From 840a414dab94d46164fe007be1b8ba1e1e23041d Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Mon, 8 Sep 2025 21:04:42 +0530
Subject: [PATCH v6 1/2] Add stats related to slot sync skip

When slot sync is performed, it can happen that it is skipped due to
various reason. This patch adds stats for synced slots regarding this
slot sync skip. This patch adds new columns slot_sync_skip_count,
last_slot_sync_skip to view pg_stat_replication_slots and new column
slot_sync_skip_reason to view pg_replication_slots.
---
 contrib/test_decoding/expected/stats.out     | 12 ++--
 doc/src/sgml/monitoring.sgml                 | 20 ++++++
 doc/src/sgml/system-views.sgml               | 40 +++++++++++
 src/backend/catalog/system_views.sql         |  5 +-
 src/backend/replication/logical/slotsync.c   | 70 ++++++++++++++++++++
 src/backend/replication/slotfuncs.c          | 26 +++++++-
 src/backend/utils/activity/pgstat_replslot.c | 25 +++++++
 src/backend/utils/adt/pgstatfuncs.c          | 18 +++--
 src/include/catalog/pg_proc.dat              | 12 ++--
 src/include/pgstat.h                         |  3 +
 src/include/replication/slot.h               | 18 +++++
 src/test/regress/expected/rules.out          |  9 ++-
 src/tools/pgindent/typedefs.list             |  1 +
 13 files changed, 238 insertions(+), 21 deletions(-)

diff --git a/contrib/test_decoding/expected/stats.out b/contrib/test_decoding/expected/stats.out
index 28da9123cc8..933dc0f08af 100644
--- a/contrib/test_decoding/expected/stats.out
+++ b/contrib/test_decoding/expected/stats.out
@@ -78,17 +78,17 @@ SELECT slot_name, spill_txns = 0 AS spill_txns, spill_count = 0 AS spill_count,
 
 -- verify accessing/resetting stats for non-existent slot does something reasonable
 SELECT * FROM pg_stat_get_replication_slot('do-not-exist');
-  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | mem_exceeded_count | total_txns | total_bytes | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+--------------------+------------+-------------+-------------
- do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |                  0 |          0 |           0 | 
+  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | mem_exceeded_count | total_txns | total_bytes | slot_sync_skip_count | last_slot_sync_skip | stats_reset 
+--------------+------------+-------------+-------------+-------------+--------------+--------------+--------------------+------------+-------------+----------------------+---------------------+-------------
+ do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |                  0 |          0 |           0 |                    0 |                     | 
 (1 row)
 
 SELECT pg_stat_reset_replication_slot('do-not-exist');
 ERROR:  replication slot "do-not-exist" does not exist
 SELECT * FROM pg_stat_get_replication_slot('do-not-exist');
-  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | mem_exceeded_count | total_txns | total_bytes | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+--------------------+------------+-------------+-------------
- do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |                  0 |          0 |           0 | 
+  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | mem_exceeded_count | total_txns | total_bytes | slot_sync_skip_count | last_slot_sync_skip | stats_reset 
+--------------+------------+-------------+-------------+-------------+--------------+--------------+--------------------+------------+-------------+----------------------+---------------------+-------------
+ do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |                  0 |          0 |           0 |                    0 |                     | 
 (1 row)
 
