How can end users know the cause of LR slot sync delays?

From 81c9e5e84302a49d44fd89cde15d4f172a752224 Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Thu, 4 Sep 2025 19:46:24 +0530
Subject: [PATCH v1] 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 and slot_sync_skip_reason to view
pg_stat_replication_view.
---
 contrib/test_decoding/expected/stats.out     | 12 +++---
 doc/src/sgml/monitoring.sgml                 | 30 ++++++++++++++
 src/backend/catalog/system_views.sql         |  3 ++
 src/backend/replication/logical/slotsync.c   | 10 +++++
 src/backend/utils/activity/pgstat_replslot.c | 36 +++++++++++++++++
 src/backend/utils/adt/pgstatfuncs.c          | 42 ++++++++++++++++++--
 src/include/catalog/pg_proc.dat              |  6 +--
 src/include/pgstat.h                         |  5 +++
 src/include/replication/slotsync.h           |  9 +++++
 src/test/regress/expected/rules.out          |  5 ++-
 10 files changed, 144 insertions(+), 14 deletions(-)

diff --git a/contrib/test_decoding/expected/stats.out b/contrib/test_decoding/expected/stats.out
index de6dc416130..aa75cdd458c 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 | total_txns | total_bytes | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+-------------
- do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 | 
+  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | total_txns | total_bytes | slot_sync_skip_count | last_slot_sync_skip | slot_sync_skip_reason | stats_reset 
+--------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+----------------------+---------------------+-----------------------+-------------
+ do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 |                    0 |                     | none                  | 
 (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 | total_txns | total_bytes | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+-------------
- do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 | 
+  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | total_txns | total_bytes | slot_sync_skip_count | last_slot_sync_skip | slot_sync_skip_reason | stats_reset 
+--------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+----------------------+---------------------+-----------------------+-------------
+ do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 |                    0 |                     | none                  | 
 (1 row)
 
 -- spilling the xact
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 3f4a27a736e..2760c2a7535 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1644,6 +1644,36 @@ 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 sync 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 sync was skipped.
+       </para>
+      </entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>slot_sync_skip_reason</structfield><type>text</type>
+       </para>
+       <para>
+        Reason of the last slot sync skip.
+       </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/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index c77fa0234bb..8276c1af2eb 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1061,6 +1061,9 @@ CREATE VIEW pg_stat_replication_slots AS
             s.stream_bytes,
             s.total_txns,
             s.total_bytes,
+            s.slot_sync_skip_count,
+            s.last_slot_sync_skip,
+			s.slot_sync_skip_reason,
             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 9d0072a49ed..dc0a23ce506 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -218,6 +218,9 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 						  LSN_FORMAT_ARGS(slot->data.restart_lsn),
 						  slot->data.catalog_xmin));
 
+		/* Update stats for slot sync skip */
+		pgstat_report_replslot_sync_skip(slot, SLOT_SYNC_SKIP_REMOTE_BEHIND);
+
 		if (remote_slot_precedes)
 			*remote_slot_precedes = true;
 
@@ -595,6 +598,9 @@ 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)));
 
+		/* Update stats for slot sync skip */
+		pgstat_report_replslot_sync_skip(slot, SLOT_SYNC_SKIP_NO_CONSISTENT_SNAPSHOT);
+
 		return false;
 	}
 
@@ -646,6 +652,10 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 					   remote_slot->name,
 					   LSN_FORMAT_ARGS(latestFlushPtr)));
 
+		/* Update stats for slot sync skip if slot exist on the standby */
+		if ((slot = SearchNamedReplicationSlot(remote_slot->name, true)))
+			pgstat_report_replslot_sync_skip(slot, SLOT_SYNC_SKIP_STANDBY_BEHIND);
+
 		return false;
 	}
 
diff --git a/src/backend/utils/activity/pgstat_replslot.c b/src/backend/utils/activity/pgstat_replslot.c
index ccfb11c49bf..ae576516e44 100644
--- a/src/backend/utils/activity/pgstat_replslot.c
+++ b/src/backend/utils/activity/pgstat_replslot.c
@@ -27,6 +27,7 @@
 
 #include "replication/slot.h"
 #include "utils/pgstat_internal.h"
+#include "replication/slotsync.h"
 
 
 static int	get_replslot_index(const char *name, bool need_lock);
@@ -101,6 +102,41 @@ 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, SlotSyncSkipReason reason)
+{
+	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;
+
+	if (reason != SLOT_SYNC_SKIP_NONE)
+	{
+		statent->slot_sync_skip_count += 1;
+		statent->last_slot_sync_skip = GetCurrentTimestamp();
+		statent->slot_sync_skip_reason = reason;
+	}
+	else
+	{
+		statent->slot_sync_skip_count = 0;
+		statent->last_slot_sync_skip = 0;
+		statent->slot_sync_skip_reason = SLOT_SYNC_SKIP_NONE;
+	}
+
+	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 c756c2bebaa..fe8feb87a3e 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -2093,6 +2093,26 @@ pg_stat_get_archiver(PG_FUNCTION_ARGS)
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/* Map a SlotSyncSkipReason enum to a human-readable string */
+static char *
+GetSlotSyncSkipReason(SlotSyncSkipReason reason)
+{
+	switch (reason)
+	{
+		case SLOT_SYNC_SKIP_NONE:
+			return pstrdup("none");
+		case SLOT_SYNC_SKIP_REMOTE_BEHIND:
+			return pstrdup("remote_behind");
+		case SLOT_SYNC_SKIP_STANDBY_BEHIND:
+			return pstrdup("standby_behind");
+		case SLOT_SYNC_SKIP_NO_CONSISTENT_SNAPSHOT:
+			return pstrdup("no_consistent_snapshot");
+	}
+
+	Assert(false);
+	return pstrdup("none");
+}
+
 /*
  * Get the statistics for the replication slot. If the slot statistics is not
  * available, return all-zeroes stats.
@@ -2100,7 +2120,7 @@ pg_stat_get_archiver(PG_FUNCTION_ARGS)
 Datum
 pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 {
-#define PG_STAT_GET_REPLICATION_SLOT_COLS 10
+#define PG_STAT_GET_REPLICATION_SLOT_COLS 13
 	text	   *slotname_text = PG_GETARG_TEXT_P(0);
 	NameData	slotname;
 	TupleDesc	tupdesc;
@@ -2129,7 +2149,13 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 9, "total_bytes",
 					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "stats_reset",
+	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "slot_sync_skip_count",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 11, "last_slot_sync_skip",
+					   TIMESTAMPTZOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 12, "slot_sync_skip_reason",
+					   TEXTOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 13, "stats_reset",
 					   TIMESTAMPTZOID, -1, 0);
 	BlessTupleDesc(tupdesc);
 
@@ -2154,11 +2180,19 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 	values[6] = Int64GetDatum(slotent->stream_bytes);
 	values[7] = Int64GetDatum(slotent->total_txns);
 	values[8] = Int64GetDatum(slotent->total_bytes);
+	values[9] = Int64GetDatum(slotent->slot_sync_skip_count);
+
+	if (slotent->last_slot_sync_skip == 0)
+		nulls[10] = true;
+	else
+		values[10] = TimestampTzGetDatum(slotent->last_slot_sync_skip);
+
+	values[11] = CStringGetTextDatum(GetSlotSyncSkipReason(slotent->slot_sync_skip_reason));
 
 	if (slotent->stat_reset_timestamp == 0)
-		nulls[9] = true;
+		nulls[12] = true;
 	else
-		values[9] = 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 118d6da1ace..147b93c7a71 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5675,9 +5675,9 @@
 { 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,timestamptz}',
-  proargmodes => '{i,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,total_txns,total_bytes,stats_reset}',
+  proallargtypes => '{text,text,int8,int8,int8,int8,int8,int8,int8,int8,int8,timestamptz,text,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,total_txns,total_bytes,slot_sync_skip_count,last_slot_sync_skip,slot_sync_skip_reason,stats_reset}',
   prosrc => 'pg_stat_get_replication_slot' },
 
 { oid => '6230', descr => 'statistics: check if a stats object exists',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index f402b17295c..4d1b8fd79a4 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -15,6 +15,7 @@
 #include "portability/instr_time.h"
 #include "postmaster/pgarch.h"	/* for MAX_XFN_CHARS */
 #include "replication/conflict.h"
+#include "replication/slotsync.h"
 #include "utils/backend_progress.h" /* for backward compatibility */	/* IWYU pragma: export */
 #include "utils/backend_status.h"	/* for backward compatibility */	/* IWYU pragma: export */
 #include "utils/pgstat_kind.h"
@@ -395,6 +396,9 @@ typedef struct PgStat_StatReplSlotEntry
 	PgStat_Counter stream_bytes;
 	PgStat_Counter total_txns;
 	PgStat_Counter total_bytes;
+	PgStat_Counter slot_sync_skip_count;
+	TimestampTz last_slot_sync_skip;
+	PgStat_Counter slot_sync_skip_reason;
 	TimestampTz stat_reset_timestamp;
 } PgStat_StatReplSlotEntry;
 
@@ -736,6 +740,7 @@ 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, SlotSyncSkipReason reason);
 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/slotsync.h b/src/include/replication/slotsync.h
index 16b721463dd..359435ff01e 100644
--- a/src/include/replication/slotsync.h
+++ b/src/include/replication/slotsync.h
@@ -23,6 +23,15 @@ extern PGDLLIMPORT bool sync_replication_slots;
 extern PGDLLIMPORT char *PrimaryConnInfo;
 extern PGDLLIMPORT char *PrimarySlotName;
 
+typedef enum SlotSyncSkipReason
+{
+	SLOT_SYNC_SKIP_NONE,		/* No skip */
+	SLOT_SYNC_SKIP_STANDBY_BEHIND,	/* Standby is behind the remote slot */
+	SLOT_SYNC_SKIP_REMOTE_BEHIND,	/* Remote slot is behind the local slot */
+	SLOT_SYNC_SKIP_NO_CONSISTENT_SNAPSHOT	/* Standby could not reach a
+											 * consistent snapshot */
+}			SlotSyncSkipReason;
+
 extern char *CheckAndGetDbnameFromConninfo(void);
 extern bool ValidateSlotSyncParams(int elevel);
 
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 35e8aad7701..ae0291c06aa 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -2140,9 +2140,12 @@ pg_stat_replication_slots| SELECT s.slot_name,
     s.stream_bytes,
     s.total_txns,
     s.total_bytes,
+    s.slot_sync_skip_count,
+    s.last_slot_sync_skip,
+    s.slot_sync_skip_reason,
     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, 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, total_txns, total_bytes, slot_sync_skip_count, last_slot_sync_skip, slot_sync_skip_reason, stats_reset)
   WHERE (r.datoid IS NOT NULL);
 pg_stat_slru| SELECT name,
     blks_zeroed,
-- 
2.34.1

From 8349ab969ec7fa015a6d5e2d4fea6d8ebda74ba0 Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Thu, 4 Sep 2025 19:46:24 +0530
Subject: [PATCH v2_approach1] 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 and slot_sync_skip_reason to view
pg_stat_replication_view.
---
 contrib/test_decoding/expected/stats.out     | 12 +++---
 doc/src/sgml/monitoring.sgml                 | 30 ++++++++++++++
 src/backend/catalog/system_views.sql         |  3 ++
 src/backend/replication/logical/slotsync.c   | 25 ++++++++++--
 src/backend/utils/activity/pgstat_replslot.c | 23 +++++++++++
 src/backend/utils/adt/pgstatfuncs.c          | 42 ++++++++++++++++++--
 src/include/catalog/pg_proc.dat              |  6 +--
 src/include/pgstat.h                         |  5 +++
 src/include/replication/slotsync.h           |  9 +++++
 src/test/regress/expected/rules.out          |  5 ++-
 10 files changed, 143 insertions(+), 17 deletions(-)

diff --git a/contrib/test_decoding/expected/stats.out b/contrib/test_decoding/expected/stats.out
index de6dc416130..aa75cdd458c 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 | total_txns | total_bytes | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+-------------
- do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 | 
+  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | total_txns | total_bytes | slot_sync_skip_count | last_slot_sync_skip | slot_sync_skip_reason | stats_reset 
+--------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+----------------------+---------------------+-----------------------+-------------
+ do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 |                    0 |                     | none                  | 
 (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 | total_txns | total_bytes | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+-------------
- do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 | 
+  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | total_txns | total_bytes | slot_sync_skip_count | last_slot_sync_skip | slot_sync_skip_reason | stats_reset 
+--------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+----------------------+---------------------+-----------------------+-------------
+ do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 |                    0 |                     | none                  | 
 (1 row)
 
 -- spilling the xact
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 3f4a27a736e..2760c2a7535 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1644,6 +1644,36 @@ 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 sync 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 sync was skipped.
+       </para>
+      </entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>slot_sync_skip_reason</structfield><type>text</type>
+       </para>
+       <para>
+        Reason of the last slot sync skip.
+       </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/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index c77fa0234bb..8276c1af2eb 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1061,6 +1061,9 @@ CREATE VIEW pg_stat_replication_slots AS
             s.stream_bytes,
             s.total_txns,
             s.total_bytes,
+            s.slot_sync_skip_count,
+            s.last_slot_sync_skip,
+			s.slot_sync_skip_reason,
             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 8c061d55bdb..75bb6346cd4 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -218,6 +218,9 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 						  LSN_FORMAT_ARGS(slot->data.restart_lsn),
 						  slot->data.catalog_xmin));
 
+		/* Update stats for slot sync skip */
+		pgstat_report_replslot_sync_skip(slot, SLOT_SYNC_SKIP_REMOTE_BEHIND);
+
 		if (remote_slot_precedes)
 			*remote_slot_precedes = true;
 
@@ -595,6 +598,9 @@ 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)));
 
+		/* Update stats for slot sync skip */
+		pgstat_report_replslot_sync_skip(slot, SLOT_SYNC_SKIP_NO_CONSISTENT_SNAPSHOT);
+
 		return false;
 	}
 
@@ -623,7 +629,7 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 static bool
 synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 {
-	ReplicationSlot *slot;
+	ReplicationSlot *slot = NULL;
 	XLogRecPtr	latestFlushPtr;
 	bool		slot_updated = false;
 
@@ -646,11 +652,13 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 					   remote_slot->name,
 					   LSN_FORMAT_ARGS(latestFlushPtr)));
 
-		return false;
+		/* If the slot is not present on the local */
+		if (!(slot = SearchNamedReplicationSlot(remote_slot->name, true)))
+			return false;
 	}
 
 	/* Search for the named slot */
-	if ((slot = SearchNamedReplicationSlot(remote_slot->name, true)))
+	if (slot || (slot = SearchNamedReplicationSlot(remote_slot->name, true)))
 	{
 		bool		synced;
 
@@ -658,6 +666,17 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		synced = slot->data.synced;
 		SpinLockRelease(&slot->mutex);
 
+		/*
+		 * If standby is behind remote slot and the synced slot is present on
+		 * local.
+		 */
+		if (remote_slot->confirmed_lsn > latestFlushPtr)
+		{
+			if (synced)
+				pgstat_report_replslot_sync_skip(slot, SLOT_SYNC_SKIP_STANDBY_BEHIND);
+			return false;
+		}
+
 		/* User-created slot with the same name exists, raise ERROR. */
 		if (!synced)
 			ereport(ERROR,
diff --git a/src/backend/utils/activity/pgstat_replslot.c b/src/backend/utils/activity/pgstat_replslot.c
index ccfb11c49bf..c43c59bcfc0 100644
--- a/src/backend/utils/activity/pgstat_replslot.c
+++ b/src/backend/utils/activity/pgstat_replslot.c
@@ -27,6 +27,7 @@
 
 #include "replication/slot.h"
 #include "utils/pgstat_internal.h"
+#include "replication/slotsync.h"
 
 
 static int	get_replslot_index(const char *name, bool need_lock);
@@ -101,6 +102,28 @@ pgstat_report_replslot(ReplicationSlot *slot, const PgStat_StatReplSlotEntry *re
 	pgstat_unlock_entry(entry_ref);
 }
 
+/*
+ * Report replication slot sync skip statistics
+ */
+void
+pgstat_report_replslot_sync_skip(ReplicationSlot *slot, SlotSyncSkipReason reason)
+{
+	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();
+	statent->slot_sync_skip_reason = reason;
+
+	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 c756c2bebaa..fe8feb87a3e 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -2093,6 +2093,26 @@ pg_stat_get_archiver(PG_FUNCTION_ARGS)
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/* Map a SlotSyncSkipReason enum to a human-readable string */
+static char *
+GetSlotSyncSkipReason(SlotSyncSkipReason reason)
+{
+	switch (reason)
+	{
+		case SLOT_SYNC_SKIP_NONE:
+			return pstrdup("none");
+		case SLOT_SYNC_SKIP_REMOTE_BEHIND:
+			return pstrdup("remote_behind");
+		case SLOT_SYNC_SKIP_STANDBY_BEHIND:
+			return pstrdup("standby_behind");
+		case SLOT_SYNC_SKIP_NO_CONSISTENT_SNAPSHOT:
+			return pstrdup("no_consistent_snapshot");
+	}
+
+	Assert(false);
+	return pstrdup("none");
+}
+
 /*
  * Get the statistics for the replication slot. If the slot statistics is not
  * available, return all-zeroes stats.
@@ -2100,7 +2120,7 @@ pg_stat_get_archiver(PG_FUNCTION_ARGS)
 Datum
 pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 {
-#define PG_STAT_GET_REPLICATION_SLOT_COLS 10
+#define PG_STAT_GET_REPLICATION_SLOT_COLS 13
 	text	   *slotname_text = PG_GETARG_TEXT_P(0);
 	NameData	slotname;
 	TupleDesc	tupdesc;
@@ -2129,7 +2149,13 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 9, "total_bytes",
 					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "stats_reset",
+	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "slot_sync_skip_count",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 11, "last_slot_sync_skip",
+					   TIMESTAMPTZOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 12, "slot_sync_skip_reason",
+					   TEXTOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 13, "stats_reset",
 					   TIMESTAMPTZOID, -1, 0);
 	BlessTupleDesc(tupdesc);
 
@@ -2154,11 +2180,19 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 	values[6] = Int64GetDatum(slotent->stream_bytes);
 	values[7] = Int64GetDatum(slotent->total_txns);
 	values[8] = Int64GetDatum(slotent->total_bytes);
+	values[9] = Int64GetDatum(slotent->slot_sync_skip_count);
+
+	if (slotent->last_slot_sync_skip == 0)
+		nulls[10] = true;
+	else
+		values[10] = TimestampTzGetDatum(slotent->last_slot_sync_skip);
+
+	values[11] = CStringGetTextDatum(GetSlotSyncSkipReason(slotent->slot_sync_skip_reason));
 
 	if (slotent->stat_reset_timestamp == 0)
-		nulls[9] = true;
+		nulls[12] = true;
 	else
-		values[9] = 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 03e82d28c87..2f95941f1ec 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5687,9 +5687,9 @@
 { 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,timestamptz}',
-  proargmodes => '{i,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,total_txns,total_bytes,stats_reset}',
+  proallargtypes => '{text,text,int8,int8,int8,int8,int8,int8,int8,int8,int8,timestamptz,text,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,total_txns,total_bytes,slot_sync_skip_count,last_slot_sync_skip,slot_sync_skip_reason,stats_reset}',
   prosrc => 'pg_stat_get_replication_slot' },
 
 { oid => '6230', descr => 'statistics: check if a stats object exists',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index f402b17295c..4d1b8fd79a4 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -15,6 +15,7 @@
 #include "portability/instr_time.h"
 #include "postmaster/pgarch.h"	/* for MAX_XFN_CHARS */
 #include "replication/conflict.h"
+#include "replication/slotsync.h"
 #include "utils/backend_progress.h" /* for backward compatibility */	/* IWYU pragma: export */
 #include "utils/backend_status.h"	/* for backward compatibility */	/* IWYU pragma: export */
 #include "utils/pgstat_kind.h"
@@ -395,6 +396,9 @@ typedef struct PgStat_StatReplSlotEntry
 	PgStat_Counter stream_bytes;
 	PgStat_Counter total_txns;
 	PgStat_Counter total_bytes;
+	PgStat_Counter slot_sync_skip_count;
+	TimestampTz last_slot_sync_skip;
+	PgStat_Counter slot_sync_skip_reason;
 	TimestampTz stat_reset_timestamp;
 } PgStat_StatReplSlotEntry;
 
@@ -736,6 +740,7 @@ 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, SlotSyncSkipReason reason);
 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/slotsync.h b/src/include/replication/slotsync.h
index 16b721463dd..359435ff01e 100644
--- a/src/include/replication/slotsync.h
+++ b/src/include/replication/slotsync.h
@@ -23,6 +23,15 @@ extern PGDLLIMPORT bool sync_replication_slots;
 extern PGDLLIMPORT char *PrimaryConnInfo;
 extern PGDLLIMPORT char *PrimarySlotName;
 
+typedef enum SlotSyncSkipReason
+{
+	SLOT_SYNC_SKIP_NONE,		/* No skip */
+	SLOT_SYNC_SKIP_STANDBY_BEHIND,	/* Standby is behind the remote slot */
+	SLOT_SYNC_SKIP_REMOTE_BEHIND,	/* Remote slot is behind the local slot */
+	SLOT_SYNC_SKIP_NO_CONSISTENT_SNAPSHOT	/* Standby could not reach a
+											 * consistent snapshot */
+}			SlotSyncSkipReason;
+
 extern char *CheckAndGetDbnameFromConninfo(void);
 extern bool ValidateSlotSyncParams(int elevel);
 
