diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index 539bc9cb0e3..a9777a7e838 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -2656,9 +2656,14 @@ asyncQueueReadAllNotifications(void)
 	 * BecomeRegisteredListener has already added us to the listener array,
 	 * so no not-yet-committed messages can be removed from the queue
 	 * before we see them.
+	 *
+	 * Since we'll only use this snapshot to check whether XIDs are still
+	 * running, and we will release it before returning, it's okay to use
+	 * GetSecondarySnapshot(); that avoids possibly blocking on other
+	 * transactions in SERIALIZABLE READ ONLY DEFERRABLE mode.
 	 *----------
 	 */
-	snapshot = RegisterSnapshot(GetLatestSnapshot());
+	snapshot = RegisterSnapshot(GetSecondarySnapshot());
 
 	/*
 	 * It is possible that we fail while trying to send a message to our
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 048450da2d3..6defd7203b1 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -376,6 +376,36 @@ GetLatestSnapshot(void)
 	return SecondarySnapshot;
 }
 
+/*
+ * GetSecondarySnapshot
+ *		Get a snapshot that is up-to-date as of the current instant.
+ *
+ * This differs from GetLatestSnapshot() in that it will never set the
+ * transaction snapshot, so that it does not need to block waiting for a safe
+ * snapshot in RO deferrable serializable transaction mode.  That makes it
+ * semantically dubious to use the result for testing tuple visibility.
+ * Current callers only use it to test whether XIDs are still running.
+ *
+ * An additional constraint is that if the caller registers the snapshot, it
+ * must unregister it before any possible call of GetTransactionSnapshot();
+ * else the empty-heap Assert therein could fire.
+ */
+Snapshot
+GetSecondarySnapshot(void)
+{
+	/*
+	 * We might be able to relax this, but nothing that could otherwise work
+	 * needs it.
+	 */
+	if (IsInParallelMode())
+		elog(ERROR,
+			 "cannot update SecondarySnapshot during a parallel operation");
+
+	SecondarySnapshot = GetSnapshotData(&SecondarySnapshotData);
+
+	return SecondarySnapshot;
+}
+
 /*
  * GetCatalogSnapshot
  *		Get a snapshot that is sufficiently up-to-date for scan of the
diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index 1c550096393..be3b2e5e5c9 100644
--- a/src/include/utils/snapmgr.h
+++ b/src/include/utils/snapmgr.h
@@ -76,6 +76,7 @@ extern PGDLLIMPORT SnapshotData SnapshotToastData;
 
 extern Snapshot GetTransactionSnapshot(void);
 extern Snapshot GetLatestSnapshot(void);
+extern Snapshot GetSecondarySnapshot(void);
 extern void SnapshotSetCommandId(CommandId curcid);
 
 extern Snapshot GetCatalogSnapshot(Oid relid);