 -- spilling the xact
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index f3bf527d5b4..bfa2dbf38fc 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1659,6 +1659,26 @@ description | Waiting for a newly initialized WAL file to reach durable storage
       </entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>slot_sync_skip_count</structfield><type>bigint</type>
+       </para>
+       <para>
+        Number of times the slot synchronization is skipped.
+       </para>
+      </entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>last_slot_sync_skip</structfield><type>timestamp with time zone</type>
+       </para>
+       <para>
+        Time at which last slot synchronization was skipped.
+       </para>
+      </entry>
+     </row>
+
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
         <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 7971498fe75..f903c75e421 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3102,6 +3102,46 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
       </para></entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>slot_sync_skip_reason</structfield><type>text</type>
+      </para>
+      <para>
+       The reason for the last slot synchronization skip. This field is set only
+       for logical slots that are being synced from a primary server (that is,
+       those whose <structfield>synced</structfield> field is
+       <literal>true</literal>). Possible values are:
+       <itemizedlist spacing="compact">
+        <listitem>
+         <para>
+          <literal>none</literal> means that the last slot synchronization
+          completed successfully.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>remote_behind</literal> means that the last slot
+          synchronization was skipped because the slot is ahead of the
+          corresponding failover slot on the primary.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>missing_wal_record</literal> means that the last slot
+          synchronization was skipped because the standby had not flushed the
+          WAL corresponding to the confirmed flush position on the remote slot.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>no_consistent_snapshot</literal> means that the last slot
+          synchronization was skipped because the standby could not build a
+          consistent snapshot.
+         </para>
+        </listitem>
+       </itemizedlist>
+      </para></entry>
+     </row>
     </tbody>
    </tgroup>
   </table>
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index dec8df4f8ee..877db2f87db 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1060,7 +1060,8 @@ CREATE VIEW pg_replication_slots AS
             L.conflicting,
             L.invalidation_reason,
             L.failover,
-            L.synced
+            L.synced,
+            L.slot_sync_skip_reason
     FROM pg_get_replication_slots() AS L
             LEFT JOIN pg_database D ON (L.datoid = D.oid);
 
@@ -1076,6 +1077,8 @@ CREATE VIEW pg_stat_replication_slots AS
             s.mem_exceeded_count,
             s.total_txns,
             s.total_bytes,
+            s.slot_sync_skip_count,
+            s.last_slot_sync_skip,
             s.stats_reset
     FROM pg_replication_slots as r,
         LATERAL pg_stat_get_replication_slot(slot_name) as s
diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index b122d99b009..47929eafcf7 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -64,6 +64,7 @@
 #include "storage/procarray.h"
 #include "tcop/tcopprot.h"
 #include "utils/builtins.h"
+#include "utils/injection_point.h"
 #include "utils/pg_lsn.h"
 #include "utils/ps_status.h"
 #include "utils/timeout.h"
@@ -148,6 +149,31 @@ typedef struct RemoteSlot
 static void slotsync_failure_callback(int code, Datum arg);
 static void update_synced_slots_inactive_since(void);
 
+/* Update slot sync skip stats */
+static void
+update_slot_sync_skip_stats(ReplicationSlot *slot, SlotSyncSkipReason skip_reason,
+							bool acquire_slot)
+{
+	/*
+	 * Update the slot sync related stats in pg_stat_replication_slot when a
+	 * slot sync is skipped
+	 */
+	if (skip_reason != SS_SKIP_NONE)
+		pgstat_report_replslot_sync_skip(slot);
+
+	if (acquire_slot)
+		ReplicationSlotAcquire(NameStr(slot->data.name), true, true);
+
+	/* Update the slot sync skip reason */
+	SpinLockAcquire(&slot->mutex);
+	if (slot->slot_sync_skip_reason != skip_reason)
+		slot->slot_sync_skip_reason = skip_reason;
+	SpinLockRelease(&slot->mutex);
+
+	if (acquire_slot)
+		ReplicationSlotRelease();
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -218,6 +244,8 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 						  LSN_FORMAT_ARGS(slot->data.restart_lsn),
 						  slot->data.catalog_xmin));
 
+		update_slot_sync_skip_stats(slot, SS_SKIP_REMOTE_BEHIND, false);
+
 		if (remote_slot_precedes)
 			*remote_slot_precedes = true;
 
@@ -261,6 +289,9 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 			slot->data.catalog_xmin = remote_slot->catalog_xmin;
 			SpinLockRelease(&slot->mutex);
 