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 35e8aad7701..ae0291c06aa 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -2140,9 +2140,12 @@ pg_stat_replication_slots| SELECT s.slot_name,
     s.stream_bytes,
     s.total_txns,
     s.total_bytes,
+    s.slot_sync_skip_count,
+    s.last_slot_sync_skip,
+    s.slot_sync_skip_reason,
     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, 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, total_txns, total_bytes, slot_sync_skip_count, last_slot_sync_skip, slot_sync_skip_reason, stats_reset)
   WHERE (r.datoid IS NOT NULL);
 pg_stat_slru| SELECT name,
     blks_zeroed,
-- 
2.34.1

From 31adb46d5263cd91d358b178139172959c86a113 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 v2_approach2] 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               |  8 +++
 src/backend/catalog/system_views.sql         |  5 +-
 src/backend/replication/logical/slotsync.c   | 61 +++++++++++++++++---
 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               | 17 ++++++
 src/test/regress/expected/rules.out          |  9 ++-
 12 files changed, 188 insertions(+), 28 deletions(-)

diff --git a/contrib/test_decoding/expected/stats.out b/contrib/test_decoding/expected/stats.out
index de6dc416130..c1ff872c08c 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 | total_txns | total_bytes | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+-------------
- do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 | 
+  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | 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 |                     | 
 (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 | total_txns | total_bytes | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+-------------
- do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 | 
+  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | 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 |                     | 
 (1 row)
 
 -- spilling the xact
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 3f4a27a736e..76bad0d7f2a 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1644,6 +1644,26 @@ 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 sync 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 sync 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 4187191ea74..6b93c97ddd9 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3036,6 +3036,14 @@ 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>
+       Reason of the last slot sync skip.
+      </para></entry>
+     </row>
     </tbody>
    </tgroup>
   </table>
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index c77fa0234bb..abca2f5f927 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1046,7 +1046,8 @@ 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);
 
@@ -1061,6 +1062,8 @@ CREATE VIEW pg_stat_replication_slots AS
             s.stream_bytes,
             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 8c061d55bdb..29fac502a8a 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -148,6 +148,23 @@ 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)
+{
+	/*
+	 * Update the slot sync related stats in pg_stat_replication_slot when a
+	 * slot sync is skipped
+	 */
+	if (skip_reason != SLOT_SYNC_SKIP_NONE)
+		pgstat_report_replslot_sync_skip(slot);
+
+	/* Update the slot sync reason */
+	SpinLockAcquire(&slot->mutex);
+	slot->slot_sync_skip_reason = skip_reason;
+	SpinLockRelease(&slot->mutex);
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -165,7 +182,8 @@ static void update_synced_slots_inactive_since(void);
 static bool
 update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 						 bool *found_consistent_snapshot,
-						 bool *remote_slot_precedes)
+						 bool *remote_slot_precedes,
+						 SlotSyncSkipReason * skip_reason)
 {
 	ReplicationSlot *slot = MyReplicationSlot;
 	bool		updated_xmin_or_lsn = false;
@@ -218,6 +236,8 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 						  LSN_FORMAT_ARGS(slot->data.restart_lsn),
 						  slot->data.catalog_xmin));
 
+		*skip_reason = SLOT_SYNC_SKIP_REMOTE_BEHIND;
+
 		if (remote_slot_precedes)
 			*remote_slot_precedes = true;
 
@@ -557,7 +577,8 @@ reserve_wal_for_local_slot(XLogRecPtr restart_lsn)
  * false.
  */
 static bool
-update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
+update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
+									 SlotSyncSkipReason * skip_reason)
 {
 	ReplicationSlot *slot = MyReplicationSlot;
 	bool		found_consistent_snapshot = false;
@@ -565,7 +586,7 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 
 	(void) update_local_synced_slot(remote_slot, remote_dbid,
 									&found_consistent_snapshot,
-									&remote_slot_precedes);
+									&remote_slot_precedes, skip_reason);
 
 	/*
 	 * Check if the primary server has caught up. Refer to the comment atop
@@ -595,6 +616,8 @@ 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)));
 
+		*skip_reason = SLOT_SYNC_SKIP_NO_CONSISTENT_SNAPSHOT;
+
 		return false;
 	}
 
@@ -626,6 +649,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 	ReplicationSlot *slot;
 	XLogRecPtr	latestFlushPtr;
 	bool		slot_updated = false;
+	SlotSyncSkipReason skip_reason = SLOT_SYNC_SKIP_NONE;
 
 	/*
 	 * Make sure that concerned WAL is received and flushed before syncing
@@ -646,7 +670,11 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 					   remote_slot->name,
 					   LSN_FORMAT_ARGS(latestFlushPtr)));
 
-		return false;
+		/* If slot is present on the local, update the slot sync skip stats */
+		if ((slot = SearchNamedReplicationSlot(remote_slot->name, true)))
+			skip_reason = SLOT_SYNC_SKIP_STANDBY_BEHIND;
+		else
+			return false;
 	}
 
 	/* Search for the named slot */
@@ -658,6 +686,17 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		synced = slot->data.synced;
 		SpinLockRelease(&slot->mutex);
 
+		/*
+		 * If standby is behind remote slot and the synced slot is present on
+		 * local.
+		 */
+		if (remote_slot->confirmed_lsn > latestFlushPtr)
+		{
+			if (synced)
+				update_slot_sync_skip_stats(slot, skip_reason);
+			return false;
+		}
+
 		/* User-created slot with the same name exists, raise ERROR. */
 		if (!synced)
 			ereport(ERROR,
@@ -715,7 +754,8 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		if (slot->data.persistency == RS_TEMPORARY)
 		{
 			slot_updated = update_and_persist_local_synced_slot(remote_slot,
-																remote_dbid);
+																remote_dbid,
+																&skip_reason);
 		}
 
 		/* Slot ready for sync, so sync it. */
@@ -737,7 +777,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 										   LSN_FORMAT_ARGS(remote_slot->confirmed_lsn)));
 
 			slot_updated = update_local_synced_slot(remote_slot, remote_dbid,
-													NULL, NULL);
+													NULL, NULL, &skip_reason);
 		}
 	}
 	/* Otherwise create the slot first. */
@@ -784,11 +824,18 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		ReplicationSlotsComputeRequiredXmin(true);
 		LWLockRelease(ProcArrayLock);
 
-		update_and_persist_local_synced_slot(remote_slot, remote_dbid);
+		update_and_persist_local_synced_slot(remote_slot, remote_dbid,
+											 &skip_reason);
 
 		slot_updated = true;
 	}
 
+	/*
+	 * If slot sync is skipped update the reason and stats. Else reset the
+	 * reason to 'none' on successful slot sync.
+	 */
+	update_slot_sync_skip_stats(slot, skip_reason);
+
 	ReplicationSlotRelease();
 
 	return slot_updated;
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index b8f21153e7b..0033c03286f 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 SLOT_SYNC_SKIP_NONE:
+			return pstrdup("none");
+		case SLOT_SYNC_SKIP_REMOTE_BEHIND:
+			return pstrdup("remote_behind");
+		case SLOT_SYNC_SKIP_STANDBY_BEHIND:
+			return pstrdup("standby_behind");
+		case SLOT_SYNC_SKIP_NO_CONSISTENT_SNAPSHOT:
+			return pstrdup("no_consistent_snapshot");
+	}
+
+	Assert(false);
+	return pstrdup("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 ccfb11c49bf..bf436472b8d 100644
--- a/src/backend/utils/activity/pgstat_replslot.c
+++ b/src/backend/utils/activity/pgstat_replslot.c
@@ -101,6 +101,31 @@ 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 c756c2bebaa..15500a77701 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -2100,7 +2100,7 @@ pg_stat_get_archiver(PG_FUNCTION_ARGS)
 Datum
 pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 {
-#define PG_STAT_GET_REPLICATION_SLOT_COLS 10
+#define PG_STAT_GET_REPLICATION_SLOT_COLS 12
 	text	   *slotname_text = PG_GETARG_TEXT_P(0);
 	NameData	slotname;
 	TupleDesc	tupdesc;
@@ -2129,7 +2129,11 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 9, "total_bytes",
 					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "stats_reset",
+	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "slot_sync_skip_count",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 11, "last_slot_sync_skip",
+					   TIMESTAMPTZOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 12, "stats_reset",
 					   TIMESTAMPTZOID, -1, 0);
 	BlessTupleDesc(tupdesc);
 
@@ -2154,11 +2158,17 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 	values[6] = Int64GetDatum(slotent->stream_bytes);
 	values[7] = Int64GetDatum(slotent->total_txns);
 	values[8] = Int64GetDatum(slotent->total_bytes);
+	values[9] = Int64GetDatum(slotent->slot_sync_skip_count);
+
+	if (slotent->last_slot_sync_skip == 0)
+		nulls[10] = true;
+	else
+		values[10] = TimestampTzGetDatum(slotent->last_slot_sync_skip);
 
 	if (slotent->stat_reset_timestamp == 0)
-		nulls[9] = true;
+		nulls[11] = true;
 	else
-		values[9] = TimestampTzGetDatum(slotent->stat_reset_timestamp);
+		values[11] = 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 03e82d28c87..d31a52b1fd4 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5687,9 +5687,9 @@
 { 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,timestamptz}',
-  proargmodes => '{i,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,total_txns,total_bytes,stats_reset}',
+  proallargtypes => '{text,text,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}',
+  proargnames => '{slot_name,slot_name,spill_txns,spill_count,spill_bytes,stream_txns,stream_count,stream_bytes,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',
@@ -11503,9 +11503,9 @@
   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 f402b17295c..906d7b7799f 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -395,6 +395,8 @@ typedef struct PgStat_StatReplSlotEntry
 	PgStat_Counter stream_bytes;
 	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;
 
@@ -736,6 +738,7 @@ 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 fe62162cde3..09932747bd3 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -71,6 +71,20 @@ 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
+{
+	SLOT_SYNC_SKIP_NONE,		/* No skip */
+	SLOT_SYNC_SKIP_STANDBY_BEHIND,	/* Standby is behind the remote slot */
+	SLOT_SYNC_SKIP_REMOTE_BEHIND,	/* Remote slot is behind the local slot */
+	SLOT_SYNC_SKIP_NO_CONSISTENT_SNAPSHOT	/* Standby could not reach a
+											 * consistent snapshot */
+}			SlotSyncSkipReason;
+
 /*
  * On-Disk data of a replication slot, preserved across restarts.
  */
@@ -249,6 +263,9 @@ 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 35e8aad7701..041d3328328 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -1499,8 +1499,9 @@ 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,
@@ -2140,9 +2141,11 @@ pg_stat_replication_slots| SELECT s.slot_name,
     s.stream_bytes,
     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, 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, 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,
-- 
2.34.1

From c248365ab0cc1dee40e236b3432306dd52ffca84 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 v3 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               |  8 +++
 src/backend/catalog/system_views.sql         |  5 +-
 src/backend/replication/logical/slotsync.c   | 59 +++++++++++++++++++-
 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, 193 insertions(+), 23 deletions(-)

diff --git a/contrib/test_decoding/expected/stats.out b/contrib/test_decoding/expected/stats.out
index de6dc416130..c1ff872c08c 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 | total_txns | total_bytes | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+-------------
- do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 | 
+  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | 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 |                     | 
 (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 | total_txns | total_bytes | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+-------------
- do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 | 
+  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | 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 |                     | 
 (1 row)
 
 -- spilling the xact
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 3f4a27a736e..65b1a8773f0 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1644,6 +1644,26 @@ 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 4187191ea74..72e9b6de1e8 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3036,6 +3036,14 @@ 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>
+       Reason of the last slot synchronization skip.
+      </para></entry>
+     </row>
     </tbody>
    </tgroup>
   </table>
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index c77fa0234bb..abca2f5f927 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1046,7 +1046,8 @@ 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);
 
@@ -1061,6 +1062,8 @@ CREATE VIEW pg_stat_replication_slots AS
             s.stream_bytes,
             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 8c061d55bdb..6259fad894c 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -148,6 +148,24 @@ 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)
+{
+	/*
+	 * 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);
+
+	/* Update the slot sync reason */
+	SpinLockAcquire(&slot->mutex);
+	if (slot->slot_sync_skip_reason != skip_reason)
+		slot->slot_sync_skip_reason = skip_reason;
+	SpinLockRelease(&slot->mutex);
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -218,6 +236,8 @@ 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);
+
 		if (remote_slot_precedes)
 			*remote_slot_precedes = true;
 
@@ -337,6 +357,15 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 		ReplicationSlotsComputeRequiredLSN();
 	}
 
+	/*
+	 * If found_consistent_snapshot is not NULL and a consistent snapshot is
+	 * found set the slot sync skip reason to none. Else, if consistent
+	 * snapshot is not found the stats will be updated in the function
+	 * update_and_persist_local_synced_slot
+	 */
+	if (!found_consistent_snapshot || *found_consistent_snapshot)
+		update_slot_sync_skip_stats(slot, SS_SKIP_NONE);
+
 	return updated_config || updated_xmin_or_lsn;
 }
 
@@ -580,6 +609,9 @@ 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 +627,21 @@ 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);
+
 		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));
@@ -623,7 +665,7 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 static bool
 synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 {
-	ReplicationSlot *slot;
+	ReplicationSlot *slot = NULL;
 	XLogRecPtr	latestFlushPtr;
 	bool		slot_updated = false;
 
@@ -634,6 +676,19 @@ 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);
+		}
+
 		/*
 		 * Can get here only if GUC 'synchronized_standby_slots' on the
 		 * primary server was not configured correctly.
@@ -650,7 +705,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 	}
 
 	/* Search for the named slot */
-	if ((slot = SearchNamedReplicationSlot(remote_slot->name, true)))
+	if (slot || (slot = SearchNamedReplicationSlot(remote_slot->name, true)))
 	{
 		bool		synced;
 
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 ccfb11c49bf..bf436472b8d 100644
--- a/src/backend/utils/activity/pgstat_replslot.c
+++ b/src/backend/utils/activity/pgstat_replslot.c
@@ -101,6 +101,31 @@ 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 c756c2bebaa..15500a77701 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -2100,7 +2100,7 @@ pg_stat_get_archiver(PG_FUNCTION_ARGS)
 Datum
 pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 {
-#define PG_STAT_GET_REPLICATION_SLOT_COLS 10
+#define PG_STAT_GET_REPLICATION_SLOT_COLS 12
 	text	   *slotname_text = PG_GETARG_TEXT_P(0);
 	NameData	slotname;
 	TupleDesc	tupdesc;
@@ -2129,7 +2129,11 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 9, "total_bytes",
 					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "stats_reset",
+	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "slot_sync_skip_count",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 11, "last_slot_sync_skip",
+					   TIMESTAMPTZOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 12, "stats_reset",
 					   TIMESTAMPTZOID, -1, 0);
 	BlessTupleDesc(tupdesc);
 
@@ -2154,11 +2158,17 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 	values[6] = Int64GetDatum(slotent->stream_bytes);
 	values[7] = Int64GetDatum(slotent->total_txns);
 	values[8] = Int64GetDatum(slotent->total_bytes);
+	values[9] = Int64GetDatum(slotent->slot_sync_skip_count);
+
+	if (slotent->last_slot_sync_skip == 0)
+		nulls[10] = true;
+	else
+		values[10] = TimestampTzGetDatum(slotent->last_slot_sync_skip);
 
 	if (slotent->stat_reset_timestamp == 0)
-		nulls[9] = true;
+		nulls[11] = true;
 	else
-		values[9] = TimestampTzGetDatum(slotent->stat_reset_timestamp);
+		values[11] = 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 01eba3b5a19..649a6c0f78c 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5687,9 +5687,9 @@
 { 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,timestamptz}',
-  proargmodes => '{i,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,total_txns,total_bytes,stats_reset}',
+  proallargtypes => '{text,text,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}',
+  proargnames => '{slot_name,slot_name,spill_txns,spill_count,spill_bytes,stream_txns,stream_count,stream_bytes,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',
@@ -11503,9 +11503,9 @@
   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 f402b17295c..906d7b7799f 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -395,6 +395,8 @@ typedef struct PgStat_StatReplSlotEntry
 	PgStat_Counter stream_bytes;
 	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;
 
@@ -736,6 +738,7 @@ 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 fe62162cde3..013f4bae942 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 35e8aad7701..041d3328328 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -1499,8 +1499,9 @@ 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,
@@ -2140,9 +2141,11 @@ pg_stat_replication_slots| SELECT s.slot_name,
     s.stream_bytes,
     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, 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, 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 3c80d49b67e..3509f40875e 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2788,6 +2788,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

From 16702f41408fec8d7a35868782fa6ca68226f715 Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Wed, 24 Sep 2025 15:26:37 +0530
Subject: [PATCH v3 2/2] Add test for new stats for slot sync skip

---
 src/backend/replication/logical/slotsync.c |   5 +
 src/test/recovery/meson.build              |   1 +
 src/test/recovery/t/049_slot_skip_stats.pl | 184 +++++++++++++++++++++
 3 files changed, 190 insertions(+)
 create mode 100644 src/test/recovery/t/049_slot_skip_stats.pl

diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 6259fad894c..4359120165e 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"
@@ -994,6 +995,10 @@ synchronize_slots(WalReceiverConn *wrconn)
 	if (started_tx)
 		CommitTransactionCommand();
 
+#ifdef USE_INJECTION_POINTS
+	INJECTION_POINT("slot-sync-skip", NULL);
+#endif
+
 	return some_slot_updated;
 }
 
diff --git a/src/test/recovery/meson.build b/src/test/recovery/meson.build
index 52993c32dbb..640cbb2b796 100644
--- a/src/test/recovery/meson.build
+++ b/src/test/recovery/meson.build
@@ -57,6 +57,7 @@ tests += {
       't/046_checkpoint_logical_slot.pl',
       't/047_checkpoint_physical_slot.pl',
       't/048_vacuum_horizon_floor.pl'
+      't/049_slot_skip_stats.pl'
     ],
   },
 }
diff --git a/src/test/recovery/t/049_slot_skip_stats.pl b/src/test/recovery/t/049_slot_skip_stats.pl
new file mode 100644
index 00000000000..21f4859f9c1
--- /dev/null
+++ b/src/test/recovery/t/049_slot_skip_stats.pl
@@ -0,0 +1,184 @@
+# Copyright (c) 2024-2025, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+my $primary = PostgreSQL::Test::Cluster->new('publisher');
+$primary->init(
+	allows_streaming => 'logical',
+	auth_extra => [ '--create-role' => 'repl_role' ]);
+$primary->append_conf(
+	'postgresql.conf', qq{
+autovacuum = off
+max_prepared_transactions = 1
+});
+$primary->start;
+
+$primary->safe_psql('postgres', q(CREATE EXTENSION injection_points));
+
+my $backup_name = 'backup';
+$primary->backup($backup_name);
+
+my $standby1 = PostgreSQL::Test::Cluster->new('standby1');
+$standby1->init_from_backup(
+	$primary, $backup_name,
+	has_streaming => 1,
+	has_restoring => 1);
+
+my $connstr_1 = $primary->connstr;
+$standby1->append_conf(
+	'postgresql.conf', qq(
+hot_standby_feedback = on
+primary_slot_name = 'sb1_slot'
+primary_conninfo = '$connstr_1 dbname=postgres'
+log_min_messages = 'debug2'
+));
+$primary->psql('postgres',
+	q{SELECT pg_create_physical_replication_slot('sb1_slot');});
+
+$standby1->start;
+
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+$primary->wait_for_replay_catchup($standby1);
+
+$standby1->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+my $result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason
+	 FROM pg_replication_slots
+	 WHERE slot_name = 'slot_sync'
+	   AND synced"
+);
+is($result, 'none', "slot sync reason is none");
+
+
+# Change pg_hba.conf so that standby cannot connect to primary
+unlink($primary->data_dir . '/pg_hba.conf');
+$primary->append_conf('pg_hba.conf',
+	qq{local   all             all                                     trust}
+);
+$primary->restart;
+
+# Advance the failover slot so that confirmed flush LSN of remote slot become
+# ahead of standby's flushed LSN
+$primary->safe_psql(
+	'postgres', qq(
+	CREATE TABLE t1(a int);
+	INSERT INTO t1 values(1);
+));
+$primary->safe_psql('postgres',
+	"SELECT pg_replication_slot_advance('slot_sync', pg_current_wal_lsn());");
+
+$standby1->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Confirm that standby is behind
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason
+	 FROM pg_replication_slots
+	 WHERE slot_name = 'slot_sync'
+	   AND synced"
+);
+is($result, 'missing_wal_record', "slot sync skip when standby is behind");
+
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_count
+	 FROM pg_stat_replication_slots
+	 WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Repeat pg_sync_replication_slots to check slot_sync_skip_count is advancing
+$standby1->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason
+	 FROM pg_replication_slots
+	 WHERE slot_name = 'slot_sync'
+	   AND synced"
+);
+is($result, 'missing_wal_record', "slot sync skip when standby is behind");
+
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_count
+	 FROM pg_stat_replication_slots
+	 WHERE slot_name = 'slot_sync'"
+);
+is($result, '2', "check slot sync skip count");
+
+# Restore the connect between primary and standby
+unlink($primary->data_dir . '/pg_hba.conf');
+$primary->append_conf(
+	'pg_hba.conf',
+	qq{
+local   all             all                                     trust
+local   replication     all                                     trust
+});
+$primary->restart;
+
+# Cleanup
+$primary->safe_psql('postgres',
+	"SELECT pg_drop_replication_slot('slot_sync')");
+$primary->wait_for_replay_catchup($standby1);
+
+$standby1->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Create a new logical slot on primary
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Attach injection point
+my $standby_psql = $standby1->background_psql('postgres');
+$standby_psql->query_safe(
+	q(select injection_points_attach('slot-sync-skip','wait')));
+
+# initiate sync the failover slots
+$standby_psql->query_until(
+	qr/slot_sync/,
+	q(
+\echo slot_sync
+select pg_sync_replication_slots();
+));
+
+$standby1->wait_for_event('client backend', 'slot-sync-skip');
+
+# the logical slot is in temporary state and the sync will skip as remote is
+# behind the freashly created slot
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason
+	 FROM pg_replication_slots
+	 WHERE slot_name = 'slot_sync'
+	   AND synced"
+);
+is($result, 'remote_behind', "slot sync skip as remote is behind");
+
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_count
+	 FROM pg_stat_replication_slots
+	 WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip cout");
+
+$standby1->safe_psql('postgres',
+	q{select injection_points_detach('slot-sync-skip')});
+$standby1->safe_psql('postgres',
+	q{select injection_points_wakeup('slot-sync-skip')});
+
+done_testing();
-- 
2.34.1

