diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index 5f690393fc..5fdcff2a3b 100644
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -254,8 +254,11 @@ InitializeParallelDSM(ParallelContext *pcxt)
 		shm_toc_estimate_chunk(&pcxt->estimator, guc_len);
 		combocidlen = EstimateComboCIDStateSpace();
 		shm_toc_estimate_chunk(&pcxt->estimator, combocidlen);
-		tsnaplen = EstimateSnapshotSpace(transaction_snapshot);
-		shm_toc_estimate_chunk(&pcxt->estimator, tsnaplen);
+		if (IsolationUsesXactSnapshot())
+		{
+			tsnaplen = EstimateSnapshotSpace(transaction_snapshot);
+			shm_toc_estimate_chunk(&pcxt->estimator, tsnaplen);
+		}
 		asnaplen = EstimateSnapshotSpace(active_snapshot);
 		shm_toc_estimate_chunk(&pcxt->estimator, asnaplen);
 		tstatelen = EstimateTransactionStateSpace();
@@ -366,11 +369,19 @@ InitializeParallelDSM(ParallelContext *pcxt)
 		SerializeComboCIDState(combocidlen, combocidspace);
 		shm_toc_insert(pcxt->toc, PARALLEL_KEY_COMBO_CID, combocidspace);
 
-		/* Serialize transaction snapshot and active snapshot. */
-		tsnapspace = shm_toc_allocate(pcxt->toc, tsnaplen);
-		SerializeSnapshot(transaction_snapshot, tsnapspace);
-		shm_toc_insert(pcxt->toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT,
-					   tsnapspace);
+		/*
+		 * Serialize the transaction snapshot if the transaction
+		 * isolation-level uses a transaction snapshot.
+		 */
+		if (IsolationUsesXactSnapshot())
+		{
+			tsnapspace = shm_toc_allocate(pcxt->toc, tsnaplen);
+			SerializeSnapshot(transaction_snapshot, tsnapspace);
+			shm_toc_insert(pcxt->toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT,
+						   tsnapspace);
+		}
+
+		/* Serialize the active snapshot. */
 		asnapspace = shm_toc_allocate(pcxt->toc, asnaplen);
 		SerializeSnapshot(active_snapshot, asnapspace);
 		shm_toc_insert(pcxt->toc, PARALLEL_KEY_ACTIVE_SNAPSHOT, asnapspace);
@@ -1261,6 +1272,8 @@ ParallelWorkerMain(Datum main_arg)
 	char	   *uncommittedenumsspace;
 	StringInfoData msgbuf;
 	char	   *session_dsm_handle_space;
+	Snapshot	tsnapshot;
+	Snapshot	asnapshot;
 
 	/* Set flag to indicate that we're initializing a parallel worker. */
 	InitializingParallelWorker = true;
@@ -1408,14 +1421,25 @@ ParallelWorkerMain(Datum main_arg)
 		shm_toc_lookup(toc, PARALLEL_KEY_SESSION_DSM, false);
 	AttachSession(*(dsm_handle *) session_dsm_handle_space);
 
-	/* Restore transaction snapshot. */
-	tsnapspace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT, false);
-	RestoreTransactionSnapshot(RestoreSnapshot(tsnapspace),
-							   fps->parallel_leader_pgproc);
-
-	/* Restore active snapshot. */
+	/*
+	 * If the transaction isolation level is REPEATABLE READ or SERIALIZABLE,
+	 * the leader has serialized the transaction snapshot and we must restore
+	 * it. At lower isolation levels, there is no transaction-lifetime
+	 * snapshot, but we need TransactionXmin to get set to a value which is
+	 * less than or equal to the xmin of every snapshot that will be used by
+	 * this worker. The easiest way to accomplish that is to install the
+	 * active snapshot as the transaction snapshot. Code running in this
+	 * parallel worker might take new snapshots via GetTransactionSnapshot()
+	 * or GetLatestSnapshot(), but it shouldn't have any way of acquiring a
+	 * snapshot older than the active snapshot.
+	 */
 	asnapspace = shm_toc_lookup(toc, PARALLEL_KEY_ACTIVE_SNAPSHOT, false);
-	PushActiveSnapshot(RestoreSnapshot(asnapspace));
+	tsnapspace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT, true);
+	asnapshot = RestoreSnapshot(asnapspace);
+	tsnapshot = tsnapspace ? RestoreSnapshot(tsnapspace) : asnapshot;
+	RestoreTransactionSnapshot(tsnapshot,
+							   fps->parallel_leader_pgproc);
+	PushActiveSnapshot(asnapshot);
 
 	/*
 	 * We've changed which tuples we can see, and must therefore invalidate