+			/* Synchronization happened, update the slot sync skip reason */
+			update_slot_sync_skip_stats(slot, SS_SKIP_NONE, false);
+
 			if (found_consistent_snapshot)
 				*found_consistent_snapshot = true;
 		}
@@ -277,6 +308,17 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 						errdetail_internal("Remote slot has LSN %X/%08X but local slot has LSN %X/%08X.",
 										   LSN_FORMAT_ARGS(remote_slot->confirmed_lsn),
 										   LSN_FORMAT_ARGS(slot->data.confirmed_flush)));
+
+			/*
+			 * If found_consistent_snapshot is not NULL, a true value means
+			 * the slot synchronization was successful, while a false value
+			 * means it was skipped (see
+			 * update_and_persist_local_synced_slot()). If
+			 * found_consistent_snapshot is NULL, no such check exists, so the
+			 * stats can be updated directly.
+			 */
+			if (!found_consistent_snapshot || *found_consistent_snapshot)
+				update_slot_sync_skip_stats(slot, SS_SKIP_NONE, false);
 		}
 
 		updated_xmin_or_lsn = true;
@@ -580,6 +622,9 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		 * current location when recreating the slot in the next cycle. It may
 		 * take more time to create such a slot. Therefore, we keep this slot
 		 * and attempt the synchronization in the next cycle.
+		 *
+		 * We do not need to update the slot sync skip stats here as it will
+		 * be already updated in function update_local_synced_slot.
 		 */
 		return false;
 	}
@@ -595,11 +640,21 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 				errdetail("Synchronization could lead to data loss, because the standby could not build a consistent snapshot to decode WALs at LSN %X/%08X.",
 						  LSN_FORMAT_ARGS(slot->data.restart_lsn)));
 
+		/*
+		 * If a consitent snapshot is not found, update the slot sync skip
+		 * stats
+		 */
+		update_slot_sync_skip_stats(slot, SS_SKIP_NO_CONSISTENT_SNAPSHOT, false);
+
 		return false;
 	}
 
 	ReplicationSlotPersist();
 
+	/*
+	 * For the success case we do not update the slot sync skip stats here as
+	 * it is already be updated in update_local_synced_slot.
+	 */
 	ereport(LOG,
 			errmsg("newly created replication slot \"%s\" is sync-ready now",
 				   remote_slot->name));
@@ -634,6 +689,19 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 	latestFlushPtr = GetStandbyFlushRecPtr(NULL);
 	if (remote_slot->confirmed_lsn > latestFlushPtr)
 	{
+		/* If slot is present on the local, update the slot sync skip stats */
+		if ((slot = SearchNamedReplicationSlot(remote_slot->name, true)))
+		{
+			bool		synced;
+
+			SpinLockAcquire(&slot->mutex);
+			synced = slot->data.synced;
+			SpinLockRelease(&slot->mutex);
+
+			if (synced)
+				update_slot_sync_skip_stats(slot, SS_SKIP_MISSING_WAL_RECORD, true);
+		}
+
 		/*
 		 * Can get here only if GUC 'synchronized_standby_slots' on the
 		 * primary server was not configured correctly.
@@ -939,6 +1007,8 @@ synchronize_slots(WalReceiverConn *wrconn)
 	if (started_tx)
 		CommitTransactionCommand();
 
+	INJECTION_POINT("slot-sync-skip", NULL);
+
 	return some_slot_updated;
 }
 
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index b8f21153e7b..4e03205c63b 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -228,6 +228,28 @@ pg_drop_replication_slot(PG_FUNCTION_ARGS)
 	PG_RETURN_VOID();
 }
 