From 99c8350040a14b38eeebf3e44d2b190fe1c44889 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 v4 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               |  8 +++
 src/backend/catalog/system_views.sql         |  5 +-
 src/backend/replication/logical/slotsync.c   | 59 +++++++++++++++++++-
 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, 193 insertions(+), 23 deletions(-)

diff --git a/contrib/test_decoding/expected/stats.out b/contrib/test_decoding/expected/stats.out
index de6dc416130..c1ff872c08c 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 | total_txns | total_bytes | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+-------------
- do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 | 
+  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | 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 |                     | 
 (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 | total_txns | total_bytes | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+------------+-------------+-------------
- do-not-exist |          0 |           0 |           0 |           0 |            0 |            0 |          0 |           0 | 
+  slot_name   | spill_txns | spill_count | spill_bytes | stream_txns | stream_count | stream_bytes | 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 |                     | 
 (1 row)
 
 -- spilling the xact
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 3f4a27a736e..65b1a8773f0 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1644,6 +1644,26 @@ 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 4187191ea74..72e9b6de1e8 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3036,6 +3036,14 @@ 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>
+       Reason of the last slot synchronization skip.
+      </para></entry>
+     </row>
     </tbody>
    </tgroup>
   </table>
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index c77fa0234bb..abca2f5f927 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1046,7 +1046,8 @@ 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);
 
@@ -1061,6 +1062,8 @@ CREATE VIEW pg_stat_replication_slots AS
             s.stream_bytes,
             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 8c061d55bdb..6259fad894c 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -148,6 +148,24 @@ 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)
+{
+	/*
+	 * 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);
+
+	/* Update the slot sync reason */
+	SpinLockAcquire(&slot->mutex);
+	if (slot->slot_sync_skip_reason != skip_reason)
+		slot->slot_sync_skip_reason = skip_reason;
+	SpinLockRelease(&slot->mutex);
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -218,6 +236,8 @@ 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);
+
 		if (remote_slot_precedes)
 			*remote_slot_precedes = true;
 
@@ -337,6 +357,15 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 		ReplicationSlotsComputeRequiredLSN();
 	}
 
+	/*
+	 * If found_consistent_snapshot is not NULL and a consistent snapshot is
+	 * found set the slot sync skip reason to none. Else, if consistent
+	 * snapshot is not found the stats will be updated in the function
+	 * update_and_persist_local_synced_slot
+	 */
+	if (!found_consistent_snapshot || *found_consistent_snapshot)
+		update_slot_sync_skip_stats(slot, SS_SKIP_NONE);
+
 	return updated_config || updated_xmin_or_lsn;
 }
 
@@ -580,6 +609,9 @@ 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 +627,21 @@ 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);
+
 		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));
@@ -623,7 +665,7 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 static bool
 synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 {
-	ReplicationSlot *slot;
+	ReplicationSlot *slot = NULL;
 	XLogRecPtr	latestFlushPtr;
 	bool		slot_updated = false;
 
@@ -634,6 +676,19 @@ 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);
+		}
+
 		/*
 		 * Can get here only if GUC 'synchronized_standby_slots' on the
 		 * primary server was not configured correctly.
@@ -650,7 +705,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 	}
 
 	/* Search for the named slot */
-	if ((slot = SearchNamedReplicationSlot(remote_slot->name, true)))
+	if (slot || (slot = SearchNamedReplicationSlot(remote_slot->name, true)))
 	{
 		bool		synced;
 
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 ccfb11c49bf..bf436472b8d 100644
--- a/src/backend/utils/activity/pgstat_replslot.c
+++ b/src/backend/utils/activity/pgstat_replslot.c
@@ -101,6 +101,31 @@ 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 c756c2bebaa..15500a77701 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -2100,7 +2100,7 @@ pg_stat_get_archiver(PG_FUNCTION_ARGS)
 Datum
 pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 {
-#define PG_STAT_GET_REPLICATION_SLOT_COLS 10
+#define PG_STAT_GET_REPLICATION_SLOT_COLS 12
 	text	   *slotname_text = PG_GETARG_TEXT_P(0);
 	NameData	slotname;
 	TupleDesc	tupdesc;
@@ -2129,7 +2129,11 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 9, "total_bytes",
 					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "stats_reset",
+	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "slot_sync_skip_count",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 11, "last_slot_sync_skip",
+					   TIMESTAMPTZOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 12, "stats_reset",
 					   TIMESTAMPTZOID, -1, 0);
 	BlessTupleDesc(tupdesc);
 
@@ -2154,11 +2158,17 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 	values[6] = Int64GetDatum(slotent->stream_bytes);
 	values[7] = Int64GetDatum(slotent->total_txns);
 	values[8] = Int64GetDatum(slotent->total_bytes);
+	values[9] = Int64GetDatum(slotent->slot_sync_skip_count);
+
+	if (slotent->last_slot_sync_skip == 0)
+		nulls[10] = true;
+	else
+		values[10] = TimestampTzGetDatum(slotent->last_slot_sync_skip);
 
 	if (slotent->stat_reset_timestamp == 0)
-		nulls[9] = true;
+		nulls[11] = true;
 	else
-		values[9] = TimestampTzGetDatum(slotent->stat_reset_timestamp);
+		values[11] = 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 01eba3b5a19..649a6c0f78c 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5687,9 +5687,9 @@
 { 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,timestamptz}',
-  proargmodes => '{i,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,total_txns,total_bytes,stats_reset}',
+  proallargtypes => '{text,text,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}',
+  proargnames => '{slot_name,slot_name,spill_txns,spill_count,spill_bytes,stream_txns,stream_count,stream_bytes,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',
@@ -11503,9 +11503,9 @@
   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 e4a59a30b8c..e9a506fa5bb 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -396,6 +396,8 @@ typedef struct PgStat_StatReplSlotEntry
 	PgStat_Counter stream_bytes;
 	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;
 
@@ -737,6 +739,7 @@ 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 fe62162cde3..013f4bae942 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 35e8aad7701..041d3328328 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -1499,8 +1499,9 @@ 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,
@@ -2140,9 +2141,11 @@ pg_stat_replication_slots| SELECT s.slot_name,
     s.stream_bytes,
     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, 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, 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 3c80d49b67e..3509f40875e 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2788,6 +2788,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

From 541bd745ea85cc0409a166eec98c0e9a4dd0868a Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Wed, 24 Sep 2025 15:26:37 +0530
Subject: [PATCH v4 2/2] Add test for new stats for slot sync skip

---
 src/backend/replication/logical/slotsync.c |   5 +
 src/test/recovery/meson.build              |   3 +-
 src/test/recovery/t/049_slot_skip_stats.pl | 180 +++++++++++++++++++++
 3 files changed, 187 insertions(+), 1 deletion(-)
 create mode 100644 src/test/recovery/t/049_slot_skip_stats.pl

diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 6259fad894c..4359120165e 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"
@@ -994,6 +995,10 @@ synchronize_slots(WalReceiverConn *wrconn)
 	if (started_tx)
 		CommitTransactionCommand();
 
+#ifdef USE_INJECTION_POINTS
+	INJECTION_POINT("slot-sync-skip", NULL);
+#endif
+
 	return some_slot_updated;
 }
 
diff --git a/src/test/recovery/meson.build b/src/test/recovery/meson.build
index 52993c32dbb..83a6c4b5c17 100644
--- a/src/test/recovery/meson.build
+++ b/src/test/recovery/meson.build
@@ -56,7 +56,8 @@ tests += {
       't/045_archive_restartpoint.pl',
       't/046_checkpoint_logical_slot.pl',
       't/047_checkpoint_physical_slot.pl',
-      't/048_vacuum_horizon_floor.pl'
+      't/048_vacuum_horizon_floor.pl',
+      't/049_slot_skip_stats.pl'
     ],
   },
 }
diff --git a/src/test/recovery/t/049_slot_skip_stats.pl b/src/test/recovery/t/049_slot_skip_stats.pl
new file mode 100644
index 00000000000..3aa63911207
--- /dev/null
+++ b/src/test/recovery/t/049_slot_skip_stats.pl
@@ -0,0 +1,180 @@
+# Copyright (c) 2024-2025, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Skip all tests if injection points are not supported in this build
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Initialize the primary cluster
+my $primary = PostgreSQL::Test::Cluster->new('publisher');
+$primary->init(
+	allows_streaming => 'logical',
+	auth_extra => [ '--create-role' => 'repl_role' ]);
+$primary->append_conf(
+	'postgresql.conf', qq{
+autovacuum = off
+max_prepared_transactions = 1
+});
+$primary->start;
+
+# Load the injection_points extension
+$primary->safe_psql('postgres', q(CREATE EXTENSION injection_points));
+
+# Take a backup of the primary for standby initialization
+my $backup_name = 'backup';
+$primary->backup($backup_name);
+
+# Initialize standby from primary backup
+my $standby1 = PostgreSQL::Test::Cluster->new('standby1');
+$standby1->init_from_backup(
+	$primary, $backup_name,
+	has_streaming => 1,
+	has_restoring => 1);
+
+my $connstr_1 = $primary->connstr;
+$standby1->append_conf(
+	'postgresql.conf', qq(
+hot_standby_feedback = on
+primary_slot_name = 'sb1_slot'
+primary_conninfo = '$connstr_1 dbname=postgres'
+));
+
+# Create a physical replication slot on primary for standby
+$primary->psql('postgres',
+	q{SELECT pg_create_physical_replication_slot('sb1_slot');});
+
+$standby1->start;
+
+# Create a logical replication slot on primary for testing
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Wait for standby to catch up
+$primary->wait_for_replay_catchup($standby1);
+
+# Initial sync of replication slots
+$standby1->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Verify that initially there is no skip reason
+my $result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced"
+);
+is($result, 'none', "slot sync reason is none");
+
+# Simulate standby connection failure by modifying pg_hba.conf
+unlink($primary->data_dir . '/pg_hba.conf');
+$primary->append_conf('pg_hba.conf',
+	qq{local   all             all                                     trust}
+);
+$primary->restart;
+
+# Advance the failover slot so that confirmed flush LSN of remote slot become
+# ahead of standby's flushed LSN
+$primary->safe_psql(
+	'postgres', qq(
+    CREATE TABLE t1(a int);
+    INSERT INTO t1 VALUES(1);
+));
+$primary->safe_psql('postgres',
+	"SELECT pg_replication_slot_advance('slot_sync', pg_current_wal_lsn());");
+
+# Attempt to sync replication slots while standby is behind
+$standby1->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Check skip reason and count when standby is behind
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'missing_wal_record', "slot sync skip when standby is behind");
+
+$result = $standby1->safe_psql('postgres',
+	"SELECT slot_sync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Repeat sync to ensure skip count increments
+$standby1->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'missing_wal_record', "slot sync skip when standby is behind");
+
+$result = $standby1->safe_psql('postgres',
+	"SELECT slot_sync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '2', "check slot sync skip count");
+
+# Restore connectivity between primary and standby
+unlink($primary->data_dir . '/pg_hba.conf');
+$primary->append_conf(
+	'pg_hba.conf',
+	qq{
+local   all             all                                     trust
+local   replication     all                                     trust
+});
+$primary->restart;
+
+# Cleanup: drop the logical slot and ensure standby catches up
+$primary->safe_psql('postgres',
+	"SELECT pg_drop_replication_slot('slot_sync')");
+$primary->wait_for_replay_catchup($standby1);
+
+$standby1->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Create a new logical slot for testing injection point
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Attach injection point to simulate wait
+my $standby_psql = $standby1->background_psql('postgres');
+$standby_psql->query_safe(
+	q(select injection_points_attach('slot-sync-skip','wait')));
+
+# Initiate sync of failover slots
+$standby_psql->query_until(
+	qr/slot_sync/,
+	q(
+\echo slot_sync
+select pg_sync_replication_slots();
+));
+
+# Wait for backend to reach injection point
+$standby1->wait_for_event('client backend', 'slot-sync-skip');
+
+# Logical slot is temporary and sync will skip because remote is behind
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND temporary"
+);
+is($result, 'remote_behind', "slot sync skip as remote is behind");
+
+$result = $standby1->safe_psql('postgres',
+	"SELECT slot_sync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Detach injection point
+$standby1->safe_psql(
+	'postgres', q{
+	SELECT injection_points_detach('slot-sync-skip');
+	SELECT injection_points_wakeup('slot-sync-skip');
+});
+
+done_testing();
-- 
2.34.1

diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 4359120165e..94063f67369 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -282,6 +282,9 @@ 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);
+
 			if (found_consistent_snapshot)
 				*found_consistent_snapshot = true;
 		}
@@ -290,6 +293,14 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 			LogicalSlotAdvanceAndCheckSnapState(remote_slot->confirmed_lsn,
 												found_consistent_snapshot);
 
+			/* Update the slot sync skip reason if snapshot could be created */
+			if (SnapBuildSnapshotExists(remote_slot->restart_lsn))
+			{
+				Assert(!found_consistent_snapshot ||
+					   *found_consistent_snapshot);
+				update_slot_sync_skip_stats(slot, SS_SKIP_NONE);
+			}
+
 			/* Sanity check */
 			if (slot->data.confirmed_flush != remote_slot->confirmed_lsn)
 				ereport(ERROR,
@@ -358,15 +369,6 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 		ReplicationSlotsComputeRequiredLSN();
 	}
 
-	/*
-	 * If found_consistent_snapshot is not NULL and a consistent snapshot is
-	 * found set the slot sync skip reason to none. Else, if consistent
-	 * snapshot is not found the stats will be updated in the function
-	 * update_and_persist_local_synced_slot
-	 */
-	if (!found_consistent_snapshot || *found_consistent_snapshot)
-		update_slot_sync_skip_stats(slot, SS_SKIP_NONE);
-
 	return updated_config || updated_xmin_or_lsn;
 }
 

From 30e93d695428789b173c7ee32ecd14fbebc5b68c 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 v5 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               |  8 +++
 src/backend/catalog/system_views.sql         |  5 +-
 src/backend/replication/logical/slotsync.c   | 61 +++++++++++++++++++-
 src/backend/replication/slotfuncs.c          | 26 ++++++++-
 src/backend/replication/walreceiver.c        |  3 +
 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 +
 14 files changed, 199 insertions(+), 22 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 dc4fc29466d..2610befbad3 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1655,6 +1655,26 @@ 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..23aa78649ee 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3102,6 +3102,14 @@ 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>
+       Reason of the last slot synchronization skip.
+      </para></entry>
+     </row>
     </tbody>
    </tgroup>
   </table>
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 823776c1498..f8e87e216b6 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 8c061d55bdb..79fb16feae2 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 +149,24 @@ 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)
+{
+	/*
+	 * 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);
+
+	/* Update the slot sync reason */
+	SpinLockAcquire(&slot->mutex);
+	if (slot->slot_sync_skip_reason != skip_reason)
+		slot->slot_sync_skip_reason = skip_reason;
+	SpinLockRelease(&slot->mutex);
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -218,6 +237,8 @@ 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);
+
 		if (remote_slot_precedes)
 			*remote_slot_precedes = true;
 
@@ -261,6 +282,9 @@ 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);
+
 			if (found_consistent_snapshot)
 				*found_consistent_snapshot = true;
 		}
@@ -277,6 +301,13 @@ 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)));
+			else
+			{
+				/* Update the slot sync stats */
+				Assert(!found_consistent_snapshot ||
+					   *found_consistent_snapshot);
+				update_slot_sync_skip_stats(slot, SS_SKIP_NONE);
+			}
 		}
 
 		updated_xmin_or_lsn = true;
@@ -580,6 +611,9 @@ 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 +629,21 @@ 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);
+
 		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));
@@ -623,7 +667,7 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 static bool
 synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 {
-	ReplicationSlot *slot;
+	ReplicationSlot *slot = NULL;
 	XLogRecPtr	latestFlushPtr;
 	bool		slot_updated = false;
 
@@ -634,6 +678,19 @@ 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);
+		}
+
 		/*
 		 * Can get here only if GUC 'synchronized_standby_slots' on the
 		 * primary server was not configured correctly.
@@ -939,6 +996,8 @@ 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/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index 7361ffc9dcf..1788aa61b61 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -75,6 +75,7 @@
 #include "utils/acl.h"
 #include "utils/builtins.h"
 #include "utils/guc.h"
+#include "utils/injection_point.h"
 #include "utils/pg_lsn.h"
 #include "utils/ps_status.h"
 #include "utils/timestamp.h"
@@ -430,6 +431,8 @@ WalReceiverMain(const void *startup_data, size_t startup_data_len)
 				/* Process any requests or signals received recently */
 				CHECK_FOR_INTERRUPTS();
 
+				INJECTION_POINT("walreceiver", NULL);
+
 				if (ConfigReloadPending)
 				{
 					ConfigReloadPending = false;
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 1fe33df2756..ba0904e3820 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -2121,7 +2121,7 @@ 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;
@@ -2152,7 +2152,11 @@ 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);
 
@@ -2178,11 +2182,17 @@ 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 b51d2b17379..de7be5fb252 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 bc8077cbae6..24f1c3ecc87 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;
 
@@ -741,6 +743,7 @@ 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 fe62162cde3..013f4bae942 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 16753b2e4c0..5f6fc151d58 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 5290b91e83e..8ac66b81f5f 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2795,6 +2795,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

From 306f570d4eff2e1437bce387da867a94ad25ae7b Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Wed, 15 Oct 2025 15:36:34 +0530
Subject: [PATCH v5 2/2] Add test for new stats for slot sync skip

---
 src/test/recovery/meson.build              |   3 +-
 src/test/recovery/t/049_slot_skip_stats.pl | 193 +++++++++++++++++++++
 2 files changed, 195 insertions(+), 1 deletion(-)
 create mode 100644 src/test/recovery/t/049_slot_skip_stats.pl

diff --git a/src/test/recovery/meson.build b/src/test/recovery/meson.build
index 52993c32dbb..83a6c4b5c17 100644
--- a/src/test/recovery/meson.build
+++ b/src/test/recovery/meson.build
@@ -56,7 +56,8 @@ tests += {
       't/045_archive_restartpoint.pl',
       't/046_checkpoint_logical_slot.pl',
       't/047_checkpoint_physical_slot.pl',
-      't/048_vacuum_horizon_floor.pl'
+      't/048_vacuum_horizon_floor.pl',
+      't/049_slot_skip_stats.pl'
     ],
   },
 }
diff --git a/src/test/recovery/t/049_slot_skip_stats.pl b/src/test/recovery/t/049_slot_skip_stats.pl
new file mode 100644
index 00000000000..74e49751348
--- /dev/null
+++ b/src/test/recovery/t/049_slot_skip_stats.pl
@@ -0,0 +1,193 @@
+# Copyright (c) 2024-2025, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Skip all tests if injection points are not supported in this build
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Initialize the primary cluster
+my $primary = PostgreSQL::Test::Cluster->new('publisher');
+$primary->init(allows_streaming => 'logical');
+$primary->append_conf(
+	'postgresql.conf', qq{
+autovacuum = off
+});
+$primary->start;
+
+# Check if the extension injection_points is available, as it may be
+# possible that this script is run with installcheck, where the module
+# would not be installed by default.
+if (!$primary->check_extension('injection_points'))
+{
+	plan skip_all => 'Extension injection_points not installed';
+}
+
+# Load the injection_points extension
+$primary->safe_psql('postgres', q(CREATE EXTENSION injection_points));
+
+# Take a backup of the primary for standby initialization
+my $backup_name = 'backup';
+$primary->backup($backup_name);
+
+# Initialize standby from primary backup
+my $standby1 = PostgreSQL::Test::Cluster->new('standby1');
+$standby1->init_from_backup($primary, $backup_name, has_streaming => 1);
+
+my $connstr = $primary->connstr;
+$standby1->append_conf(
+	'postgresql.conf', qq(
+hot_standby_feedback = on
+primary_slot_name = 'sb1_slot'
+primary_conninfo = '$connstr dbname=postgres'
+));
+
+# Create a physical replication slot on primary for standby
+$primary->safe_psql('postgres',
+	q{SELECT pg_create_physical_replication_slot('sb1_slot');});
+
+$standby1->start;
+
+# Create a logical replication slot on primary for testing
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Wait for standby to catch up
+$primary->wait_for_replay_catchup($standby1);
+
+# Initial sync of replication slots
+$standby1->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Verify that initially there is no skip reason
+my $result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced"
+);
+is($result, 'none', "slot sync reason is none");
+
+# Hold walreciever so WAL is not flushed on standby
+my $standby_psql = $standby1->background_psql('postgres');
+$standby_psql->query_safe(
+	q(select injection_points_attach('walreceiver','wait')));
+
+# Advance the failover slot so that confirmed flush LSN of remote slot become
+# ahead of standby's flushed LSN
+$primary->safe_psql(
+	'postgres', qq(
+    CREATE TABLE t1(a int);
+    INSERT INTO t1 VALUES(1);
+));
+
+# Wait for backend to reach injection point
+$standby1->wait_for_event('walreceiver', 'walreceiver');
+
+$primary->safe_psql('postgres',
+	"SELECT pg_replication_slot_advance('slot_sync', pg_current_wal_lsn());");
+
+my ($stdout, $stderr);
+# Attempt to sync replication slots while standby is behind
+($result, $stdout, $stderr) =
+  $standby1->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Check skip reason and count when standby is behind
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'missing_wal_record', "slot sync skip when standby is behind");
+
+$result = $standby1->safe_psql('postgres',
+	"SELECT slot_sync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Repeat sync to ensure skip count increments
+($result, $stdout, $stderr) =
+  $standby1->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'missing_wal_record', "slot sync skip when standby is behind");
+
+$result = $standby1->safe_psql('postgres',
+	"SELECT slot_sync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '2', "check slot sync skip count");
+
+# Detach injection point
+$standby1->safe_psql(
+	'postgres', q{
+	SELECT injection_points_detach('walreceiver');
+	SELECT injection_points_wakeup('walreceiver');
+});
+
+# Check that skip reason is reset after successful sync
+$standby1->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'none', "slot_sync_skip_reason is reset after successful sync");
+
+# Cleanup: drop the logical slot and ensure standby catches up
+$primary->safe_psql('postgres',
+	"SELECT pg_drop_replication_slot('slot_sync')");
+$primary->wait_for_replay_catchup($standby1);
+
+$standby1->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Test for case when slot sync is skipped when the remote slot is
+# behind the local slot.
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Attach injection point to simulate wait
+$standby_psql->query_safe(
+	q(select injection_points_attach('slot-sync-skip','wait')));
+
+# Initiate sync of failover slots
+$standby_psql->query_until(
+	qr/slot_sync/,
+	q(
+\echo slot_sync
+select pg_sync_replication_slots();
+));
+
+# Wait for backend to reach injection point
+$standby1->wait_for_event('client backend', 'slot-sync-skip');
+
+# Logical slot is temporary and sync will skip because remote is behind
+$result = $standby1->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND temporary"
+);
+is($result, 'remote_behind', "slot sync skip as remote is behind");
+
+$result = $standby1->safe_psql('postgres',
+	"SELECT slot_sync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Detach injection point
+$standby1->safe_psql(
+	'postgres', q{
+	SELECT injection_points_detach('slot-sync-skip');
+	SELECT injection_points_wakeup('slot-sync-skip');
+});
+
+done_testing();
-- 
2.34.1

