1 files changed, 16 insertions, 37 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index a87f09ee47f..1ac1c0550dd 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -6330,7 +6330,6 @@ StartupXLOG(void)
 	SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB);
 	SetCommitTsLimit(checkPoint.oldestCommitTs,
 					 checkPoint.newestCommitTs);
-	MultiXactSetSafeTruncate(checkPoint.oldestMulti);
 	XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
 	XLogCtl->ckptXid = checkPoint.nextXid;
 
@@ -6347,10 +6346,8 @@ StartupXLOG(void)
 	StartupReorderBuffer();
 
 	/*
-	 * Startup MultiXact.  We need to do this early for two reasons: one is
-	 * that we might try to access multixacts when we do tuple freezing, and
-	 * the other is we need its state initialized because we attempt
-	 * truncation during restartpoints.
+	 * Startup MultiXact. We need to do this early to be able to replay
+	 * truncations.
 	 */
 	StartupMultiXact();
 
@@ -8508,12 +8505,6 @@ CreateCheckPoint(int flags)
 	END_CRIT_SECTION();
 
 	/*
-	 * Now that the checkpoint is safely on disk, we can update the point to
-	 * which multixact can be truncated.
-	 */
-	MultiXactSetSafeTruncate(checkPoint.oldestMulti);
-
-	/*
 	 * Let smgr do post-checkpoint cleanup (eg, deleting old files).
 	 */
 	smgrpostckpt();
@@ -8552,11 +8543,6 @@ CreateCheckPoint(int flags)
 	if (!RecoveryInProgress())
 		TruncateSUBTRANS(GetOldestXmin(NULL, false));
 
-	/*
-	 * Truncate pg_multixact too.
-	 */
-	TruncateMultiXact();
-
 	/* Real work is done, but log and update stats before releasing lock. */
 	LogCheckpointEnd(false);
 
@@ -8887,21 +8873,6 @@ CreateRestartPoint(int flags)
 	}
 
 	/*
-	 * Due to a historical accident multixact truncations are not WAL-logged,
-	 * but just performed everytime the mxact horizon is increased. So, unless
-	 * we explicitly execute truncations on a standby it will never clean out
-	 * /pg_multixact which obviously is bad, both because it uses space and
-	 * because we can wrap around into pre-existing data...
-	 *
-	 * We can only do the truncation here, after the UpdateControlFile()
-	 * above, because we've now safely established a restart point.  That
-	 * guarantees we will not need to access those multis.
-	 *
-	 * It's probably worth improving this.
-	 */
-	TruncateMultiXact();
-
-	/*
 	 * Truncate pg_subtrans if possible.  We can throw away all data before
 	 * the oldest XMIN of any running transaction.  No future transaction will
 	 * attempt to reference any pg_subtrans entry older than that (see Asserts
@@ -9261,9 +9232,14 @@ xlog_redo(XLogReaderState *record)
 		LWLockRelease(OidGenLock);
 		MultiXactSetNextMXact(checkPoint.nextMulti,
 							  checkPoint.nextMultiOffset);
+
+		/*
+		 * NB: This may perform multixact truncation when replaying WAL
+		 * generated by an older primary.
+		 */
+		MultiXactAdvanceOldest(checkPoint.oldestMulti,
+							   checkPoint.oldestMultiDB);
 		SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
-		SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB);
-		MultiXactSetSafeTruncate(checkPoint.oldestMulti);
 
 		/*
 		 * If we see a shutdown checkpoint while waiting for an end-of-backup
@@ -9353,14 +9329,17 @@ xlog_redo(XLogReaderState *record)
 		LWLockRelease(OidGenLock);
 		MultiXactAdvanceNextMXact(checkPoint.nextMulti,
 								  checkPoint.nextMultiOffset);
+
+		/*
+		 * NB: This may perform multixact truncation when replaying WAL
+		 * generated by an older primary.
+		 */
+		MultiXactAdvanceOldest(checkPoint.oldestMulti,
+							   checkPoint.oldestMultiDB);
 		if (TransactionIdPrecedes(ShmemVariableCache->oldestXid,
 								  checkPoint.oldestXid))
 			SetTransactionIdLimit(checkPoint.oldestXid,
 								  checkPoint.oldestXidDB);
-		MultiXactAdvanceOldest(checkPoint.oldestMulti,
-							   checkPoint.oldestMultiDB);
-		MultiXactSetSafeTruncate(checkPoint.oldestMulti);
-
 		/* ControlFile->checkPointCopy always tracks the latest ckpt XID */
 		ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
 		ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;