+/*
+ * Map a SlotSyncSkipReason enum to a human-readable string
+ */
+static char *
+GetSlotSyncSkipReason(SlotSyncSkipReason reason)
+{
+	switch (reason)
+	{
+		case SS_SKIP_NONE:
+			return "none";
+		case SS_SKIP_REMOTE_BEHIND:
+			return "remote_behind";
+		case SS_SKIP_MISSING_WAL_RECORD:
+			return "missing_wal_record";
+		case SS_SKIP_NO_CONSISTENT_SNAPSHOT:
+			return "no_consistent_snapshot";
+	}
+
+	Assert(false);
+	return "none";
+}
+
 /*
  * pg_get_replication_slots - SQL SRF showing all replication slots
  * that currently exist on the database cluster.
@@ -235,7 +257,7 @@ pg_drop_replication_slot(PG_FUNCTION_ARGS)
 Datum
 pg_get_replication_slots(PG_FUNCTION_ARGS)
 {
-#define PG_GET_REPLICATION_SLOTS_COLS 20
+#define PG_GET_REPLICATION_SLOTS_COLS 21
 	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
 	XLogRecPtr	currlsn;
 	int			slotno;
@@ -443,6 +465,8 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 
 		values[i++] = BoolGetDatum(slot_contents.data.synced);
 
+		values[i++] = CStringGetTextDatum(GetSlotSyncSkipReason(slot_contents.slot_sync_skip_reason));
+
 		Assert(i == PG_GET_REPLICATION_SLOTS_COLS);
 
 		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
diff --git a/src/backend/utils/activity/pgstat_replslot.c b/src/backend/utils/activity/pgstat_replslot.c
index d210c261ac6..ddfbe97d87d 100644
--- a/src/backend/utils/activity/pgstat_replslot.c
+++ b/src/backend/utils/activity/pgstat_replslot.c
@@ -102,6 +102,31 @@ pgstat_report_replslot(ReplicationSlot *slot, const PgStat_StatReplSlotEntry *re
 	pgstat_unlock_entry(entry_ref);
 }
 
+/*
+ * Report replication slot sync skip statistics.
+ *
+ * We can rely on the stats for the slot to exist and to belong to this
+ * slot. We can only get here if pgstat_create_replslot() or
+ * pgstat_acquire_replslot() have already been called.
+ */
+void
+pgstat_report_replslot_sync_skip(ReplicationSlot *slot)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_ReplSlot *shstatent;
+	PgStat_StatReplSlotEntry *statent;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_REPLSLOT, InvalidOid,
+											ReplicationSlotIndex(slot), false);
+	shstatent = (PgStatShared_ReplSlot *) entry_ref->shared_stats;
+	statent = &shstatent->stats;
+
+	statent->slot_sync_skip_count += 1;
+	statent->last_slot_sync_skip = GetCurrentTimestamp();
+
+	pgstat_unlock_entry(entry_ref);
+}
+
 /*
  * Report replication slot creation.
  *
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index a710508979e..59de7c0355e 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -2129,7 +2129,7 @@ pg_stat_get_archiver(PG_FUNCTION_ARGS)
 Datum
 pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 {
-#define PG_STAT_GET_REPLICATION_SLOT_COLS 11
+#define PG_STAT_GET_REPLICATION_SLOT_COLS 13
 	text	   *slotname_text = PG_GETARG_TEXT_P(0);
 	NameData	slotname;
 	TupleDesc	tupdesc;
@@ -2160,7 +2160,11 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "total_bytes",
 					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 11, "stats_reset",
+	TupleDescInitEntry(tupdesc, (AttrNumber) 11, "slot_sync_skip_count",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 12, "last_slot_sync_skip",
+					   TIMESTAMPTZOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 13, "stats_reset",
 					   TIMESTAMPTZOID, -1, 0);
 	BlessTupleDesc(tupdesc);
 
@@ -2186,11 +2190,17 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 	values[7] = Int64GetDatum(slotent->mem_exceeded_count);
 	values[8] = Int64GetDatum(slotent->total_txns);
 	values[9] = Int64GetDatum(slotent->total_bytes);
+	values[10] = Int64GetDatum(slotent->slot_sync_skip_count);
+
+	if (slotent->last_slot_sync_skip == 0)
+		nulls[11] = true;
+	else
+		values[11] = TimestampTzGetDatum(slotent->last_slot_sync_skip);
 
 	if (slotent->stat_reset_timestamp == 0)
-		nulls[10] = true;
+		nulls[12] = true;
 	else
-		values[10] = TimestampTzGetDatum(slotent->stat_reset_timestamp);
+		values[12] = TimestampTzGetDatum(slotent->stat_reset_timestamp);
 