From c8b96465e8b58496fb861ee6451fed093625f750 Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Wed, 15 Oct 2025 15:36:34 +0530
Subject: [PATCH v6 2/2] Add test for new stats for slot sync skip

---
 src/test/recovery/meson.build              |   3 +-
 src/test/recovery/t/049_slot_skip_stats.pl | 199 +++++++++++++++++++++
 2 files changed, 201 insertions(+), 1 deletion(-)
 create mode 100644 src/test/recovery/t/049_slot_skip_stats.pl

diff --git a/src/test/recovery/meson.build b/src/test/recovery/meson.build
index 52993c32dbb..83a6c4b5c17 100644
--- a/src/test/recovery/meson.build
+++ b/src/test/recovery/meson.build
@@ -56,7 +56,8 @@ tests += {
       't/045_archive_restartpoint.pl',
       't/046_checkpoint_logical_slot.pl',
       't/047_checkpoint_physical_slot.pl',
-      't/048_vacuum_horizon_floor.pl'
+      't/048_vacuum_horizon_floor.pl',
+      't/049_slot_skip_stats.pl'
     ],
   },
 }
diff --git a/src/test/recovery/t/049_slot_skip_stats.pl b/src/test/recovery/t/049_slot_skip_stats.pl
new file mode 100644
index 00000000000..0f87960e94b
--- /dev/null
+++ b/src/test/recovery/t/049_slot_skip_stats.pl
@@ -0,0 +1,199 @@
+# Copyright (c) 2024-2025, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Skip all tests if injection points are not supported in this build
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Initialize the primary cluster
+my $primary = PostgreSQL::Test::Cluster->new('publisher');
+$primary->init(allows_streaming => 'logical');
+$primary->append_conf(
+	'postgresql.conf', qq{
+autovacuum = off
+});
+$primary->start;
+
+# Check if the extension injection_points is available, as it may be
+# possible that this script is run with installcheck, where the module
+# would not be installed by default.
+if (!$primary->check_extension('injection_points'))
+{
+	plan skip_all => 'Extension injection_points not installed';
+}
+
+# Load the injection_points extension
+$primary->safe_psql('postgres', q(CREATE EXTENSION injection_points));
+
+# Take a backup of the primary for standby initialization
+my $backup_name = 'backup';
+$primary->backup($backup_name);
+
+# Initialize standby from primary backup
+my $standby = PostgreSQL::Test::Cluster->new('standby');
+$standby->init_from_backup($primary, $backup_name, has_streaming => 1);
+
+my $connstr = $primary->connstr;
+$standby->append_conf(
+	'postgresql.conf', qq(
+hot_standby_feedback = on
+primary_slot_name = 'sb1_slot'
+primary_conninfo = '$connstr dbname=postgres'
+));
+
+# Create a physical replication slot on primary for standby
+$primary->safe_psql('postgres',
+	q{SELECT pg_create_physical_replication_slot('sb1_slot');});
+
+$standby->start;
+
+# Create a logical replication slot on primary for testing
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Wait for standby to catch up
+$primary->wait_for_replay_catchup($standby);
+
+# Initial sync of replication slots
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Verify that initially there is no skip reason
+my $result = $standby->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced"
+);
+is($result, 'none', "slot sync reason is none");
+
+# Pause steaming replication connection so that standby can lag behind
+unlink($primary->data_dir . '/pg_hba.conf');
+$primary->append_conf(
+	'pg_hba.conf', qq{
+local   all             all                                     trust
+host    all             all             127.0.0.1/32            trust
+host    all             all             ::1/128                 trust
+});
+$primary->restart;
+
+# Advance the failover slot so that confirmed flush LSN of remote slot become
+# ahead of standby's flushed LSN
+$primary->safe_psql(
+	'postgres', qq(
+    CREATE TABLE t1(a int);
+    INSERT INTO t1 VALUES(1);
+	SELECT pg_replication_slot_advance('slot_sync', pg_current_wal_lsn());
+));
+
+my ($stdout, $stderr);
+# Attempt to sync replication slots while standby is behind
+($result, $stdout, $stderr) =
+  $standby->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Check skip reason and count when standby is behind
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'missing_wal_record', "slot sync skip when standby is behind");
+
+$result = $standby->safe_psql('postgres',
+	"SELECT slot_sync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Repeat sync to ensure skip count increments
+($result, $stdout, $stderr) =
+  $standby->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'missing_wal_record', "slot sync skip when standby is behind");
+
+$result = $standby->safe_psql('postgres',
+	"SELECT slot_sync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '2', "check slot sync skip count");
+
+# Restore streaming replication connection
+$primary->append_conf(
+	'pg_hba.conf', qq{
+local   replication     all                                     trust
+host    replication     all             127.0.0.1/32            trust
+host    replication     all             ::1/128                 trust
+});
+$primary->restart;
+
+# Wait for standby to catch up
+$primary->wait_for_replay_catchup($standby);
+
+# Check that skip reason is reset after successful sync
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'none', "slot_sync_skip_reason is reset after successful sync");
+
+# Cleanup: drop the logical slot and ensure standby catches up
+$primary->safe_psql('postgres',
+	"SELECT pg_drop_replication_slot('slot_sync')");
+$primary->wait_for_replay_catchup($standby);
+
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Test for case when slot sync is skipped when the remote slot is
+# behind the local slot.
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Attach injection point to simulate wait
+my $standby_psql = $standby->background_psql('postgres');
+$standby_psql->query_safe(
+	q(select injection_points_attach('slot-sync-skip','wait')));
+
+# Initiate sync of failover slots
+$standby_psql->query_until(
+	qr/slot_sync/,
+	q(
+\echo slot_sync
+select pg_sync_replication_slots();
+));
+
+# Wait for backend to reach injection point
+$standby->wait_for_event('client backend', 'slot-sync-skip');
+
+# Logical slot is temporary and sync will skip because remote is behind
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slot_sync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND temporary"
+);
+is($result, 'remote_behind', "slot sync skip as remote is behind");
+
+$result = $standby->safe_psql('postgres',
+	"SELECT slot_sync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Detach injection point
+$standby->safe_psql(
+	'postgres', q{
+	SELECT injection_points_detach('slot-sync-skip');
+	SELECT injection_points_wakeup('slot-sync-skip');
+});
+
+done_testing();
-- 
2.34.1

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

From 9d6b7d7ec4a9d4abfe0b3153f02921946fdbd63a 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 v7 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                 | 25 +++++++
 doc/src/sgml/system-views.sgml               | 43 +++++++++++
 src/backend/catalog/system_views.sql         |  5 +-
 src/backend/replication/logical/slotsync.c   | 77 ++++++++++++++++++++
 src/backend/replication/slot.c               |  1 +
 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               | 19 ++++-
 src/test/regress/expected/rules.out          |  9 ++-
 src/tools/pgindent/typedefs.list             |  1 +
 14 files changed, 254 insertions(+), 22 deletions(-)

diff --git a/contrib/test_decoding/expected/stats.out b/contrib/test_decoding/expected/stats.out
index 28da9123cc8..e5117f88a14 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 | slotsync_skip_count | slotsync_skip_at | 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 | slotsync_skip_count | slotsync_skip_at | 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 2741c138593..5332328e07b 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1659,6 +1659,31 @@ 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>slotsync_skip_count</structfield><type>bigint</type>
+       </para>
+       <para>
+        Number of times the slot synchronization is skipped. The value of this
+        column has no meaning on the primary server; it defaults to 0 for all
+        slots, but may (if leftover from a promoted standby) also have a
+        positive value.
+       </para>
+      </entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>slotsync_skip_at</structfield><type>timestamp with time zone</type>
+       </para>
+       <para>
+        Time at which last slot synchronization was skipped. The value of this
+        column has no meaning on the primary server; it defaults to NULL for all
+        slots, but may (if leftover from a promotedstandby) contain a timestamp.
+       </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..08ed9d609e1 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3102,6 +3102,49 @@ 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>slotsync_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 synchronized from a primary server (that
+       is, those whose <structfield>synced</structfield> field is
+       <literal>true</literal>). The value of this column has no meaning on the
+       primary server; it defaults to <literal>none</literal> for all slots, but
+       may (if leftover from a promoted standby) also have a value other than
+       <literal>none</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>wal_not_flushed</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 059e8778ca7..56f754c9973 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.slotsync_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.slotsync_skip_count,
+            s.slotsync_skip_at,
             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 8b4afd87dc9..fef00d0805d 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. This function requires the caller to acquire
+ * the slot.
+ */
+static void
+update_slot_sync_skip_stats(ReplicationSlot *slot, SlotSyncSkipReason skip_reason)
+{
+	Assert(MyReplicationSlot);
+
+	/*
+	 * 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);
+
+	/* Update the slot sync skip reason */
+	if (slot->data.slotsync_skip_reason != skip_reason)
+	{
+		SpinLockAcquire(&slot->mutex);
+		slot->data.slotsync_skip_reason = skip_reason;
+		SpinLockRelease(&slot->mutex);
+	}
+}
+
 /*
  * 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);
+
 		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);
+
 			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);
 		}
 
 		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);
+
 		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 +689,26 @@ 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)
+			{
+				ReplicationSlotAcquire(NameStr(slot->data.name), true, false);
+
+				if (slot->data.invalidated == RS_INVAL_NONE)
+					update_slot_sync_skip_stats(slot, SS_SKIP_WAL_NOT_FLUSHED);
+
+				ReplicationSlotRelease();
+			}
+		}
+
 		/*
 		 * Can get here only if GUC 'synchronized_standby_slots' on the
 		 * primary server was not configured correctly.
@@ -939,6 +1014,8 @@ synchronize_slots(WalReceiverConn *wrconn)
 	if (started_tx)
 		CommitTransactionCommand();
 
+	INJECTION_POINT("slot-sync-skip", NULL);
+
 	return some_slot_updated;
 }
 
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 1ec1e997b27..0da07b2873e 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -478,6 +478,7 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 	slot->data.two_phase_at = InvalidXLogRecPtr;
 	slot->data.failover = failover;
 	slot->data.synced = synced;
+	slot->data.slotsync_skip_reason = SS_SKIP_NONE;
 
 	/* and then data only present in shared memory */
 	slot->just_dirtied = false;
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 0478fc9c977..109185ce5a2 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_WAL_NOT_FLUSHED:
+			return "wal_not_flushed";
+		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.data.slotsync_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..89865d615c8 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->slotsync_skip_count += 1;
+	statent->slotsync_skip_at = 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 1521d6e2ab4..580a238fccd 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, "slotsync_skip_count",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 12, "slotsync_skip_at",
+					   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->slotsync_skip_count);
+
+	if (slotent->slotsync_skip_at == 0)
+		nulls[11] = true;
+	else
+		values[11] = TimestampTzGetDatum(slotent->slotsync_skip_at);
 
 	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 5cf9e12fcb9..0e209cc43e1 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,slotsync_skip_count,slotsync_skip_at,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,slotsync_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 a0610bb3e31..e9205700aa3 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -400,6 +400,8 @@ typedef struct PgStat_StatReplSlotEntry
 	PgStat_Counter mem_exceeded_count;
 	PgStat_Counter total_txns;
 	PgStat_Counter total_bytes;
+	PgStat_Counter slotsync_skip_count;
+	TimestampTz slotsync_skip_at;
 	TimestampTz stat_reset_timestamp;
 } PgStat_StatReplSlotEntry;
 
@@ -745,6 +747,7 @@ 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..33a0ad38163 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_WAL_NOT_FLUSHED,	/* 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.
  */
@@ -141,6 +156,9 @@ typedef struct ReplicationSlotPersistentData
 	 * for logical slots on the primary server.
 	 */
 	bool		failover;
+
+	/* The reason for last slot sync skip */
+	SlotSyncSkipReason slotsync_skip_reason;
 } ReplicationSlotPersistentData;
 
 /*
@@ -248,7 +266,6 @@ typedef struct ReplicationSlot
 	 * position.
 	 */
 	XLogRecPtr	last_saved_restart_lsn;
-
 } 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 7c52181cbcb..6e6425d01fe 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.slotsync_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, slotsync_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.slotsync_skip_count,
+    s.slotsync_skip_at,
     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, slotsync_skip_count, slotsync_skip_at, 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 432509277c9..17f1446cfd4 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2799,6 +2799,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

From b20f5558c26e9e9f2836d4d7792415c542ed9840 Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Wed, 12 Nov 2025 15:13:17 +0530
Subject: [PATCH v7 2/2] Add test for new stats for slot sync skip

---
 src/test/recovery/meson.build                 |   1 +
 .../recovery/t/050_slotsync_skip_stats.pl     | 198 ++++++++++++++++++
 2 files changed, 199 insertions(+)
 create mode 100644 src/test/recovery/t/050_slotsync_skip_stats.pl

diff --git a/src/test/recovery/meson.build b/src/test/recovery/meson.build
index 523a5cd5b52..17551cf114a 100644
--- a/src/test/recovery/meson.build
+++ b/src/test/recovery/meson.build
@@ -58,6 +58,7 @@ tests += {
       't/047_checkpoint_physical_slot.pl',
       't/048_vacuum_horizon_floor.pl',
       't/049_wait_for_lsn.pl',
+      't/050_slotsync_skip_stats.pl',
     ],
   },
 }
diff --git a/src/test/recovery/t/050_slotsync_skip_stats.pl b/src/test/recovery/t/050_slotsync_skip_stats.pl
new file mode 100644
index 00000000000..3f9d235c2b6
--- /dev/null
+++ b/src/test/recovery/t/050_slotsync_skip_stats.pl
@@ -0,0 +1,198 @@
+# Copyright (c) 2025, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Skip all tests if injection points are not supported in this build
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Initialize the primary cluster
+my $primary = PostgreSQL::Test::Cluster->new('primary');
+$primary->init(allows_streaming => 'logical');
+$primary->append_conf(
+	'postgresql.conf', qq{
+autovacuum = off
+});
+$primary->start;
+
+# Check if the extension injection_points is available, as it may be
+# possible that this script is run with installcheck, where the module
+# would not be installed by default.
+if (!$primary->check_extension('injection_points'))
+{
+	plan skip_all => 'Extension injection_points not installed';
+}
+
+# Load the injection_points extension
+$primary->safe_psql('postgres', q(CREATE EXTENSION injection_points));
+
+# Take a backup of the primary for standby initialization
+my $backup_name = 'backup';
+$primary->backup($backup_name);
+
+# Initialize standby from primary backup
+my $standby = PostgreSQL::Test::Cluster->new('standby');
+$standby->init_from_backup($primary, $backup_name, has_streaming => 1);
+
+my $connstr = $primary->connstr;
+$standby->append_conf(
+	'postgresql.conf', qq(
+hot_standby_feedback = on
+primary_slot_name = 'sb1_slot'
+primary_conninfo = '$connstr dbname=postgres'
+));
+
+# Create a physical replication slot on primary for standby
+$primary->safe_psql('postgres',
+	q{SELECT pg_create_physical_replication_slot('sb1_slot');});
+
+$standby->start;
+
+# Create a logical replication slot on primary for testing
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Wait for standby to catch up
+$primary->wait_for_replay_catchup($standby);
+
+# Initial sync of replication slots
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Verify that initially there is no skip reason
+my $result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced"
+);
+is($result, 'none', "slot sync reason is none");
+
+# Update pg_hba.conf and restart the primary to reject streaming replication
+# connections. WAL records won't be replicated to the standby until the
+# configuration is restored.
+unlink($primary->data_dir . '/pg_hba.conf');
+$primary->append_conf(
+	'pg_hba.conf', qq{
+local   all             all                                     trust
+host    all             all             127.0.0.1/32            trust
+host    all             all             ::1/128                 trust
+});
+$primary->restart;
+
+# Advance the failover slot so that confirmed flush LSN of remote slot become
+# ahead of standby's flushed LSN
+$primary->safe_psql(
+	'postgres', qq(
+    CREATE TABLE t1(a int);
+    INSERT INTO t1 VALUES(1);
+	SELECT pg_replication_slot_advance('slot_sync', pg_current_wal_lsn());
+));
+
+my ($stdout, $stderr);
+# Attempt to sync replication slots while standby is behind
+($result, $stdout, $stderr) =
+  $standby->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Verify pg_sync_replication_slots is failing
+ok( $stderr =~ /skipping slot synchronization because the received slot sync/,
+	'pg_sync_replication_slots failed as expected');
+
+# Check skip reason and count when standby is behind
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'wal_not_flushed', "slot sync skip when standby is behind");
+
+$result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Repeat sync to ensure skip count increments
+($result, $stdout, $stderr) =
+  $standby->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+$result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '2', "check slot sync skip count");
+
+# Restore streaming replication connection
+$primary->append_conf(
+	'pg_hba.conf', qq{
+local   replication     all                                     trust
+host    replication     all             127.0.0.1/32            trust
+host    replication     all             ::1/128                 trust
+});
+$primary->restart;
+
+# Wait for standby to catch up
+$primary->wait_for_replay_catchup($standby);
+
+# Check that skip reason is reset after successful sync
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'none', "slotsync_skip_reason is reset after successful sync");
+
+# Cleanup: drop the logical slot and ensure standby catches up
+$primary->safe_psql('postgres',
+	"SELECT pg_drop_replication_slot('slot_sync')");
+$primary->wait_for_replay_catchup($standby);
+
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Test for case when slot sync is skipped when the remote slot is
+# behind the local slot.
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Attach injection point to simulate wait
+my $standby_psql = $standby->background_psql('postgres');
+$standby_psql->query_safe(
+	q(select injection_points_attach('slot-sync-skip','wait')));
+
+# Initiate sync of failover slots
+$standby_psql->query_until(
+	qr/slot_sync/,
+	q(
+\echo slot_sync
+select pg_sync_replication_slots();
+));
+
+# Wait for backend to reach injection point
+$standby->wait_for_event('client backend', 'slot-sync-skip');
+
+# Logical slot is temporary and sync will skip because remote is behind
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND temporary"
+);
+is($result, 'remote_behind', "slot sync skip as remote is behind");
+
+$result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Detach injection point
+$standby->safe_psql(
+	'postgres', q{
+	SELECT injection_points_detach('slot-sync-skip');
+	SELECT injection_points_wakeup('slot-sync-skip');
+});
+
+done_testing();
-- 
2.34.1

