lslot_advance_comments.patch

diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index 61588d626f..76bafca41c 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -335,11 +335,14 @@ CreateInitDecodingContext(char *plugin,
  *		The LSN at which to start decoding.  If InvalidXLogRecPtr, restart
  *		from the slot's confirmed_flush; otherwise, start from the specified
  *		location (but move it forwards to confirmed_flush if it's older than
- *		that, see below).
+ *		that, see below). Doesn't matter in fast_forward mode.
  *
  * 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..0a4985ef8c 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -341,12 +341,10 @@ 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 do LogicalConfirmReceivedLocation updating
+ * confirmed_flush_lsn, we'd better digest WAL to advance restart_lsn
+ * (allowing to recycle WAL) and xmin (allowing to vacuum old catalog tuples).
+ * We do it in special fast_forward mode without actual replay.
  */
 static XLogRecPtr
 pg_logical_replication_slot_advance(XLogRecPtr moveto)
@@ -358,7 +356,6 @@ pg_logical_replication_slot_advance(XLogRecPtr moveto)
 
 	PG_TRY();
 	{
-		/* restart at slot's confirmed_flush */
 		ctx = CreateDecodingContext(InvalidXLogRecPtr,
 									NIL,
 									true,
@@ -388,10 +385,7 @@ 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.
-			 */
+			/* Changes are not actually produced in fast_forward mode. */
 			if (record != NULL)
 				LogicalDecodingProcessRecord(ctx, ctx->reader);