 	/* Returns the record as Datum */
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 9121a382f76..577d90e4e0e 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5691,9 +5691,9 @@
 { oid => '6169', descr => 'statistics: information about replication slot',
   proname => 'pg_stat_get_replication_slot', provolatile => 's',
   proparallel => 'r', prorettype => 'record', proargtypes => 'text',
-  proallargtypes => '{text,text,int8,int8,int8,int8,int8,int8,int8,int8,int8,timestamptz}',
-  proargmodes => '{i,o,o,o,o,o,o,o,o,o,o,o}',
-  proargnames => '{slot_name,slot_name,spill_txns,spill_count,spill_bytes,stream_txns,stream_count,stream_bytes,mem_exceeded_count,total_txns,total_bytes,stats_reset}',
+  proallargtypes => '{text,text,int8,int8,int8,int8,int8,int8,int8,int8,int8,int8,timestamptz,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o,o,o,o,o,o,o,o,o}',
+  proargnames => '{slot_name,slot_name,spill_txns,spill_count,spill_bytes,stream_txns,stream_count,stream_bytes,mem_exceeded_count,total_txns,total_bytes,slot_sync_skip_count,last_slot_sync_skip,stats_reset}',
   prosrc => 'pg_stat_get_replication_slot' },
 
 { oid => '6230', descr => 'statistics: check if a stats object exists',
@@ -11511,9 +11511,9 @@
   proname => 'pg_get_replication_slots', prorows => '10', proisstrict => 'f',
   proretset => 't', provolatile => 's', prorettype => 'record',
   proargtypes => '',
-  proallargtypes => '{name,name,text,oid,bool,bool,int4,xid,xid,pg_lsn,pg_lsn,text,int8,bool,pg_lsn,timestamptz,bool,text,bool,bool}',
-  proargmodes => '{o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o}',
-  proargnames => '{slot_name,plugin,slot_type,datoid,temporary,active,active_pid,xmin,catalog_xmin,restart_lsn,confirmed_flush_lsn,wal_status,safe_wal_size,two_phase,two_phase_at,inactive_since,conflicting,invalidation_reason,failover,synced}',
+  proallargtypes => '{name,name,text,oid,bool,bool,int4,xid,xid,pg_lsn,pg_lsn,text,int8,bool,pg_lsn,timestamptz,bool,text,bool,bool,text}',
+  proargmodes => '{o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o}',
+  proargnames => '{slot_name,plugin,slot_type,datoid,temporary,active,active_pid,xmin,catalog_xmin,restart_lsn,confirmed_flush_lsn,wal_status,safe_wal_size,two_phase,two_phase_at,inactive_since,conflicting,invalidation_reason,failover,synced,slot_sync_skip_reason}',
   prosrc => 'pg_get_replication_slots' },
 { oid => '3786', descr => 'set up a logical replication slot',
   proname => 'pg_create_logical_replication_slot', provolatile => 'v',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 7ae503e71a2..6d75e47f9e5 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -398,6 +398,8 @@ typedef struct PgStat_StatReplSlotEntry
 	PgStat_Counter mem_exceeded_count;
 	PgStat_Counter total_txns;
 	PgStat_Counter total_bytes;
+	PgStat_Counter slot_sync_skip_count;
+	TimestampTz last_slot_sync_skip;
 	TimestampTz stat_reset_timestamp;
 } PgStat_StatReplSlotEntry;
 