From 3041fdff5a05f918807a8f2154d56cbb616a8d7d 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 v8 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 slotsync_skip_count,
last_slotsync_skip_at to view pg_stat_replication_slots and new column
slotsync_skip_reason to view pg_replication_slots.
---
 contrib/test_decoding/expected/stats.out     | 12 +--
 doc/src/sgml/monitoring.sgml                 | 25 ++++++
 doc/src/sgml/system-views.sgml               | 43 +++++++++++
 src/backend/catalog/system_views.sql         |  5 +-
 src/backend/replication/logical/slotsync.c   | 81 ++++++++++++++++++++
 src/backend/replication/slot.c               |  1 +
 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               | 17 ++++
 src/test/regress/expected/rules.out          |  9 ++-
 src/tools/pgindent/typedefs.list             |  1 +
 14 files changed, 257 insertions(+), 21 deletions(-)

diff --git a/contrib/test_decoding/expected/stats.out b/contrib/test_decoding/expected/stats.out
index 28da9123cc8..7426c48040a 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 | slotsync_skip_count | last_slotsync_skip_at | 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 | slotsync_skip_count | last_slotsync_skip_at | 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 436ef0e8bd0..894a972b3c1 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1659,6 +1659,31 @@ 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>slotsync_skip_count</structfield><type>bigint</type>
+       </para>
+       <para>
+        Number of times the slot synchronization is skipped. The value of this
+        column has no meaning on the primary server; it defaults to 0 for all
+        slots, but may (if leftover from a promoted standby) also have a
+        positive value.
+       </para>
+      </entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>last_slotsync_skip_at</structfield><type>timestamp with time zone</type>
+       </para>
+       <para>
+        Time at which last slot synchronization was skipped. The value of this
+        column has no meaning on the primary server; it defaults to NULL for all
+        slots, but may (if leftover from a promotedstandby) contain a timestamp.
+       </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..08ed9d609e1 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3102,6 +3102,49 @@ 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>slotsync_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 synchronized from a primary server (that
+       is, those whose <structfield>synced</structfield> field is
+       <literal>true</literal>). The value of this column has no meaning on the
+       primary server; it defaults to <literal>none</literal> for all slots, but
+       may (if leftover from a promoted standby) also have a value other than
+       <literal>none</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>wal_not_flushed</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 95ad29a64b9..4257e7db9be 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.slotsync_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.slotsync_skip_count,
+            s.last_slotsync_skip_at,
             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 8b4afd87dc9..b7d8b991d71 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 +149,35 @@ 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. This function requires the caller to acquire
+ * the slot.
+ */
+static void
+update_slotsync_skip_stats(SlotSyncSkipReason skip_reason)
+{
+	ReplicationSlot *slot;
+
+	Assert(MyReplicationSlot);
+
+	slot = MyReplicationSlot;
+
+	/*
+	 * 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_replslotsync_skip(slot);
+
+	/* Update the slot sync skip reason */
+	if (slot->slotsync_skip_reason != skip_reason)
+	{
+		SpinLockAcquire(&slot->mutex);
+		slot->slotsync_skip_reason = skip_reason;
+		SpinLockRelease(&slot->mutex);
+	}
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -218,6 +248,8 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 						  LSN_FORMAT_ARGS(slot->data.restart_lsn),
 						  slot->data.catalog_xmin));
 
+		update_slotsync_skip_stats(SS_SKIP_REMOTE_BEHIND);
+
 		if (remote_slot_precedes)
 			*remote_slot_precedes = true;
 
@@ -261,6 +293,9 @@ 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_slotsync_skip_stats(SS_SKIP_NONE);
+
 			if (found_consistent_snapshot)
 				*found_consistent_snapshot = true;
 		}
@@ -277,6 +312,17 @@ 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_slotsync_skip_stats(SS_SKIP_NONE);
 		}
 
 		updated_xmin_or_lsn = true;
@@ -580,6 +626,9 @@ 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 +644,21 @@ 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_slotsync_skip_stats(SS_SKIP_NO_CONSISTENT_SNAPSHOT);
+
 		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 +693,26 @@ 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)
+			{
+				ReplicationSlotAcquire(NameStr(slot->data.name), true, false);
+
+				if (slot->data.invalidated == RS_INVAL_NONE)
+					update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
+
+				ReplicationSlotRelease();
+			}
+		}
+
 		/*
 		 * Can get here only if GUC 'synchronized_standby_slots' on the
 		 * primary server was not configured correctly.
@@ -939,6 +1018,8 @@ synchronize_slots(WalReceiverConn *wrconn)
 	if (started_tx)
 		CommitTransactionCommand();
 
+	INJECTION_POINT("slot-sync-skip", NULL);
+
 	return some_slot_updated;
 }
 
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 1ec1e997b27..86ae99a3ca9 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -491,6 +491,7 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 	slot->last_saved_confirmed_flush = InvalidXLogRecPtr;
 	slot->last_saved_restart_lsn = InvalidXLogRecPtr;
 	slot->inactive_since = 0;
+	slot->slotsync_skip_reason = SS_SKIP_NONE;
 
 	/*
 	 * Create the slot on disk.  We haven't actually marked the slot allocated
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 0478fc9c977..66b989027a5 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_WAL_NOT_FLUSHED:
+			return "wal_not_flushed";
+		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.slotsync_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..10cf97bd29a 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_replslotsync_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->slotsync_skip_count += 1;
+	statent->last_slotsync_skip_at = 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 3d98d064a94..607aa39fd89 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, "slotsync_skip_count",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 12, "last_slotsync_skip_at",
+					   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->slotsync_skip_count);
+
+	if (slotent->last_slotsync_skip_at == 0)
+		nulls[11] = true;
+	else
+		values[11] = TimestampTzGetDatum(slotent->last_slotsync_skip_at);
 
 	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 aaadfd8c748..e1dc1d2e8b1 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,slotsync_skip_count,last_slotsync_skip_at,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,slotsync_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 a68e725259a..1cb96e6d0a6 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -400,6 +400,8 @@ typedef struct PgStat_StatReplSlotEntry
 	PgStat_Counter mem_exceeded_count;
 	PgStat_Counter total_txns;
 	PgStat_Counter total_bytes;
+	PgStat_Counter slotsync_skip_count;
+	TimestampTz last_slotsync_skip_at;
 	TimestampTz stat_reset_timestamp;
 } PgStat_StatReplSlotEntry;
 
@@ -745,6 +747,7 @@ 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_replslotsync_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..aebc40f486d 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_WAL_NOT_FLUSHED,	/* 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 +264,8 @@ typedef struct ReplicationSlot
 	 */
 	XLogRecPtr	last_saved_restart_lsn;
 
+	/* The reason for last slot sync skip */
+	SlotSyncSkipReason slotsync_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 372a2188c22..872116cb02a 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.slotsync_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, slotsync_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.slotsync_skip_count,
+    s.last_slotsync_skip_at,
     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, slotsync_skip_count, last_slotsync_skip_at, 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 23bce72ae64..eb19e2c2363 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2799,6 +2799,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

From bd9df395c639b478df5e62888a01d29eace3e221 Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Tue, 18 Nov 2025 15:42:10 +0530
Subject: [PATCH v8 2/2] Add test for new stats for slot sync skip

---
 src/test/recovery/meson.build                 |   1 +
 .../recovery/t/050_slotsync_skip_stats.pl     | 201 ++++++++++++++++++
 2 files changed, 202 insertions(+)
 create mode 100644 src/test/recovery/t/050_slotsync_skip_stats.pl

diff --git a/src/test/recovery/meson.build b/src/test/recovery/meson.build
index 523a5cd5b52..17551cf114a 100644
--- a/src/test/recovery/meson.build
+++ b/src/test/recovery/meson.build
@@ -58,6 +58,7 @@ tests += {
       't/047_checkpoint_physical_slot.pl',
       't/048_vacuum_horizon_floor.pl',
       't/049_wait_for_lsn.pl',
+      't/050_slotsync_skip_stats.pl',
     ],
   },
 }
diff --git a/src/test/recovery/t/050_slotsync_skip_stats.pl b/src/test/recovery/t/050_slotsync_skip_stats.pl
new file mode 100644
index 00000000000..9313b9cb45b
--- /dev/null
+++ b/src/test/recovery/t/050_slotsync_skip_stats.pl
@@ -0,0 +1,201 @@
+# Copyright (c) 2025, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Skip all tests if injection points are not supported in this build
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Initialize the primary cluster
+my $primary = PostgreSQL::Test::Cluster->new('primary');
+$primary->init(allows_streaming => 'logical');
+$primary->append_conf(
+	'postgresql.conf', qq{
+autovacuum = off
+});
+$primary->start;
+
+# Check if the extension injection_points is available, as it may be
+# possible that this script is run with installcheck, where the module
+# would not be installed by default.
+if (!$primary->check_extension('injection_points'))
+{
+	plan skip_all => 'Extension injection_points not installed';
+}
+
+# Load the injection_points extension
+$primary->safe_psql('postgres', q(CREATE EXTENSION injection_points));
+
+# Take a backup of the primary for standby initialization
+my $backup_name = 'backup';
+$primary->backup($backup_name);
+
+# Initialize standby from primary backup
+my $standby = PostgreSQL::Test::Cluster->new('standby');
+$standby->init_from_backup($primary, $backup_name, has_streaming => 1);
+
+my $connstr = $primary->connstr;
+$standby->append_conf(
+	'postgresql.conf', qq(
+hot_standby_feedback = on
+primary_slot_name = 'sb1_slot'
+primary_conninfo = '$connstr dbname=postgres'
+));
+
+# Create a physical replication slot on primary for standby
+$primary->safe_psql('postgres',
+	q{SELECT pg_create_physical_replication_slot('sb1_slot');});
+
+$standby->start;
+
+# Create a logical replication slot on primary for testing
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Wait for standby to catch up
+$primary->wait_for_replay_catchup($standby);
+
+# Initial sync of replication slots
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Verify that initially there is no skip reason
+my $result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced"
+);
+is($result, 'none', "slot sync reason is none");
+
+# Update pg_hba.conf and restart the primary to reject streaming replication
+# connections. WAL records won't be replicated to the standby until the
+# configuration is restored.
+unlink($primary->data_dir . '/pg_hba.conf');
+$primary->append_conf(
+	'pg_hba.conf', qq{
+local   all             all                                     trust
+host    all             all             127.0.0.1/32            trust
+host    all             all             ::1/128                 trust
+});
+$primary->restart;
+
+# Advance the failover slot so that confirmed flush LSN of remote slot become
+# ahead of standby's flushed LSN
+$primary->safe_psql(
+	'postgres', qq(
+    CREATE TABLE t1(a int);
+    INSERT INTO t1 VALUES(1);
+	SELECT pg_replication_slot_advance('slot_sync', pg_current_wal_lsn());
+));
+
+my ($stdout, $stderr);
+# Attempt to sync replication slots while standby is behind
+($result, $stdout, $stderr) =
+  $standby->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Verify pg_sync_replication_slots is failing
+ok( $stderr =~
+	  qr/skipping slot synchronization because the received slot sync.*is ahead of the standby position/,
+	'pg_sync_replication_slots failed as expected');
+
+# Check skip reason and count when standby is behind
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'wal_not_flushed', "slot sync skip when standby is behind");
+
+$result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Repeat sync to ensure skip count increments
+($result, $stdout, $stderr) =
+  $standby->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+$result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '2', "check slot sync skip count");
+
+# Restore streaming replication connection
+$primary->append_conf(
+	'pg_hba.conf', qq{
+local   replication     all                                     trust
+host    replication     all             127.0.0.1/32            trust
+host    replication     all             ::1/128                 trust
+});
+$primary->restart;
+
+# Wait for standby to catch up
+$primary->wait_for_replay_catchup($standby);
+
+# Check that skip reason is reset after successful sync
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'none', "slotsync_skip_reason is reset after successful sync");
+
+# Cleanup: drop the logical slot and ensure standby catches up
+$primary->safe_psql('postgres',
+	"SELECT pg_drop_replication_slot('slot_sync')");
+$primary->wait_for_replay_catchup($standby);
+
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Test for case when slot sync is skipped when the remote slot is
+# behind the local slot.
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Attach injection point to simulate wait
+my $standby_psql = $standby->background_psql('postgres');
+$standby_psql->query_safe(
+	q(select injection_points_attach('slot-sync-skip','wait')));
+
+# Initiate sync of failover slots
+$standby_psql->query_until(
+	qr/slot_sync/,
+	q(
+\echo slot_sync
+select pg_sync_replication_slots();
+));
+
+# Wait for backend to reach injection point
+$standby->wait_for_event('client backend', 'slot-sync-skip');
+
+# Logical slot is temporary and sync will skip because remote is behind
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND temporary"
+);
+is($result, 'remote_behind', "slot sync skip as remote is behind");
+
+$result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Detach injection point
+$standby->safe_psql(
+	'postgres', q{
+	SELECT injection_points_detach('slot-sync-skip');
+	SELECT injection_points_wakeup('slot-sync-skip');
+});
+
+$standby_psql->quit;
+
+done_testing();
-- 
2.34.1

From d7133f583cf5fd101777b91bd85d4f0ecad8897a Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Fri, 21 Nov 2025 12:35:46 +0530
Subject: [PATCH v9 1/2] Add slotsync skip statistics

This patch introduces two new columns slotsync_skip_count,
slotsync_last_skip to pg_stat_replication_slots view. These columns
indicates the number of time the slotsync was skipped and the last time
at which slotsync was skipped.
---
 contrib/test_decoding/expected/stats.out      |  12 +-
 doc/src/sgml/monitoring.sgml                  |  25 +++
 src/backend/catalog/system_views.sql          |   2 +
 src/backend/replication/logical/slotsync.c    |  49 +++++
 src/backend/utils/activity/pgstat_replslot.c  |  25 +++
 src/backend/utils/adt/pgstatfuncs.c           |  18 +-
 src/include/catalog/pg_proc.dat               |   6 +-
 src/include/pgstat.h                          |   3 +
 src/test/recovery/meson.build                 |   1 +
 .../recovery/t/050_slotsync_skip_stats.pl     | 176 ++++++++++++++++++
 src/test/regress/expected/rules.out           |   4 +-
 11 files changed, 307 insertions(+), 14 deletions(-)
 create mode 100644 src/test/recovery/t/050_slotsync_skip_stats.pl

diff --git a/contrib/test_decoding/expected/stats.out b/contrib/test_decoding/expected/stats.out
index 28da9123cc8..d0749bc0daf 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 | slotsync_skip_count | slotsync_last_skip_at | 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 | slotsync_skip_count | slotsync_last_skip_at | 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 436ef0e8bd0..b054ebb4ade 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1659,6 +1659,31 @@ 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>slotsync_skip_count</structfield><type>bigint</type>
+       </para>
+       <para>
+        Number of times the slot synchronization is skipped. The value of this
+        column has no meaning on the primary server; it defaults to 0 for all
+        slots, but may (if leftover from a promoted standby) also have a
+        positive value.
+       </para>
+      </entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>slotsync_last_skip_at</structfield><type>timestamp with time zone</type>
+       </para>
+       <para>
+        Time at which last slot synchronization was skipped. The value of this
+        column has no meaning on the primary server; it defaults to NULL for all
+        slots, but may (if leftover from a promoted standby) contain a timestamp.
+       </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/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 95ad29a64b9..c77a5d15a2e 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1076,6 +1076,8 @@ CREATE VIEW pg_stat_replication_slots AS
             s.mem_exceeded_count,
             s.total_txns,
             s.total_bytes,
+            s.slotsync_skip_count,
+            s.slotsync_last_skip_at,
             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 8b4afd87dc9..052117f0481 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"
@@ -218,6 +219,9 @@ 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 */
+		pgstat_report_replslotsync_skip(slot);
+
 		if (remote_slot_precedes)
 			*remote_slot_precedes = true;
 
@@ -277,6 +281,17 @@ 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,
+			 * indicating slot synchronization is successful.
+			 */
+			if (found_consistent_snapshot && !(*found_consistent_snapshot))
+				pgstat_report_replslotsync_skip(slot);
 		}
 
 		updated_xmin_or_lsn = true;
@@ -580,6 +595,9 @@ 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;
 	}
@@ -590,6 +608,10 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 	 */
 	if (!found_consistent_snapshot)
 	{
+		/*
+		 * We do not need to update the slot sync skip stats here as it will
+		 * be already updated in function update_local_synced_slot.
+		 */
 		ereport(LOG,
 				errmsg("could not synchronize replication slot \"%s\"", remote_slot->name),
 				errdetail("Synchronization could lead to data loss, because the standby could not build a consistent snapshot to decode WALs at LSN %X/%08X.",
@@ -600,6 +622,10 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 
 	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 +660,25 @@ 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)
+			{
+				ReplicationSlotAcquire(NameStr(slot->data.name), true, false);
+
+				pgstat_report_replslotsync_skip(slot);
+
+				ReplicationSlotRelease();
+			}
+		}
+
 		/*
 		 * Can get here only if GUC 'synchronized_standby_slots' on the
 		 * primary server was not configured correctly.
@@ -707,6 +752,8 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		/* Skip the sync of an invalidated slot */
 		if (slot->data.invalidated != RS_INVAL_NONE)
 		{
+			pgstat_report_replslotsync_skip(slot);
+
 			ReplicationSlotRelease();
 			return slot_updated;
 		}
@@ -939,6 +986,8 @@ synchronize_slots(WalReceiverConn *wrconn)
 	if (started_tx)
 		CommitTransactionCommand();
 
+	INJECTION_POINT("slot-sync-skip", NULL);
+
 	return some_slot_updated;
 }
 
diff --git a/src/backend/utils/activity/pgstat_replslot.c b/src/backend/utils/activity/pgstat_replslot.c
index d210c261ac6..d9cc4ec2314 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_replslotsync_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->slotsync_skip_count += 1;
+	statent->slotsync_last_skip_at = 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 3d98d064a94..46e103ce7c3 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, "slotsync_skip_count",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 12, "slotsync_last_skip_at",
+					   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->slotsync_skip_count);
+
+	if (slotent->slotsync_last_skip_at == 0)
+		nulls[11] = true;
+	else
+		values[11] = TimestampTzGetDatum(slotent->slotsync_last_skip_at);
 
 	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 aaadfd8c748..b10809ba9b6 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,slotsync_skip_count,slotsync_last_skip_at,stats_reset}',
   prosrc => 'pg_stat_get_replication_slot' },
 
 { oid => '6230', descr => 'statistics: check if a stats object exists',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index a68e725259a..144042c1940 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -400,6 +400,8 @@ typedef struct PgStat_StatReplSlotEntry
 	PgStat_Counter mem_exceeded_count;
 	PgStat_Counter total_txns;
 	PgStat_Counter total_bytes;
+	PgStat_Counter slotsync_skip_count;
+	TimestampTz slotsync_last_skip_at;
 	TimestampTz stat_reset_timestamp;
 } PgStat_StatReplSlotEntry;
 
@@ -745,6 +747,7 @@ 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_replslotsync_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/test/recovery/meson.build b/src/test/recovery/meson.build
index 523a5cd5b52..17551cf114a 100644
--- a/src/test/recovery/meson.build
+++ b/src/test/recovery/meson.build
@@ -58,6 +58,7 @@ tests += {
       't/047_checkpoint_physical_slot.pl',
       't/048_vacuum_horizon_floor.pl',
       't/049_wait_for_lsn.pl',
+      't/050_slotsync_skip_stats.pl',
     ],
   },
 }
diff --git a/src/test/recovery/t/050_slotsync_skip_stats.pl b/src/test/recovery/t/050_slotsync_skip_stats.pl
new file mode 100644
index 00000000000..39ce9ef702b
--- /dev/null
+++ b/src/test/recovery/t/050_slotsync_skip_stats.pl
@@ -0,0 +1,176 @@
+# Copyright (c) 2025, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Skip all tests if injection points are not supported in this build
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Initialize the primary cluster
+my $primary = PostgreSQL::Test::Cluster->new('primary');
+$primary->init(allows_streaming => 'logical');
+$primary->append_conf(
+	'postgresql.conf', qq{
+autovacuum = off
+});
+$primary->start;
+
+# Check if the extension injection_points is available, as it may be
+# possible that this script is run with installcheck, where the module
+# would not be installed by default.
+if (!$primary->check_extension('injection_points'))
+{
+	plan skip_all => 'Extension injection_points not installed';
+}
+
+# Load the injection_points extension
+$primary->safe_psql('postgres', q(CREATE EXTENSION injection_points));
+
+# Take a backup of the primary for standby initialization
+my $backup_name = 'backup';
+$primary->backup($backup_name);
+
+# Initialize standby from primary backup
+my $standby = PostgreSQL::Test::Cluster->new('standby');
+$standby->init_from_backup($primary, $backup_name, has_streaming => 1);
+
+my $connstr = $primary->connstr;
+$standby->append_conf(
+	'postgresql.conf', qq(
+hot_standby_feedback = on
+primary_slot_name = 'sb1_slot'
+primary_conninfo = '$connstr dbname=postgres'
+));
+
+# Create a physical replication slot on primary for standby
+$primary->safe_psql('postgres',
+	q{SELECT pg_create_physical_replication_slot('sb1_slot');});
+
+$standby->start;
+
+# Create a logical replication slot on primary for testing
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Wait for standby to catch up
+$primary->wait_for_replay_catchup($standby);
+
+# Initial sync of replication slots
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Verify slot is synced successfully
+my $result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '0', "check slot sync skip count after initial sync");
+
+# Update pg_hba.conf and restart the primary to reject streaming replication
+# connections. WAL records won't be replicated to the standby until the
+# configuration is restored.
+unlink($primary->data_dir . '/pg_hba.conf');
+$primary->append_conf(
+	'pg_hba.conf', qq{
+local   all             all                                     trust
+host    all             all             127.0.0.1/32            trust
+host    all             all             ::1/128                 trust
+});
+$primary->restart;
+
+# Advance the failover slot so that confirmed flush LSN of remote slot become
+# ahead of standby's flushed LSN
+$primary->safe_psql(
+	'postgres', qq(
+    CREATE TABLE t1(a int);
+    INSERT INTO t1 VALUES(1);
+	SELECT pg_replication_slot_advance('slot_sync', pg_current_wal_lsn());
+));
+
+my ($stdout, $stderr);
+# Attempt to sync replication slots while standby is behind
+($result, $stdout, $stderr) =
+  $standby->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Verify pg_sync_replication_slots is failing
+ok( $stderr =~
+	  qr/skipping slot synchronization because the received slot sync.*is ahead of the standby position/,
+	'pg_sync_replication_slots failed as expected');
+
+# Check slot sync skip count when standby is behind
+$result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Repeat sync to ensure skip count increments
+($result, $stdout, $stderr) =
+  $standby->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+$result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '2', "check slot sync skip count");
+
+# Restore streaming replication connection
+$primary->append_conf(
+	'pg_hba.conf', qq{
+local   replication     all                                     trust
+host    replication     all             127.0.0.1/32            trust
+host    replication     all             ::1/128                 trust
+});
+$primary->restart;
+
+# Wait for standby to catch up
+$primary->wait_for_replay_catchup($standby);
+
+# Cleanup: drop the logical slot and ensure standby catches up
+$primary->safe_psql('postgres',
+	"SELECT pg_drop_replication_slot('slot_sync')");
+$primary->wait_for_replay_catchup($standby);
+
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Test for case when slot sync is skipped when the remote slot is
+# behind the local slot.
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Attach injection point to simulate wait
+my $standby_psql = $standby->background_psql('postgres');
+$standby_psql->query_safe(
+	q(select injection_points_attach('slot-sync-skip','wait')));
+
+# Initiate sync of failover slots
+$standby_psql->query_until(
+	qr/slot_sync/,
+	q(
+\echo slot_sync
+select pg_sync_replication_slots();
+));
+
+# Wait for backend to reach injection point
+$standby->wait_for_event('client backend', 'slot-sync-skip');
+
+# Logical slot is temporary and sync will skip because remote is behind
+$result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Detach injection point
+$standby->safe_psql(
+	'postgres', q{
+	SELECT injection_points_detach('slot-sync-skip');
+	SELECT injection_points_wakeup('slot-sync-skip');
+});
+
+$standby_psql->quit;
+
+done_testing();
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 372a2188c22..adda7f425e2 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -2151,9 +2151,11 @@ pg_stat_replication_slots| SELECT s.slot_name,
     s.mem_exceeded_count,
     s.total_txns,
     s.total_bytes,
+    s.slotsync_skip_count,
+    s.slotsync_last_skip_at,
     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, slotsync_skip_count, slotsync_last_skip_at, stats_reset)
   WHERE (r.datoid IS NOT NULL);
 pg_stat_slru| SELECT name,
     blks_zeroed,
-- 
2.34.1

