slot-advance-comment-v2.patch

diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index c2d0e0c723..7e10a027ca 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -340,6 +340,9 @@ CreateInitDecodingContext(char *plugin,
  * output_plugin_options
  *		contains options passed to the output plugin.
  *
+ * fast_forward
+ *		bypasses the generation of logical changes.
+ *
  * read_page, prepare_write, do_write, update_progress
  *		callbacks that have to be filled to perform the use-case dependent,
  *		actual work.
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 2806e1076c..6c2addd5b7 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -343,10 +343,10 @@ pg_physical_replication_slot_advance(XLogRecPtr moveto)
  * 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.
+ * While we could just use LogicalConfirmReceivedLocation to update
+ * confirmed_flush_lsn, we had better digest WAL to advance restart_lsn
+ * allowing to recycle WAL and old catalog tuples.  As decoding is done
+ * with fast_forward mode, no changes are generated.
  */
 static XLogRecPtr
 pg_logical_replication_slot_advance(XLogRecPtr moveto)
@@ -358,7 +358,10 @@ pg_logical_replication_slot_advance(XLogRecPtr moveto)
 
 	PG_TRY();
 	{
-		/* restart at slot's confirmed_flush */
+		/*
+		 * Note that start_lsn does not matter here, as with fast_forward mode
+		 * no transactions are replayed.
+		 */
 		ctx = CreateDecodingContext(InvalidXLogRecPtr,
 									NIL,
 									true,
@@ -378,6 +381,10 @@ pg_logical_replication_slot_advance(XLogRecPtr moveto)
 			XLogRecord *record;
 			char	   *errm = NULL;
 
+			/*
+			 * Start reading WAL at the slot's restart_lsn, which certainly
+			 * points to the valid record.
+			 */
 			record = XLogReadRecord(ctx->reader, startlsn, &errm);
 			if (errm)
 				elog(ERROR, "%s", errm);
@@ -389,8 +396,8 @@ pg_logical_replication_slot_advance(XLogRecPtr moveto)
 			startlsn = InvalidXLogRecPtr;
 
 			/*
-			 * The {begin_txn,change,commit_txn}_wrapper callbacks above will
-			 * store the description into our tuplestore.
+			 * Note that changes are not generated in fast_forward mode, and
+			 * that the slot's data is still updated.
 			 */
 			if (record != NULL)
 				LogicalDecodingProcessRecord(ctx, ctx->reader);