@@ -742,6 +744,7 @@ extern PgStat_TableStatus *find_tabstat_entry(Oid rel_id);
 extern void pgstat_reset_replslot(const char *name);
 struct ReplicationSlot;
 extern void pgstat_report_replslot(struct ReplicationSlot *slot, const PgStat_StatReplSlotEntry *repSlotStat);
+extern void pgstat_report_replslot_sync_skip(struct ReplicationSlot *slot);
 extern void pgstat_create_replslot(struct ReplicationSlot *slot);
 extern void pgstat_acquire_replslot(struct ReplicationSlot *slot);
 extern void pgstat_drop_replslot(struct ReplicationSlot *slot);
diff --git a/src/include/replication/slot.h b/src/include/replication/slot.h
index 09c69f83d57..9d7aabfb894 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -71,6 +71,21 @@ typedef enum ReplicationSlotInvalidationCause
 /* Maximum number of invalidation causes */
 #define	RS_INVAL_MAX_CAUSES 4
 
+/*
+ * When slot sync worker is running or pg_sync_replication_slots is run, the
+ * slot sync can be skipped. This enum keeps a list of reasons of slot sync
+ * skip.
+ */
+typedef enum SlotSyncSkipReason
+{
+	SS_SKIP_NONE,				/* No skip */
+	SS_SKIP_MISSING_WAL_RECORD, /* Standby did not flush the wal coresponding
+								 * to confirmed flush on remote slot */
+	SS_SKIP_REMOTE_BEHIND,		/* Remote slot is behind the local slot */
+	SS_SKIP_NO_CONSISTENT_SNAPSHOT	/* Standby could not build a consistent
+									 * snapshot */
+} SlotSyncSkipReason;
+
 /*
  * On-Disk data of a replication slot, preserved across restarts.
  */
@@ -249,6 +264,9 @@ typedef struct ReplicationSlot
 	 */
 	XLogRecPtr	last_saved_restart_lsn;
 
+	/* The reason for last slot sync skip */
+	SlotSyncSkipReason slot_sync_skip_reason;
+
 } ReplicationSlot;
 
 #define SlotIsPhysical(slot) ((slot)->data.database == InvalidOid)
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 77e25ca029e..fcbcf45d80f 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -1507,8 +1507,9 @@ pg_replication_slots| SELECT l.slot_name,
     l.conflicting,
     l.invalidation_reason,
     l.failover,
-    l.synced
-   FROM (pg_get_replication_slots() l(slot_name, plugin, slot_type, datoid, temporary, active, active_pid, xmin, catalog_xmin, restart_lsn, confirmed_flush_lsn, wal_status, safe_wal_size, two_phase, two_phase_at, inactive_since, conflicting, invalidation_reason, failover, synced)
+    l.synced,
+    l.slot_sync_skip_reason
+   FROM (pg_get_replication_slots() l(slot_name, plugin, slot_type, datoid, temporary, active, active_pid, xmin, catalog_xmin, restart_lsn, confirmed_flush_lsn, wal_status, safe_wal_size, two_phase, two_phase_at, inactive_since, conflicting, invalidation_reason, failover, synced, slot_sync_skip_reason)
      LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
 pg_roles| SELECT pg_authid.rolname,
     pg_authid.rolsuper,
@@ -2151,9 +2152,11 @@ pg_stat_replication_slots| SELECT s.slot_name,
     s.mem_exceeded_count,
     s.total_txns,
     s.total_bytes,
+    s.slot_sync_skip_count,
+    s.last_slot_sync_skip,
     s.stats_reset
    FROM pg_replication_slots r,
-    LATERAL pg_stat_get_replication_slot((r.slot_name)::text) s(slot_name, spill_txns, spill_count, spill_bytes, stream_txns, stream_count, stream_bytes, mem_exceeded_count, total_txns, total_bytes, stats_reset)
+    LATERAL pg_stat_get_replication_slot((r.slot_name)::text) s(slot_name, spill_txns, spill_count, spill_bytes, stream_txns, stream_count, stream_bytes, mem_exceeded_count, total_txns, total_bytes, slot_sync_skip_count, last_slot_sync_skip, stats_reset)
   WHERE (r.datoid IS NOT NULL);
 pg_stat_slru| SELECT name,
     blks_zeroed,
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 018b5919cf6..ed3298fe7ca 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2797,6 +2797,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