From 7739ec27aa8ac66471a1018f0753bf970ba973fd Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Fri, 21 Nov 2025 14:39:47 +0530
Subject: [PATCH v9 2/2] Add slotsync_skip_reason to pg_replication_slots

This patch introduces a new column slotsync_skip_reson to
pg_replication_slots view. This indicates the reason for last slot
synchronization skip.
---
 doc/src/sgml/system-views.sgml                | 48 +++++++++++++++++++
 src/backend/catalog/system_views.sql          |  3 +-
 src/backend/replication/logical/slotsync.c    | 44 +++++++++++++++--
 src/backend/replication/slot.c                |  1 +
 src/backend/replication/slotfuncs.c           | 28 ++++++++++-
 src/include/catalog/pg_proc.dat               |  6 +--
 src/include/replication/slot.h                | 26 ++++++++++
 .../recovery/t/050_slotsync_skip_stats.pl     | 33 ++++++++++++-
 src/test/regress/expected/rules.out           |  5 +-
 src/tools/pgindent/typedefs.list              |  1 +
 10 files changed, 182 insertions(+), 13 deletions(-)

diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 7971498fe75..79fce8ed697 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3102,6 +3102,54 @@ 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>slotsync_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 synchronized from a primary server (that
+       is, those whose <structfield>synced</structfield> field is
+       <literal>true</literal>). The value of this column has no meaning on the
+       primary server; it defaults to <literal>none</literal> for all slots, but
+       may (if leftover from a promoted standby) also have a value other than
+       <literal>none</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>wal_not_flushed</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>
+        <listitem>
+         <para>
+          <literal>slot_invalidated</literal> means that the last slot
+          synchronization was skipped because the slot is invalidated.
+        </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 c77a5d15a2e..1445ac5a78c 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.slotsync_skip_reason
     FROM pg_get_replication_slots() AS L
             LEFT JOIN pg_database D ON (L.datoid = D.oid);
 
diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 052117f0481..83cc37239c7 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -149,6 +149,35 @@ 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. This function requires the caller to acquire
+ * the slot.
+ */
+static void
+update_slotsync_skip_stats(SlotSyncSkipReason skip_reason)
+{
+	ReplicationSlot *slot;
+
+	Assert(MyReplicationSlot);
+
+	slot = MyReplicationSlot;
+
+	/*
+	 * 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_replslotsync_skip(slot);
+
+	/* Update the slot sync skip reason */
+	if (slot->slotsync_skip_reason != skip_reason)
+	{
+		SpinLockAcquire(&slot->mutex);
+		slot->slotsync_skip_reason = skip_reason;
+		SpinLockRelease(&slot->mutex);
+	}
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -171,6 +200,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 	ReplicationSlot *slot = MyReplicationSlot;
 	bool		updated_xmin_or_lsn = false;
 	bool		updated_config = false;
+	SlotSyncSkipReason skip_reason = SS_SKIP_NONE;
 
 	Assert(slot->data.invalidated == RS_INVAL_NONE);
 
@@ -220,7 +250,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 						  slot->data.catalog_xmin));
 
 		/* Update slot sync skip stats */
-		pgstat_report_replslotsync_skip(slot);
+		update_slotsync_skip_stats(SS_SKIP_REMOTE_BEHIND);
 
 		if (remote_slot_precedes)
 			*remote_slot_precedes = true;
@@ -291,12 +321,15 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 			 * indicating slot synchronization is successful.
 			 */
 			if (found_consistent_snapshot && !(*found_consistent_snapshot))
-				pgstat_report_replslotsync_skip(slot);
+				skip_reason = SS_SKIP_NO_CONSISTENT_SNAPSHOT;
 		}
 
 		updated_xmin_or_lsn = true;
 	}
 
+	/* Update slot sync skip stats */
+	update_slotsync_skip_stats(skip_reason);
+
 	if (remote_dbid != slot->data.database ||
 		remote_slot->two_phase != slot->data.two_phase ||
 		remote_slot->failover != slot->data.failover ||
@@ -673,7 +706,10 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 			{
 				ReplicationSlotAcquire(NameStr(slot->data.name), true, false);
 
-				pgstat_report_replslotsync_skip(slot);
+				if (slot->data.invalidated == RS_INVAL_NONE)
+					update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
+				else
+					update_slotsync_skip_stats(SS_SKIP_INVALID);
 
 				ReplicationSlotRelease();
 			}
@@ -752,7 +788,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		/* Skip the sync of an invalidated slot */
 		if (slot->data.invalidated != RS_INVAL_NONE)
 		{
-			pgstat_report_replslotsync_skip(slot);
+			update_slotsync_skip_stats(SS_SKIP_INVALID);
 
 			ReplicationSlotRelease();
 			return slot_updated;
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 1ec1e997b27..86ae99a3ca9 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -491,6 +491,7 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 	slot->last_saved_confirmed_flush = InvalidXLogRecPtr;
 	slot->last_saved_restart_lsn = InvalidXLogRecPtr;
 	slot->inactive_since = 0;
+	slot->slotsync_skip_reason = SS_SKIP_NONE;
 
 	/*
 	 * Create the slot on disk.  We haven't actually marked the slot allocated
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 0478fc9c977..7200c7f071d 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -228,6 +228,30 @@ 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_WAL_NOT_FLUSHED:
+			return "wal_not_flushed";
+		case SS_SKIP_NO_CONSISTENT_SNAPSHOT:
+			return "no_consistent_snapshot";
+		case SS_SKIP_INVALID:
+			return "slot_invalidated";
+	}
+
+	Assert(false);
+	return "none";
+}
+
 /*
  * pg_get_replication_slots - SQL SRF showing all replication slots
  * that currently exist on the database cluster.
@@ -235,7 +259,7 @@ 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 +467,8 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 
 		values[i++] = BoolGetDatum(slot_contents.data.synced);
 
+		values[i++] = CStringGetTextDatum(GetSlotSyncSkipReason(slot_contents.slotsync_skip_reason));
+
 		Assert(i == PG_GET_REPLICATION_SLOTS_COLS);
 
 		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index b10809ba9b6..4205d565df3 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -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,slotsync_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/replication/slot.h b/src/include/replication/slot.h
index 09c69f83d57..054e81b6c43 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -71,6 +71,22 @@ 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_WAL_NOT_FLUSHED,	/* Standby did not flush the wal corresponding
+								 * to confirmed flush of 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 */
+	SS_SKIP_INVALID				/* Local slot is invalid */
+} SlotSyncSkipReason;
+
 /*
  * On-Disk data of a replication slot, preserved across restarts.
  */
@@ -249,6 +265,16 @@ typedef struct ReplicationSlot
 	 */
 	XLogRecPtr	last_saved_restart_lsn;
 
+	/*
+	 * The reason for last slot sync skip.
+	 *
+	 * A slotsync skip typically occurs only for temporary slots. For
+	 * persistent slots it is extremely rare (e.g., cases like
+	 * SS_SKIP_WAL_NOT_FLUSHED or SS_SKIP_REMOTE_BEHIND). Also, temporary
+	 * slots are dropped after server restart, so there is no value in
+	 * persisting the slotsync_skip_reason.
+	 */
+	SlotSyncSkipReason slotsync_skip_reason;
 } ReplicationSlot;
 
 #define SlotIsPhysical(slot) ((slot)->data.database == InvalidOid)
diff --git a/src/test/recovery/t/050_slotsync_skip_stats.pl b/src/test/recovery/t/050_slotsync_skip_stats.pl
index 39ce9ef702b..59b512bc116 100644
--- a/src/test/recovery/t/050_slotsync_skip_stats.pl
+++ b/src/test/recovery/t/050_slotsync_skip_stats.pl
@@ -66,7 +66,13 @@ $primary->wait_for_replay_catchup($standby);
 $standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
 
 # Verify slot is synced successfully
-my $result = $standby->safe_psql('postgres',
+my $result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced"
+);
+is($result, 'none', "slot sync reason is none");
+$result = $standby->safe_psql('postgres',
 	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
 );
 is($result, '0', "check slot sync skip count after initial sync");
@@ -102,7 +108,14 @@ ok( $stderr =~
 	  qr/skipping slot synchronization because the received slot sync.*is ahead of the standby position/,
 	'pg_sync_replication_slots failed as expected');
 
-# Check slot sync skip count when standby is behind
+# Check skip reason and count when standby is behind
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'wal_not_flushed', "slot sync skip when standby is behind");
+
 $result = $standby->safe_psql('postgres',
 	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
 );
@@ -129,6 +142,15 @@ $primary->restart;
 # Wait for standby to catch up
 $primary->wait_for_replay_catchup($standby);
 
+# Check that skip reason is reset after successful sync
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'none', "slotsync_skip_reason is reset after successful sync");
+
 # Cleanup: drop the logical slot and ensure standby catches up
 $primary->safe_psql('postgres',
 	"SELECT pg_drop_replication_slot('slot_sync')");
@@ -159,6 +181,13 @@ select pg_sync_replication_slots();
 $standby->wait_for_event('client backend', 'slot-sync-skip');
 
 # Logical slot is temporary and sync will skip because remote is behind
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND temporary"
+);
+is($result, 'remote_behind', "slot sync skip as remote is behind");
+
 $result = $standby->safe_psql('postgres',
 	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
 );
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index adda7f425e2..feac3e4c089 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.slotsync_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, slotsync_skip_reason)
      LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
 pg_roles| SELECT pg_authid.rolname,
     pg_authid.rolsuper,
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 57f2a9ccdc5..435927e5638 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2802,6 +2802,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

From d7133f583cf5fd101777b91bd85d4f0ecad8897a Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Fri, 21 Nov 2025 12:35:46 +0530
Subject: [PATCH v10 1/2] Add slotsync skip statistics

This patch introduces two new columns slotsync_skip_count,
slotsync_last_skip to pg_stat_replication_slots view. These columns
indicates the number of time the slotsync was skipped and the last time
at which slotsync was skipped.
---
 contrib/test_decoding/expected/stats.out      |  12 +-
 doc/src/sgml/monitoring.sgml                  |  25 +++
 src/backend/catalog/system_views.sql          |   2 +
 src/backend/replication/logical/slotsync.c    |  49 +++++
 src/backend/utils/activity/pgstat_replslot.c  |  25 +++
 src/backend/utils/adt/pgstatfuncs.c           |  18 +-
 src/include/catalog/pg_proc.dat               |   6 +-
 src/include/pgstat.h                          |   3 +
 src/test/recovery/meson.build                 |   1 +
 .../recovery/t/050_slotsync_skip_stats.pl     | 176 ++++++++++++++++++
 src/test/regress/expected/rules.out           |   4 +-
 11 files changed, 307 insertions(+), 14 deletions(-)
 create mode 100644 src/test/recovery/t/050_slotsync_skip_stats.pl

diff --git a/contrib/test_decoding/expected/stats.out b/contrib/test_decoding/expected/stats.out
index 28da9123cc8..d0749bc0daf 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 | slotsync_skip_count | slotsync_last_skip_at | 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 | slotsync_skip_count | slotsync_last_skip_at | 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 436ef0e8bd0..b054ebb4ade 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1659,6 +1659,31 @@ 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>slotsync_skip_count</structfield><type>bigint</type>
+       </para>
+       <para>
+        Number of times the slot synchronization is skipped. The value of this
+        column has no meaning on the primary server; it defaults to 0 for all
+        slots, but may (if leftover from a promoted standby) also have a
+        positive value.
+       </para>
+      </entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>slotsync_last_skip_at</structfield><type>timestamp with time zone</type>
+       </para>
+       <para>
+        Time at which last slot synchronization was skipped. The value of this
+        column has no meaning on the primary server; it defaults to NULL for all
+        slots, but may (if leftover from a promoted standby) contain a timestamp.
+       </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/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 95ad29a64b9..c77a5d15a2e 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1076,6 +1076,8 @@ CREATE VIEW pg_stat_replication_slots AS
             s.mem_exceeded_count,
             s.total_txns,
             s.total_bytes,
+            s.slotsync_skip_count,
+            s.slotsync_last_skip_at,
             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 8b4afd87dc9..052117f0481 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"
@@ -218,6 +219,9 @@ 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 */
+		pgstat_report_replslotsync_skip(slot);
+
 		if (remote_slot_precedes)
 			*remote_slot_precedes = true;
 
@@ -277,6 +281,17 @@ 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,
+			 * indicating slot synchronization is successful.
+			 */
+			if (found_consistent_snapshot && !(*found_consistent_snapshot))
+				pgstat_report_replslotsync_skip(slot);
 		}
 
 		updated_xmin_or_lsn = true;
@@ -580,6 +595,9 @@ 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;
 	}
@@ -590,6 +608,10 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 	 */
 	if (!found_consistent_snapshot)
 	{
+		/*
+		 * We do not need to update the slot sync skip stats here as it will
+		 * be already updated in function update_local_synced_slot.
+		 */
 		ereport(LOG,
 				errmsg("could not synchronize replication slot \"%s\"", remote_slot->name),
 				errdetail("Synchronization could lead to data loss, because the standby could not build a consistent snapshot to decode WALs at LSN %X/%08X.",
@@ -600,6 +622,10 @@ update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 
 	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 +660,25 @@ 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)
+			{
+				ReplicationSlotAcquire(NameStr(slot->data.name), true, false);
+
+				pgstat_report_replslotsync_skip(slot);
+
+				ReplicationSlotRelease();
+			}
+		}
+
 		/*
 		 * Can get here only if GUC 'synchronized_standby_slots' on the
 		 * primary server was not configured correctly.
@@ -707,6 +752,8 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		/* Skip the sync of an invalidated slot */
 		if (slot->data.invalidated != RS_INVAL_NONE)
 		{
+			pgstat_report_replslotsync_skip(slot);
+
 			ReplicationSlotRelease();
 			return slot_updated;
 		}
@@ -939,6 +986,8 @@ synchronize_slots(WalReceiverConn *wrconn)
 	if (started_tx)
 		CommitTransactionCommand();
 
+	INJECTION_POINT("slot-sync-skip", NULL);
+
 	return some_slot_updated;
 }
 
diff --git a/src/backend/utils/activity/pgstat_replslot.c b/src/backend/utils/activity/pgstat_replslot.c
index d210c261ac6..d9cc4ec2314 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_replslotsync_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->slotsync_skip_count += 1;
+	statent->slotsync_last_skip_at = 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 3d98d064a94..46e103ce7c3 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, "slotsync_skip_count",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 12, "slotsync_last_skip_at",
+					   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->slotsync_skip_count);
+
+	if (slotent->slotsync_last_skip_at == 0)
+		nulls[11] = true;
+	else
+		values[11] = TimestampTzGetDatum(slotent->slotsync_last_skip_at);
 
 	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 aaadfd8c748..b10809ba9b6 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,slotsync_skip_count,slotsync_last_skip_at,stats_reset}',
   prosrc => 'pg_stat_get_replication_slot' },
 
 { oid => '6230', descr => 'statistics: check if a stats object exists',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index a68e725259a..144042c1940 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -400,6 +400,8 @@ typedef struct PgStat_StatReplSlotEntry
 	PgStat_Counter mem_exceeded_count;
 	PgStat_Counter total_txns;
 	PgStat_Counter total_bytes;
+	PgStat_Counter slotsync_skip_count;
+	TimestampTz slotsync_last_skip_at;
 	TimestampTz stat_reset_timestamp;
 } PgStat_StatReplSlotEntry;
 
@@ -745,6 +747,7 @@ 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_replslotsync_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/test/recovery/meson.build b/src/test/recovery/meson.build
index 523a5cd5b52..17551cf114a 100644
--- a/src/test/recovery/meson.build
+++ b/src/test/recovery/meson.build
@@ -58,6 +58,7 @@ tests += {
       't/047_checkpoint_physical_slot.pl',
       't/048_vacuum_horizon_floor.pl',
       't/049_wait_for_lsn.pl',
+      't/050_slotsync_skip_stats.pl',
     ],
   },
 }
diff --git a/src/test/recovery/t/050_slotsync_skip_stats.pl b/src/test/recovery/t/050_slotsync_skip_stats.pl
new file mode 100644
index 00000000000..39ce9ef702b
--- /dev/null
+++ b/src/test/recovery/t/050_slotsync_skip_stats.pl
@@ -0,0 +1,176 @@
+# Copyright (c) 2025, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Skip all tests if injection points are not supported in this build
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Initialize the primary cluster
+my $primary = PostgreSQL::Test::Cluster->new('primary');
+$primary->init(allows_streaming => 'logical');
+$primary->append_conf(
+	'postgresql.conf', qq{
+autovacuum = off
+});
+$primary->start;
+
+# Check if the extension injection_points is available, as it may be
+# possible that this script is run with installcheck, where the module
+# would not be installed by default.
+if (!$primary->check_extension('injection_points'))
+{
+	plan skip_all => 'Extension injection_points not installed';
+}
+
+# Load the injection_points extension
+$primary->safe_psql('postgres', q(CREATE EXTENSION injection_points));
+
+# Take a backup of the primary for standby initialization
+my $backup_name = 'backup';
+$primary->backup($backup_name);
+
+# Initialize standby from primary backup
+my $standby = PostgreSQL::Test::Cluster->new('standby');
+$standby->init_from_backup($primary, $backup_name, has_streaming => 1);
+
+my $connstr = $primary->connstr;
+$standby->append_conf(
+	'postgresql.conf', qq(
+hot_standby_feedback = on
+primary_slot_name = 'sb1_slot'
+primary_conninfo = '$connstr dbname=postgres'
+));
+
+# Create a physical replication slot on primary for standby
+$primary->safe_psql('postgres',
+	q{SELECT pg_create_physical_replication_slot('sb1_slot');});
+
+$standby->start;
+
+# Create a logical replication slot on primary for testing
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Wait for standby to catch up
+$primary->wait_for_replay_catchup($standby);
+
+# Initial sync of replication slots
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Verify slot is synced successfully
+my $result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '0', "check slot sync skip count after initial sync");
+
+# Update pg_hba.conf and restart the primary to reject streaming replication
+# connections. WAL records won't be replicated to the standby until the
+# configuration is restored.
+unlink($primary->data_dir . '/pg_hba.conf');
+$primary->append_conf(
+	'pg_hba.conf', qq{
+local   all             all                                     trust
+host    all             all             127.0.0.1/32            trust
+host    all             all             ::1/128                 trust
+});
+$primary->restart;
+
+# Advance the failover slot so that confirmed flush LSN of remote slot become
+# ahead of standby's flushed LSN
+$primary->safe_psql(
+	'postgres', qq(
+    CREATE TABLE t1(a int);
+    INSERT INTO t1 VALUES(1);
+	SELECT pg_replication_slot_advance('slot_sync', pg_current_wal_lsn());
+));
+
+my ($stdout, $stderr);
+# Attempt to sync replication slots while standby is behind
+($result, $stdout, $stderr) =
+  $standby->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Verify pg_sync_replication_slots is failing
+ok( $stderr =~
+	  qr/skipping slot synchronization because the received slot sync.*is ahead of the standby position/,
+	'pg_sync_replication_slots failed as expected');
+
+# Check slot sync skip count when standby is behind
+$result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Repeat sync to ensure skip count increments
+($result, $stdout, $stderr) =
+  $standby->psql('postgres', "SELECT pg_sync_replication_slots();");
+
+$result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '2', "check slot sync skip count");
+
+# Restore streaming replication connection
+$primary->append_conf(
+	'pg_hba.conf', qq{
+local   replication     all                                     trust
+host    replication     all             127.0.0.1/32            trust
+host    replication     all             ::1/128                 trust
+});
+$primary->restart;
+
+# Wait for standby to catch up
+$primary->wait_for_replay_catchup($standby);
+
+# Cleanup: drop the logical slot and ensure standby catches up
+$primary->safe_psql('postgres',
+	"SELECT pg_drop_replication_slot('slot_sync')");
+$primary->wait_for_replay_catchup($standby);
+
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+# Test for case when slot sync is skipped when the remote slot is
+# behind the local slot.
+$primary->safe_psql('postgres',
+	"SELECT pg_create_logical_replication_slot('slot_sync', 'test_decoding', false, false, true)"
+);
+
+# Attach injection point to simulate wait
+my $standby_psql = $standby->background_psql('postgres');
+$standby_psql->query_safe(
+	q(select injection_points_attach('slot-sync-skip','wait')));
+
+# Initiate sync of failover slots
+$standby_psql->query_until(
+	qr/slot_sync/,
+	q(
+\echo slot_sync
+select pg_sync_replication_slots();
+));
+
+# Wait for backend to reach injection point
+$standby->wait_for_event('client backend', 'slot-sync-skip');
+
+# Logical slot is temporary and sync will skip because remote is behind
+$result = $standby->safe_psql('postgres',
+	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
+);
+is($result, '1', "check slot sync skip count");
+
+# Detach injection point
+$standby->safe_psql(
+	'postgres', q{
+	SELECT injection_points_detach('slot-sync-skip');
+	SELECT injection_points_wakeup('slot-sync-skip');
+});
+
+$standby_psql->quit;
+
+done_testing();
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 372a2188c22..adda7f425e2 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -2151,9 +2151,11 @@ pg_stat_replication_slots| SELECT s.slot_name,
     s.mem_exceeded_count,
     s.total_txns,
     s.total_bytes,
+    s.slotsync_skip_count,
+    s.slotsync_last_skip_at,
     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, slotsync_skip_count, slotsync_last_skip_at, stats_reset)
   WHERE (r.datoid IS NOT NULL);
 pg_stat_slru| SELECT name,
     blks_zeroed,
-- 
2.34.1

