diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index d9e10263bb..2806e1076c 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -318,32 +318,43 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 
 /*
  * Helper function for advancing physical replication slot forward.
+ * The LSN position to move to is compared simply to the slot's
+ * restart_lsn, knowing that any position older than that would be
+ * removed by successive checkpoints.
  */
 static XLogRecPtr
-pg_physical_replication_slot_advance(XLogRecPtr startlsn, XLogRecPtr moveto)
+pg_physical_replication_slot_advance(XLogRecPtr moveto)
 {
-	XLogRecPtr	retlsn = InvalidXLogRecPtr;
+	XLogRecPtr	startlsn = MyReplicationSlot->data.restart_lsn;
+	XLogRecPtr	retlsn = startlsn;
 
-	SpinLockAcquire(&MyReplicationSlot->mutex);
-	if (MyReplicationSlot->data.restart_lsn < moveto)
+	if (startlsn < moveto)
 	{
+		SpinLockAcquire(&MyReplicationSlot->mutex);
 		MyReplicationSlot->data.restart_lsn = moveto;
+		SpinLockRelease(&MyReplicationSlot->mutex);
 		retlsn = moveto;
 	}
-	SpinLockRelease(&MyReplicationSlot->mutex);
 
 	return retlsn;
 }
 
 /*
  * Helper function for advancing logical replication slot forward.
+ * The slot's restart_lsn is used as start point for reading records,
+ * while confirmed_lsn is used as base point for the decoding context.
+ * The LSN position to move to is checked by doing a per-record scan and
+ * logical decoding which makes sure that confirmed_lsn is updated to a
+ * LSN which allows the future slot consumer to get consistent logical
+ * changes.
  */
 static XLogRecPtr
-pg_logical_replication_slot_advance(XLogRecPtr startlsn, XLogRecPtr moveto)
+pg_logical_replication_slot_advance(XLogRecPtr moveto)
 {
 	LogicalDecodingContext *ctx;
 	ResourceOwner old_resowner = CurrentResourceOwner;
-	XLogRecPtr	retlsn = InvalidXLogRecPtr;
+	XLogRecPtr	startlsn = MyReplicationSlot->data.restart_lsn;
+	XLogRecPtr	retlsn = MyReplicationSlot->data.confirmed_flush;
 
 	PG_TRY();
 	{
@@ -384,7 +395,7 @@ pg_logical_replication_slot_advance(XLogRecPtr startlsn, XLogRecPtr moveto)
 			if (record != NULL)
 				LogicalDecodingProcessRecord(ctx, ctx->reader);
 
-			/* check limits */
+			/* Stop once the moving point wanted by caller has been reached */
 			if (moveto <= ctx->reader->EndRecPtr)
 				break;
 
@@ -441,7 +452,7 @@ pg_replication_slot_advance(PG_FUNCTION_ARGS)
 	Name		slotname = PG_GETARG_NAME(0);
 	XLogRecPtr	moveto = PG_GETARG_LSN(1);
 	XLogRecPtr	endlsn;
-	XLogRecPtr	startlsn;
+	XLogRecPtr	minlsn;
 	TupleDesc	tupdesc;
 	Datum		values[2];
 	bool		nulls[2];
@@ -472,21 +483,32 @@ pg_replication_slot_advance(PG_FUNCTION_ARGS)
 	/* Acquire the slot so we "own" it */
 	ReplicationSlotAcquire(NameStr(*slotname), true);
 
-	startlsn = MyReplicationSlot->data.confirmed_flush;
-	if (moveto < startlsn)
+	/*
+	 * Check if the slot is not moving backwards.  Physical slots rely simply
+	 * on restart_lsn as a minimum point, while logical slots have confirmed
+	 * consumption up to confirmed_lsn, meaning that in both cases data older
+	 * than that is not available anymore.
+	 */
+	if (OidIsValid(MyReplicationSlot->data.database))
+		minlsn = MyReplicationSlot->data.confirmed_flush;
+	else
+		minlsn = MyReplicationSlot->data.restart_lsn;
+
+	if (moveto < minlsn)
 	{
 		ReplicationSlotRelease();
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("cannot move slot to %X/%X, minimum is %X/%X",
 						(uint32) (moveto >> 32), (uint32) moveto,
-						(uint32) (startlsn >> 32), (uint32) startlsn)));
+						(uint32) (minlsn >> 32), (uint32) minlsn)));
 	}
 
+	/* Do the actual slot update, depending on the slot type */
 	if (OidIsValid(MyReplicationSlot->data.database))
-		endlsn = pg_logical_replication_slot_advance(startlsn, moveto);
+		endlsn = pg_logical_replication_slot_advance(moveto);
 	else
-		endlsn = pg_physical_replication_slot_advance(startlsn, moveto);
+		endlsn = pg_physical_replication_slot_advance(moveto);
 
 	values[0] = NameGetDatum(&MyReplicationSlot->data.name);
 	nulls[0] = false;