diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 8aa1f49..62811f7 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -1685,10 +1685,18 @@ MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin)
 	if (old_snapshot_threshold < 0)
 		return;
 
-	/* Keep track of the latest xmin seen by any process. */
+	/*
+	 * Keep track of the latest xmin seen by any process.  We need to
+	 * update time mapping only if xmin is changed since last time.
+	 */
 	SpinLockAcquire(&oldSnapshotControl->mutex_latest_xmin);
 	if (TransactionIdFollows(xmin, oldSnapshotControl->latest_xmin))
 		oldSnapshotControl->latest_xmin = xmin;
+	else
+	{
+		SpinLockRelease(&oldSnapshotControl->mutex_latest_xmin);
+		return;
+	}
 	SpinLockRelease(&oldSnapshotControl->mutex_latest_xmin);
 
 	/* No further tracking needed for 0 (used for testing). */