From a7c6526eb30b1f5a47f210f950b3876d7b536ac9 Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Fri, 21 Nov 2025 14:39:47 +0530
Subject: [PATCH v10 2/2] Add slotsync_skip_reason to pg_replication_slots

This patch introduces a new column slotsync_skip_reson to
pg_replication_slots view. This indicates the reason for last slot
synchronization skip.
---
 doc/src/sgml/system-views.sgml                | 49 +++++++++++++++++++
 src/backend/catalog/system_views.sql          |  3 +-
 src/backend/replication/logical/slotsync.c    | 44 +++++++++++++++--
 src/backend/replication/slot.c                |  1 +
 src/backend/replication/slotfuncs.c           | 28 ++++++++++-
 src/include/catalog/pg_proc.dat               |  6 +--
 src/include/replication/slot.h                | 26 ++++++++++
 .../recovery/t/050_slotsync_skip_stats.pl     | 33 ++++++++++++-
 src/test/regress/expected/rules.out           |  5 +-
 src/tools/pgindent/typedefs.list              |  1 +
 10 files changed, 183 insertions(+), 13 deletions(-)

diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 7971498fe75..ec797d2d916 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3102,6 +3102,55 @@ 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>slotsync_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 synchronized from a primary server (that
+       is, those whose <structfield>synced</structfield> field is
+       <literal>true</literal>). The value of this column has no meaning on the
+       primary server; it defaults to <literal>none</literal> for all slots, but
+       may (if leftover from a promoted standby) also have a value other than
+       <literal>none</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>wal_not_flushed</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>
+        <listitem>
+         <para>
+          <literal>slot_invalidated</literal> means that the last slot
+          synchronization was skipped because the slot is invalidated.
+         </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 c77a5d15a2e..1445ac5a78c 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.slotsync_skip_reason
     FROM pg_get_replication_slots() AS L
             LEFT JOIN pg_database D ON (L.datoid = D.oid);
 
diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 052117f0481..83cc37239c7 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -149,6 +149,35 @@ 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. This function requires the caller to acquire
+ * the slot.
+ */
+static void
+update_slotsync_skip_stats(SlotSyncSkipReason skip_reason)
+{
+	ReplicationSlot *slot;
+
+	Assert(MyReplicationSlot);
+
+	slot = MyReplicationSlot;
+
+	/*
+	 * 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_replslotsync_skip(slot);
+
+	/* Update the slot sync skip reason */
+	if (slot->slotsync_skip_reason != skip_reason)
+	{
+		SpinLockAcquire(&slot->mutex);
+		slot->slotsync_skip_reason = skip_reason;
+		SpinLockRelease(&slot->mutex);
+	}
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -171,6 +200,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 	ReplicationSlot *slot = MyReplicationSlot;
 	bool		updated_xmin_or_lsn = false;
 	bool		updated_config = false;
+	SlotSyncSkipReason skip_reason = SS_SKIP_NONE;
 
 	Assert(slot->data.invalidated == RS_INVAL_NONE);
 
@@ -220,7 +250,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 						  slot->data.catalog_xmin));
 
 		/* Update slot sync skip stats */
-		pgstat_report_replslotsync_skip(slot);
+		update_slotsync_skip_stats(SS_SKIP_REMOTE_BEHIND);
 
 		if (remote_slot_precedes)
 			*remote_slot_precedes = true;
@@ -291,12 +321,15 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 			 * indicating slot synchronization is successful.
 			 */
 			if (found_consistent_snapshot && !(*found_consistent_snapshot))
-				pgstat_report_replslotsync_skip(slot);
+				skip_reason = SS_SKIP_NO_CONSISTENT_SNAPSHOT;
 		}
 
 		updated_xmin_or_lsn = true;
 	}
 
+	/* Update slot sync skip stats */
+	update_slotsync_skip_stats(skip_reason);
+
 	if (remote_dbid != slot->data.database ||
 		remote_slot->two_phase != slot->data.two_phase ||
 		remote_slot->failover != slot->data.failover ||
@@ -673,7 +706,10 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 			{
 				ReplicationSlotAcquire(NameStr(slot->data.name), true, false);
 
-				pgstat_report_replslotsync_skip(slot);
+				if (slot->data.invalidated == RS_INVAL_NONE)
+					update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
+				else
+					update_slotsync_skip_stats(SS_SKIP_INVALID);
 
 				ReplicationSlotRelease();
 			}
@@ -752,7 +788,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		/* Skip the sync of an invalidated slot */
 		if (slot->data.invalidated != RS_INVAL_NONE)
 		{
-			pgstat_report_replslotsync_skip(slot);
+			update_slotsync_skip_stats(SS_SKIP_INVALID);
 
 			ReplicationSlotRelease();
 			return slot_updated;
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 1ec1e997b27..86ae99a3ca9 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -491,6 +491,7 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 	slot->last_saved_confirmed_flush = InvalidXLogRecPtr;
 	slot->last_saved_restart_lsn = InvalidXLogRecPtr;
 	slot->inactive_since = 0;
+	slot->slotsync_skip_reason = SS_SKIP_NONE;
 
 	/*
 	 * Create the slot on disk.  We haven't actually marked the slot allocated
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 0478fc9c977..7200c7f071d 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -228,6 +228,30 @@ 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_WAL_NOT_FLUSHED:
+			return "wal_not_flushed";
+		case SS_SKIP_NO_CONSISTENT_SNAPSHOT:
+			return "no_consistent_snapshot";
+		case SS_SKIP_INVALID:
+			return "slot_invalidated";
+	}
+
+	Assert(false);
+	return "none";
+}
+
 /*
  * pg_get_replication_slots - SQL SRF showing all replication slots
  * that currently exist on the database cluster.
@@ -235,7 +259,7 @@ 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 +467,8 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 
 		values[i++] = BoolGetDatum(slot_contents.data.synced);
 
+		values[i++] = CStringGetTextDatum(GetSlotSyncSkipReason(slot_contents.slotsync_skip_reason));
+
 		Assert(i == PG_GET_REPLICATION_SLOTS_COLS);
 
 		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index b10809ba9b6..4205d565df3 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -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,slotsync_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/replication/slot.h b/src/include/replication/slot.h
index 09c69f83d57..054e81b6c43 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -71,6 +71,22 @@ 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_WAL_NOT_FLUSHED,	/* Standby did not flush the wal corresponding
+								 * to confirmed flush of 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 */
+	SS_SKIP_INVALID				/* Local slot is invalid */
+} SlotSyncSkipReason;
+
 /*
  * On-Disk data of a replication slot, preserved across restarts.
  */
@@ -249,6 +265,16 @@ typedef struct ReplicationSlot
 	 */
 	XLogRecPtr	last_saved_restart_lsn;
 
+	/*
+	 * The reason for last slot sync skip.
+	 *
+	 * A slotsync skip typically occurs only for temporary slots. For
+	 * persistent slots it is extremely rare (e.g., cases like
+	 * SS_SKIP_WAL_NOT_FLUSHED or SS_SKIP_REMOTE_BEHIND). Also, temporary
+	 * slots are dropped after server restart, so there is no value in
+	 * persisting the slotsync_skip_reason.
+	 */
+	SlotSyncSkipReason slotsync_skip_reason;
 } ReplicationSlot;
 
 #define SlotIsPhysical(slot) ((slot)->data.database == InvalidOid)
diff --git a/src/test/recovery/t/050_slotsync_skip_stats.pl b/src/test/recovery/t/050_slotsync_skip_stats.pl
index 39ce9ef702b..59b512bc116 100644
--- a/src/test/recovery/t/050_slotsync_skip_stats.pl
+++ b/src/test/recovery/t/050_slotsync_skip_stats.pl
@@ -66,7 +66,13 @@ $primary->wait_for_replay_catchup($standby);
 $standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
 
 # Verify slot is synced successfully
-my $result = $standby->safe_psql('postgres',
+my $result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced"
+);
+is($result, 'none', "slot sync reason is none");
+$result = $standby->safe_psql('postgres',
 	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
 );
 is($result, '0', "check slot sync skip count after initial sync");
@@ -102,7 +108,14 @@ ok( $stderr =~
 	  qr/skipping slot synchronization because the received slot sync.*is ahead of the standby position/,
 	'pg_sync_replication_slots failed as expected');
 
-# Check slot sync skip count when standby is behind
+# Check skip reason and count when standby is behind
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'wal_not_flushed', "slot sync skip when standby is behind");
+
 $result = $standby->safe_psql('postgres',
 	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
 );
@@ -129,6 +142,15 @@ $primary->restart;
 # Wait for standby to catch up
 $primary->wait_for_replay_catchup($standby);
 
+# Check that skip reason is reset after successful sync
+$standby->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND NOT temporary"
+);
+is($result, 'none', "slotsync_skip_reason is reset after successful sync");
+
 # Cleanup: drop the logical slot and ensure standby catches up
 $primary->safe_psql('postgres',
 	"SELECT pg_drop_replication_slot('slot_sync')");
@@ -159,6 +181,13 @@ select pg_sync_replication_slots();
 $standby->wait_for_event('client backend', 'slot-sync-skip');
 
 # Logical slot is temporary and sync will skip because remote is behind
+$result = $standby->safe_psql(
+	'postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots
+     WHERE slot_name = 'slot_sync' AND synced AND temporary"
+);
+is($result, 'remote_behind', "slot sync skip as remote is behind");
+
 $result = $standby->safe_psql('postgres',
 	"SELECT slotsync_skip_count FROM pg_stat_replication_slots WHERE slot_name = 'slot_sync'"
 );
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index adda7f425e2..feac3e4c089 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.slotsync_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, slotsync_skip_reason)
      LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
 pg_roles| SELECT pg_authid.rolname,
     pg_authid.rolsuper,
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 57f2a9ccdc5..435927e5638 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2802,6 +2802,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

From 2fc8dc66a5c77a3b86f25daefdbbc8015dd7ac2f Mon Sep 17 00:00:00 2001
From: Zhijie Hou <houzj.fnst@fujitsu.com>
Date: Tue, 25 Nov 2025 16:08:35 +0800
Subject: [PATCH v1] Fix a BF failure where a replication slot was not released
 in time

The commit 76b7872 did not release the replication slot in the slotsync worker
when a cycle of slot synchronization was skipped due to the required WAL not
being received and flushed on the standby server. This commit fixes that
issue.
---
 src/backend/replication/logical/slotsync.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 7e9dc7f18bd..1f4f06d467b 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -725,6 +725,8 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 						   remote_slot->name,
 						   LSN_FORMAT_ARGS(latestFlushPtr)));
 
+			ReplicationSlotRelease();
+
 			return slot_updated;
 		}
 
@@ -824,6 +826,8 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 						   remote_slot->name,
 						   LSN_FORMAT_ARGS(latestFlushPtr)));
 
+			ReplicationSlotRelease();
+
 			return false;
 		}
 
-- 
2.51.1.windows.1

From cd5a916631b4d34884cde3aa6f29f12427146bc9 Mon Sep 17 00:00:00 2001
From: Zhijie Hou <houzj.fnst@fujitsu.com>
Date: Tue, 25 Nov 2025 18:42:55 +0800
Subject: [PATCH v1] Fix test failure caused by commit 76b78721ca

The test failed because it assumed that a newly created logical
replication slot could be synced to the standby by the slotsync worker.
However, the presence of an existing physical slot caused the new logical
slot to use a non-latest xmin. On the standby, the DDL had already been
replayed, advancing xmin, which led to the slotsync worker failing to sync
the lagging logical slot.

To resolve this, we moved the slot sync statistics tests to run after the
tests that do not require the newly created slot to be sync-ready.
---
 .../t/040_standby_failover_slots_sync.pl      | 128 ++++++++++--------
 1 file changed, 70 insertions(+), 58 deletions(-)

diff --git a/src/test/recovery/t/040_standby_failover_slots_sync.pl b/src/test/recovery/t/040_standby_failover_slots_sync.pl
index b2bf5072bbf..8e682b88ead 100644
--- a/src/test/recovery/t/040_standby_failover_slots_sync.pl
+++ b/src/test/recovery/t/040_standby_failover_slots_sync.pl
@@ -213,75 +213,19 @@ is( $standby1->safe_psql(
 ##################################################
 # Test that the synchronized slot will be dropped if the corresponding remote
 # slot on the primary server has been dropped.
-#
-# Note: Both slots need to be dropped for the next test to work
 ##################################################
 
 $primary->psql('postgres', "SELECT pg_drop_replication_slot('lsub2_slot');");
-$primary->psql('postgres', "SELECT pg_drop_replication_slot('lsub1_slot');");
 
 $standby1->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
 
 is( $standby1->safe_psql(
 		'postgres',
-		q{SELECT count(*) = 0 FROM pg_replication_slots WHERE slot_name IN ('lsub1_slot', 'lsub2_slot');}
+		q{SELECT count(*) = 0 FROM pg_replication_slots WHERE slot_name = 'lsub2_slot';}
 	),
 	"t",
 	'synchronized slot has been dropped');
 
-##################################################
-# Verify that slotsync skip statistics are correctly updated when the
-# slotsync operation is skipped.
-##################################################
-
-# Create a logical replication slot and create some DDL on the primary so
-# that the slot lags behind the standby.
-$primary->safe_psql(
-	'postgres', qq(
-	SELECT pg_create_logical_replication_slot('lsub1_slot', 'pgoutput', false, false, true);
-	CREATE TABLE wal_push(a int);
-));
-$primary->wait_for_replay_catchup($standby1);
-
-my $log_offset = -s $standby1->logfile;
-
-# Enable slot sync worker.
-$standby1->append_conf('postgresql.conf', qq(sync_replication_slots = on));
-$standby1->reload;
-
-# Confirm that the slot sync worker is able to start.
-$standby1->wait_for_log(qr/slot sync worker started/, $log_offset);
-
-# Confirm that the slot sync is skipped due to the remote slot lagging behind
-$standby1->wait_for_log(
-	qr/could not synchronize replication slot \"lsub1_slot\"/, $log_offset);
-
-# Confirm that the slotsync skip statistics is updated
-$result = $standby1->safe_psql('postgres',
-	"SELECT slotsync_skip_count > 0 FROM pg_stat_replication_slots WHERE slot_name = 'lsub1_slot'"
-);
-is($result, 't', "check slot sync skip count increments");
-
-# Clean the table
-$primary->safe_psql(
-	'postgres', qq(
-    DROP TABLE wal_push;
-));
-$primary->wait_for_replay_catchup($standby1);
-
-# Re-create the logical replication slot and sync it to standby for further tests
-$primary->safe_psql(
-	'postgres', qq(
-	SELECT pg_drop_replication_slot('lsub1_slot');
-	SELECT pg_create_logical_replication_slot('lsub1_slot', 'pgoutput', false, false, true);
-));
-$standby1->wait_for_log(
-	qr/newly created replication slot \"lsub1_slot\" is sync-ready now/,
-	$log_offset);
-
-$standby1->append_conf('postgresql.conf', qq(sync_replication_slots = off));
-$standby1->reload;
-
 ##################################################
 # Test that if the synchronized slot is invalidated while the remote slot is
 # still valid, the slot will be dropped and re-created on the standby by
@@ -337,7 +281,7 @@ $inactive_since_on_primary =
 # the failover slots.
 $primary->wait_for_replay_catchup($standby1);
 
-$log_offset = -s $standby1->logfile;
+my $log_offset = -s $standby1->logfile;
 
 # Synchronize the primary server slots to the standby.
 $standby1->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
@@ -1043,4 +987,72 @@ $result = $standby1->safe_psql('postgres',
 
 is($result, '1', "data can be consumed using snap_test_slot");
 
+##################################################
+# Verify that slotsync skip statistics are correctly updated when the
+# slotsync operation is skipped.
+##################################################
+
+# Enable slot sync on standby2
+$standby2->append_conf('postgresql.conf', qq(
+hot_standby_feedback = on
+primary_conninfo = '$connstr_1 dbname=postgres'
+log_min_messages = 'debug2'
+));
+
+$standby2->reload;
+
+# Commit the pending prepared transaction
+$primary->safe_psql('postgres', "COMMIT PREPARED 'test_twophase_slotsync';");
+$primary->wait_for_replay_catchup($standby2);
+
+# Remove all logical replication slots on the primary server to ensure the
+# corresponding synced slots are also removed. This guarantees that the safest
+# catalog_xmin on the standby is not preserved by existing slots, allowing newly
+# created slots to have a fresher initial catalog_xmin.
+$primary->psql('postgres', qq(
+	SELECT pg_drop_replication_slot('lsub1_slot');
+	SELECT pg_drop_replication_slot('snap_test_slot');
+));
+
+$subscriber2->safe_psql(
+	'postgres', 'DROP SUBSCRIPTION regress_mysub2;');
+
+$standby2->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
+
+is( $standby2->safe_psql(
+		'postgres',
+		q{SELECT count(*) = 0 FROM pg_replication_slots WHERE slot_name IN ('lsub1_slot', 'lsub2_slot', 'snap_test_slot');}
+	),
+	"t",
+	'synchronized slot has been dropped');
+
+# Create a logical replication slot and create some DDL on the primary so
+# that the slot lags behind the standby.
+$primary->safe_psql(
+	'postgres', qq(
+	SELECT pg_create_logical_replication_slot('lsub1_slot', 'pgoutput', false, false, true);
+	CREATE TABLE wal_push(a int);
+));
+$primary->wait_for_replay_catchup($standby2);
+
+$log_offset = -s $standby2->logfile;
+
+# Enable slot sync worker
+$standby2->append_conf('postgresql.conf', qq(sync_replication_slots = on));
+
+$standby2->reload;
+
+# Confirm that the slot sync worker is able to start.
+$standby2->wait_for_log(qr/slot sync worker started/, $log_offset);
+
+# Confirm that the slot sync is skipped due to the remote slot lagging behind
+$standby2->wait_for_log(
+	qr/could not synchronize replication slot \"lsub1_slot\"/, $log_offset);
+
+# Confirm that the slotsync skip statistics is updated
+$result = $standby2->safe_psql('postgres',
+	"SELECT slotsync_skip_count > 0 FROM pg_stat_replication_slots WHERE slot_name = 'lsub1_slot'"
+);
+is($result, 't', "check slot sync skip count increments");
+
 done_testing();
-- 
2.51.1.windows.1

From de3d8dc82cb4eefc1d516091f6fc5bd5dc9252cd Mon Sep 17 00:00:00 2001
From: Zhijie Hou <houzj.fnst@fujitsu.com>
Date: Tue, 25 Nov 2025 18:42:55 +0800
Subject: [PATCH v2] Fix test failure caused by commit 76b78721ca

The test failed because it assumed that a newly created logical
replication slot could be synced to the standby by the slotsync worker.
However, the presence of an existing physical slot caused the new logical
slot to use a non-latest xmin. On the standby, the DDL had already been
replayed, advancing xmin, which led to the slotsync worker failing to sync
the lagging logical slot.

