2825,8 @@ pg_get_multixact_members(PG_FUNCTION_ARGS)
 
 		multi = palloc(sizeof(mxact));
 		/* no need to allow for old values here */
-		multi->nmembers = GetMultiXactIdMembers(mxid, &multi->members, false);
+		multi->nmembers = GetMultiXactIdMembers(mxid, &multi->members, false,
+												false);
 		multi->iter = 0;
 
 		tupdesc = CreateTemplateTupleDesc(2, false);

src/backend/utils/time/tqual.c

@@ -607,14 +607,19 @@ HeapTupleSatisfiesUpdate(HeapTuple htup, CommandId curcid,
 			 */
 			if ((tuple->t_infomask & (HEAP_XMAX_EXCL_LOCK |
 									  HEAP_XMAX_KEYSHR_LOCK)) &&
-				MultiXactIdIsRunning(HeapTupleHeaderGetRawXmax(tuple)))
+				MultiXactIdIsRunning(HeapTupleHeaderGetRawXmax(tuple), true))
 				return HeapTupleBeingUpdated;
 
 			SetHintBits(tuple, buffer, HEAP_XMAX_INVALID, InvalidTransactionId);
 			return HeapTupleMayBeUpdated;
 		}
 
 		xmax = HeapTupleGetUpdateXid(tuple);
+		if (!TransactionIdIsValid(xmax))
+		{
+			if (MultiXactIdIsRunning(HeapTupleHeaderGetRawXmax(tuple), false))
+				return HeapTupleBeingUpdated;
+		}
 
 		/* not LOCKED_ONLY, so it has to have an xmax */
 		Assert(TransactionIdIsValid(xmax));
@@ -627,7 +632,7 @@ HeapTupleSatisfiesUpdate(HeapTuple htup, CommandId curcid,
 				return HeapTupleInvisible;		/* updated before scan started */
 		}
 
-		if (TransactionIdIsInProgress(xmax))
+		if (MultiXactIdIsRunning(HeapTupleHeaderGetRawXmax(tuple), false))
 			return HeapTupleBeingUpdated;
 
 		if (TransactionIdDidCommit(xmax))
@@ -638,7 +643,7 @@ HeapTupleSatisfiesUpdate(HeapTuple htup, CommandId curcid,
 		 * what about the other members?
 		 */
 
-		if (!MultiXactIdIsRunning(HeapTupleHeaderGetRawXmax(tuple)))
+		if (!MultiXactIdIsRunning(HeapTupleHeaderGetRawXmax(tuple), false))
 		{
 			/*
 			 * There's no member, even just a locker, alive anymore, so we can
@@ -1240,7 +1245,8 @@ HeapTupleSatisfiesVacuum(HeapTuple htup, TransactionId OldestXmin,
 				 */
 				if ((tuple->t_infomask & (HEAP_XMAX_EXCL_LOCK |
 										  HEAP_XMAX_KEYSHR_LOCK)) &&
-					MultiXactIdIsRunning(HeapTupleHeaderGetRawXmax(tuple)))
+					MultiXactIdIsRunning(HeapTupleHeaderGetRawXmax(tuple),
+										 true))
 					return HEAPTUPLE_LIVE;
 				SetHintBits(tuple, buffer, HEAP_XMAX_INVALID, InvalidTransactionId);
 
@@ -1267,7 +1273,7 @@ HeapTupleSatisfiesVacuum(HeapTuple htup, TransactionId OldestXmin,
 	{
 		TransactionId xmax;
 
-		if (MultiXactIdIsRunning(HeapTupleHeaderGetRawXmax(tuple)))
+		if (MultiXactIdIsRunning(HeapTupleHeaderGetRawXmax(tuple), false))
 		{
 			/* already checked above */
 			Assert(!HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask));

src/include/access/multixact.h

@@ -91,10 +91,10 @@ extern MultiXactId MultiXactIdCreateFromMembers(int nmembers,
 							 MultiXactMember *members);
 
 extern MultiXactId ReadNextMultiXactId(void);
-extern bool MultiXactIdIsRunning(MultiXactId multi);
+extern bool MultiXactIdIsRunning(MultiXactId multi, bool isLockOnly);
 extern void MultiXactIdSetOldestMember(void);
 extern int GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **xids,
-					  bool allow_old);
+					  bool allow_old, bool isLockOnly);
 extern bool MultiXactHasRunningRemoteMembers(MultiXactId multi);
 extern bool MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2);
 extern bool MultiXactIdPrecedesOrEquals(MultiXactId multi1,