To resolve this, we moved the slot sync statistics tests to run after the
tests that do not require the newly created slot to be sync-ready.
---
 .../t/040_standby_failover_slots_sync.pl      | 128 ++++++++++--------
 1 file changed, 70 insertions(+), 58 deletions(-)

diff --git a/src/test/recovery/t/040_standby_failover_slots_sync.pl b/src/test/recovery/t/040_standby_failover_slots_sync.pl
index b2bf5072bbf..7d3c82e0a29 100644
--- a/src/test/recovery/t/040_standby_failover_slots_sync.pl
+++ b/src/test/recovery/t/040_standby_failover_slots_sync.pl
@@ -213,75 +213,19 @@ is( $standby1->safe_psql(
 ##################################################
 # Test that the synchronized slot will be dropped if the corresponding remote
 # slot on the primary server has been dropped.
-#
-# Note: Both slots need to be dropped for the next test to work
 ##################################################
 
 $primary->psql('postgres', "SELECT pg_drop_replication_slot('lsub2_slot');");
-$primary->psql('postgres', "SELECT pg_drop_replication_slot('lsub1_slot');");
 
 $standby1->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
 
 is( $standby1->safe_psql(
 		'postgres',
-		q{SELECT count(*) = 0 FROM pg_replication_slots WHERE slot_name IN ('lsub1_slot', 'lsub2_slot');}
+		q{SELECT count(*) = 0 FROM pg_replication_slots WHERE slot_name = 'lsub2_slot';}
 	),
 	"t",
 	'synchronized slot has been dropped');
 
-##################################################
-# Verify that slotsync skip statistics are correctly updated when the
-# slotsync operation is skipped.
-##################################################
-
-# Create a logical replication slot and create some DDL on the primary so
-# that the slot lags behind the standby.
-$primary->safe_psql(
-	'postgres', qq(
-	SELECT pg_create_logical_replication_slot('lsub1_slot', 'pgoutput', false, false, true);
-	CREATE TABLE wal_push(a int);
-));
-$primary->wait_for_replay_catchup($standby1);
-
-my $log_offset = -s $standby1->logfile;
-
-# Enable slot sync worker.
-$standby1->append_conf('postgresql.conf', qq(sync_replication_slots = on));
-$standby1->reload;
-
-# Confirm that the slot sync worker is able to start.
-$standby1->wait_for_log(qr/slot sync worker started/, $log_offset);
-
-# Confirm that the slot sync is skipped due to the remote slot lagging behind
-$standby1->wait_for_log(
-	qr/could not synchronize replication slot \"lsub1_slot\"/, $log_offset);
-
-# Confirm that the slotsync skip statistics is updated
-$result = $standby1->safe_psql('postgres',
-	"SELECT slotsync_skip_count > 0 FROM pg_stat_replication_slots WHERE slot_name = 'lsub1_slot'"
-);
-is($result, 't', "check slot sync skip count increments");
-
-# Clean the table
-$primary->safe_psql(
-	'postgres', qq(
-    DROP TABLE wal_push;
-));
-$primary->wait_for_replay_catchup($standby1);
-
-# Re-create the logical replication slot and sync it to standby for further tests
-$primary->safe_psql(
-	'postgres', qq(
-	SELECT pg_drop_replication_slot('lsub1_slot');
-	SELECT pg_create_logical_replication_slot('lsub1_slot', 'pgoutput', false, false, true);
-));
-$standby1->wait_for_log(
-	qr/newly created replication slot \"lsub1_slot\" is sync-ready now/,
-	$log_offset);
-
-$standby1->append_conf('postgresql.conf', qq(sync_replication_slots = off));
-$standby1->reload;
-
 ##################################################
 # Test that if the synchronized slot is invalidated while the remote slot is
 # still valid, the slot will be dropped and re-created on the standby by
@@ -337,7 +281,7 @@ $inactive_since_on_primary =
 # the failover slots.
 $primary->wait_for_replay_catchup($standby1);
 
-$log_offset = -s $standby1->logfile;
+my $log_offset = -s $standby1->logfile;
 
 # Synchronize the primary server slots to the standby.
 $standby1->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
@@ -1043,4 +987,72 @@ $result = $standby1->safe_psql('postgres',
 
 is($result, '1', "data can be consumed using snap_test_slot");
 
+##################################################
+# Remove any unnecessary replication slots and clear pending transactions on the
+# primary server to ensure a clean environment.
+##################################################
+
+$primary->psql(
+	'postgres', qq(
+	SELECT pg_drop_replication_slot('sb1_slot');
+	SELECT pg_drop_replication_slot('lsub1_slot');
+	SELECT pg_drop_replication_slot('snap_test_slot');
+));
+
+$subscriber2->safe_psql('postgres', 'DROP SUBSCRIPTION regress_mysub2;');
+
+# Verify that all slots have been removed except the one necessary for standby2,
+# which is needed for further testing.
+is( $primary->safe_psql(
+		'postgres',
+		q{SELECT count(*) = 0 FROM pg_replication_slots WHERE slot_name != 'sb2_slot';}
+	),
+	"t",
+	'all replication slots have been dropped except the physical slot used by standby2'
+);
+
+# Commit the pending prepared transaction
+$primary->safe_psql('postgres', "COMMIT PREPARED 'test_twophase_slotsync';");
+$primary->wait_for_replay_catchup($standby2);
+
+##################################################
+# Verify that slotsync skip statistics are correctly updated when the
+# slotsync operation is skipped.
+##################################################
+
+# Create a logical replication slot and create some DDL on the primary so
+# that the slot lags behind the standby.
+$primary->safe_psql(
+	'postgres', qq(
+	SELECT pg_create_logical_replication_slot('lsub1_slot', 'pgoutput', false, false, true);
+	CREATE TABLE wal_push(a int);
+));
+$primary->wait_for_replay_catchup($standby2);
+
+$log_offset = -s $standby2->logfile;
+
+# Enable slot sync worker
+$standby2->append_conf(
+	'postgresql.conf', qq(
+hot_standby_feedback = on
+primary_conninfo = '$connstr_1 dbname=postgres'
+log_min_messages = 'debug2'
+sync_replication_slots = on
+));
+
+$standby2->reload;
+
+# Confirm that the slot sync worker is able to start.
+$standby2->wait_for_log(qr/slot sync worker started/, $log_offset);
+
+# Confirm that the slot sync is skipped due to the remote slot lagging behind
+$standby2->wait_for_log(
+	qr/could not synchronize replication slot \"lsub1_slot\"/, $log_offset);
+
+# Confirm that the slotsync skip statistics is updated
+$result = $standby2->safe_psql('postgres',
+	"SELECT slotsync_skip_count > 0 FROM pg_stat_replication_slots WHERE slot_name = 'lsub1_slot'"
+);
+is($result, 't', "check slot sync skip count increments");
+
 done_testing();
-- 
2.51.1.windows.1

From 727d1df40a9cc38ebb5cb525e4dfa714799ae6e0 Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Wed, 26 Nov 2025 08:52:29 +0530
Subject: [PATCH v12] Add slotsync_skip_reason to pg_replication_slots

This patch introduces a new column slotsync_skip_reason to
pg_replication_slots view. This indicates the reason for last slot
synchronization skip.
---
 doc/src/sgml/monitoring.sgml                  |  4 +-
 doc/src/sgml/system-views.sgml                | 49 +++++++++++++++++++
 src/backend/catalog/system_views.sql          |  3 +-
 src/backend/replication/logical/slotsync.c    | 43 ++++++++++++++--
 src/backend/replication/slot.c                |  1 +
 src/backend/replication/slotfuncs.c           | 28 ++++++++++-
 src/include/catalog/pg_proc.dat               |  6 +--
 src/include/replication/slot.h                | 26 ++++++++++
 .../t/040_standby_failover_slots_sync.pl      |  6 +++
 src/test/regress/expected/rules.out           |  5 +-
 src/tools/pgindent/typedefs.list              |  1 +
 11 files changed, 158 insertions(+), 14 deletions(-)

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index dcc8474a7f7..e0556b6baac 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1665,7 +1665,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para>
        <para>
         Number of times the slot synchronization is skipped. Slot
-        synchronization occur only on standby servers and thus this column has
+        synchronization occurs only on standby servers and thus this column has
         no meaning on the primary server.
        </para>
       </entry>
@@ -1677,7 +1677,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para>
        <para>
         Time at which last slot synchronization was skipped. Slot
-        synchronization occur only on standby servers and thus this column has
+        synchronization occurs only on standby servers and thus this column has
         no meaning on the primary server.
        </para>
       </entry>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 0e623e7fb86..4dd5174ba1e 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3102,6 +3102,55 @@ 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>slotsync_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 synchronized from a primary server (that
+       is, those whose <structfield>synced</structfield> field is
+       <literal>true</literal>). The value of this column has no meaning on the
+       primary server; it defaults to <literal>none</literal> for all slots, but
+       may (if leftover from a promoted standby) also have a value other than
+       <literal>none</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>wal_not_flushed</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>
+        <listitem>
+         <para>
+          <literal>slot_invalidated</literal> means that the last slot
+          synchronization was skipped because the slot is invalidated.
+         </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 6fffdb9398e..086c4c8fb6f 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.slotsync_skip_reason
     FROM pg_get_replication_slots() AS L
             LEFT JOIN pg_database D ON (L.datoid = D.oid);
 
diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 1f4f06d467b..818f432172c 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -148,6 +148,35 @@ 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. This function requires the caller to acquire
+ * the slot.
+ */
+static void
+update_slotsync_skip_stats(SlotSyncSkipReason skip_reason)
+{
+	ReplicationSlot *slot;
+
+	Assert(MyReplicationSlot);
+
+	slot = MyReplicationSlot;
+
+	/*
+	 * 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_replslotsync(slot);
+
+	/* Update the slot sync skip reason */
+	if (slot->slotsync_skip_reason != skip_reason)
+	{
+		SpinLockAcquire(&slot->mutex);
+		slot->slotsync_skip_reason = skip_reason;
+		SpinLockRelease(&slot->mutex);
+	}
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -170,6 +199,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 	ReplicationSlot *slot = MyReplicationSlot;
 	bool		updated_xmin_or_lsn = false;
 	bool		updated_config = false;
+	SlotSyncSkipReason skip_reason = SS_SKIP_NONE;
 
 	Assert(slot->data.invalidated == RS_INVAL_NONE);
 
@@ -188,7 +218,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 							  slot->data.catalog_xmin))
 	{
 		/* Update slot sync skip stats */
-		pgstat_report_replslotsync(slot);
+		update_slotsync_skip_stats(SS_SKIP_REMOTE_BEHIND);
 
 		/*
 		 * This can happen in following situations:
@@ -286,12 +316,15 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 			 * persisted. See update_and_persist_local_synced_slot().
 			 */
 			if (found_consistent_snapshot && !(*found_consistent_snapshot))
-				pgstat_report_replslotsync(slot);
+				skip_reason = SS_SKIP_NO_CONSISTENT_SNAPSHOT;
 		}
 
 		updated_xmin_or_lsn = true;
 	}
 
+	/* Update slot sync skip stats */
+	update_slotsync_skip_stats(skip_reason);
+
 	if (remote_dbid != slot->data.database ||
 		remote_slot->two_phase != slot->data.two_phase ||
 		remote_slot->failover != slot->data.failover ||
@@ -696,7 +729,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		/* Skip the sync of an invalidated slot */
 		if (slot->data.invalidated != RS_INVAL_NONE)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_INVALID);
 
 			ReplicationSlotRelease();
 			return slot_updated;
@@ -711,7 +744,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		 */
 		if (remote_slot->confirmed_lsn > latestFlushPtr)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
 
 			/*
 			 * Can get here only if GUC 'synchronized_standby_slots' on the
@@ -812,7 +845,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		 */
 		if (remote_slot->confirmed_lsn > latestFlushPtr)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
 
 			/*
 			 * Can get here only if GUC 'synchronized_standby_slots' on the
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 1ec1e997b27..86ae99a3ca9 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -491,6 +491,7 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 	slot->last_saved_confirmed_flush = InvalidXLogRecPtr;
 	slot->last_saved_restart_lsn = InvalidXLogRecPtr;
 	slot->inactive_since = 0;
+	slot->slotsync_skip_reason = SS_SKIP_NONE;
 
 	/*
 	 * Create the slot on disk.  We haven't actually marked the slot allocated
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 0478fc9c977..7200c7f071d 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -228,6 +228,30 @@ 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_WAL_NOT_FLUSHED:
+			return "wal_not_flushed";
+		case SS_SKIP_NO_CONSISTENT_SNAPSHOT:
+			return "no_consistent_snapshot";
+		case SS_SKIP_INVALID:
+			return "slot_invalidated";
+	}
+
+	Assert(false);
+	return "none";
+}
+
 /*
  * pg_get_replication_slots - SQL SRF showing all replication slots
  * that currently exist on the database cluster.
@@ -235,7 +259,7 @@ 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 +467,8 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 
 		values[i++] = BoolGetDatum(slot_contents.data.synced);
 
+		values[i++] = CStringGetTextDatum(GetSlotSyncSkipReason(slot_contents.slotsync_skip_reason));
+
 		Assert(i == PG_GET_REPLICATION_SLOTS_COLS);
 
 		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 66431940700..66af2d96d67 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -11519,9 +11519,9 @@
   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,slotsync_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/replication/slot.h b/src/include/replication/slot.h
index 09c69f83d57..054e81b6c43 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -71,6 +71,22 @@ 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_WAL_NOT_FLUSHED,	/* Standby did not flush the wal corresponding
+								 * to confirmed flush of 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 */
+	SS_SKIP_INVALID				/* Local slot is invalid */
+} SlotSyncSkipReason;
+
 /*
  * On-Disk data of a replication slot, preserved across restarts.
  */
@@ -249,6 +265,16 @@ typedef struct ReplicationSlot
 	 */
 	XLogRecPtr	last_saved_restart_lsn;
 
+	/*
+	 * The reason for last slot sync skip.
+	 *
+	 * A slotsync skip typically occurs only for temporary slots. For
+	 * persistent slots it is extremely rare (e.g., cases like
+	 * SS_SKIP_WAL_NOT_FLUSHED or SS_SKIP_REMOTE_BEHIND). Also, temporary
+	 * slots are dropped after server restart, so there is no value in
+	 * persisting the slotsync_skip_reason.
+	 */
+	SlotSyncSkipReason slotsync_skip_reason;
 } ReplicationSlot;
 
 #define SlotIsPhysical(slot) ((slot)->data.database == InvalidOid)
diff --git a/src/test/recovery/t/040_standby_failover_slots_sync.pl b/src/test/recovery/t/040_standby_failover_slots_sync.pl
index 7d3c82e0a29..da1c3aee65d 100644
--- a/src/test/recovery/t/040_standby_failover_slots_sync.pl
+++ b/src/test/recovery/t/040_standby_failover_slots_sync.pl
@@ -1049,6 +1049,12 @@ $standby2->wait_for_log(qr/slot sync worker started/, $log_offset);
 $standby2->wait_for_log(
 	qr/could not synchronize replication slot \"lsub1_slot\"/, $log_offset);
 
+# Confirm that the slotsync skip reason is updated
+$result = $standby2->safe_psql('postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots WHERE slot_name = 'lsub1_slot'"
+);
+is($result, 'remote_behind', "check slot sync skip reason");
+
 # Confirm that the slotsync skip statistics is updated
 $result = $standby2->safe_psql('postgres',
 	"SELECT slotsync_skip_count > 0 FROM pg_stat_replication_slots WHERE slot_name = 'lsub1_slot'"
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index c337f0bc30d..94e45dd4d57 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.slotsync_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, slotsync_skip_reason)
      LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
 pg_roles| SELECT pg_authid.rolname,
     pg_authid.rolsuper,
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index dfcd619bfee..c7e52cc2191 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2807,6 +2807,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

From 2e54b6f9b83827822d05c8edd429e14477209b3f Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Wed, 26 Nov 2025 08:52:29 +0530
Subject: [PATCH v13] Add slotsync_skip_reason to pg_replication_slots

This patch introduces a new column slotsync_skip_reason to
pg_replication_slots view. This indicates the reason for last slot
synchronization skip.
---
 doc/src/sgml/monitoring.sgml                  |  4 +-
 doc/src/sgml/system-views.sgml                | 39 +++++++++++++++++
 src/backend/catalog/system_views.sql          |  3 +-
 src/backend/replication/logical/slotsync.c    | 43 ++++++++++++++++---
 src/backend/replication/slot.c                |  1 +
 src/backend/replication/slotfuncs.c           | 31 ++++++++++++-
 src/include/catalog/pg_proc.dat               |  6 +--
 src/include/replication/slot.h                | 26 +++++++++++
 .../t/040_standby_failover_slots_sync.pl      |  6 +++
 src/test/regress/expected/rules.out           |  5 ++-
 src/tools/pgindent/typedefs.list              |  1 +
 11 files changed, 151 insertions(+), 14 deletions(-)

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index dcc8474a7f7..e0556b6baac 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1665,7 +1665,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para>
        <para>
         Number of times the slot synchronization is skipped. Slot
-        synchronization occur only on standby servers and thus this column has
+        synchronization occurs only on standby servers and thus this column has
         no meaning on the primary server.
        </para>
       </entry>
@@ -1677,7 +1677,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para>
        <para>
         Time at which last slot synchronization was skipped. Slot
-        synchronization occur only on standby servers and thus this column has
+        synchronization occurs only on standby servers and thus this column has
         no meaning on the primary server.
        </para>
       </entry>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 0e623e7fb86..5a08a222ad3 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3102,6 +3102,45 @@ 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>slotsync_skip_reason</structfield><type>text</type>
+      </para>
+      <para>
+       The reason for the last slot synchronization skip. Slot synchronization
+       occurs only on standby servers and for synced slots (that is, those whose
+       <structfield>synced</structfield> field is <literal>true</literal>).
+       Thus, this column has no meaning on the primary server and slot which are
+       not synced. <literal>NULL</literal> if slot synchronization is not
+       performed yet or is successful. Possible values are:
+       <itemizedlist spacing="compact">
+        <listitem>
+         <para>
+          <literal>remote_behind</literal> means that the slot is ahead of the
+          corresponding failover slot on the primary.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>wal_not_flushed</literal> means that the standby had not
+          flushed the WAL corresponding to the position reserved on the remote
+          slot.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>no_consistent_snapshot</literal> means that the standby could
+          not build a consistent snapshot.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>slot_invalidated</literal> means that the slot is invalidated.
+         </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 6fffdb9398e..086c4c8fb6f 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.slotsync_skip_reason
     FROM pg_get_replication_slots() AS L
             LEFT JOIN pg_database D ON (L.datoid = D.oid);
 
diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 1f4f06d467b..818f432172c 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -148,6 +148,35 @@ 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. This function requires the caller to acquire
+ * the slot.
+ */
+static void
+update_slotsync_skip_stats(SlotSyncSkipReason skip_reason)
+{
+	ReplicationSlot *slot;
+
+	Assert(MyReplicationSlot);
+
+	slot = MyReplicationSlot;
+
+	/*
+	 * 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_replslotsync(slot);
+
+	/* Update the slot sync skip reason */
+	if (slot->slotsync_skip_reason != skip_reason)
+	{
+		SpinLockAcquire(&slot->mutex);
+		slot->slotsync_skip_reason = skip_reason;
+		SpinLockRelease(&slot->mutex);
+	}
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -170,6 +199,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 	ReplicationSlot *slot = MyReplicationSlot;
 	bool		updated_xmin_or_lsn = false;
 	bool		updated_config = false;
+	SlotSyncSkipReason skip_reason = SS_SKIP_NONE;
 
 	Assert(slot->data.invalidated == RS_INVAL_NONE);
 
@@ -188,7 +218,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 							  slot->data.catalog_xmin))
 	{
 		/* Update slot sync skip stats */
-		pgstat_report_replslotsync(slot);
+		update_slotsync_skip_stats(SS_SKIP_REMOTE_BEHIND);
 
 		/*
 		 * This can happen in following situations:
@@ -286,12 +316,15 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 			 * persisted. See update_and_persist_local_synced_slot().
 			 */
 			if (found_consistent_snapshot && !(*found_consistent_snapshot))
-				pgstat_report_replslotsync(slot);
+				skip_reason = SS_SKIP_NO_CONSISTENT_SNAPSHOT;
 		}
 
 		updated_xmin_or_lsn = true;
 	}
 
+	/* Update slot sync skip stats */
+	update_slotsync_skip_stats(skip_reason);
+
 	if (remote_dbid != slot->data.database ||
 		remote_slot->two_phase != slot->data.two_phase ||
 		remote_slot->failover != slot->data.failover ||
@@ -696,7 +729,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		/* Skip the sync of an invalidated slot */
 		if (slot->data.invalidated != RS_INVAL_NONE)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_INVALID);
 
 			ReplicationSlotRelease();
 			return slot_updated;
@@ -711,7 +744,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		 */
 		if (remote_slot->confirmed_lsn > latestFlushPtr)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
 
 			/*
 			 * Can get here only if GUC 'synchronized_standby_slots' on the
@@ -812,7 +845,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		 */
 		if (remote_slot->confirmed_lsn > latestFlushPtr)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
 
 			/*
 			 * Can get here only if GUC 'synchronized_standby_slots' on the
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 1ec1e997b27..86ae99a3ca9 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -491,6 +491,7 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 	slot->last_saved_confirmed_flush = InvalidXLogRecPtr;
 	slot->last_saved_restart_lsn = InvalidXLogRecPtr;
 	slot->inactive_since = 0;
+	slot->slotsync_skip_reason = SS_SKIP_NONE;
 
 	/*
 	 * Create the slot on disk.  We haven't actually marked the slot allocated
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 0478fc9c977..58262e1a9d8 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -228,6 +228,30 @@ pg_drop_replication_slot(PG_FUNCTION_ARGS)
 	PG_RETURN_VOID();
 }
 
+/*
+ * Map a SlotSyncSkipReason enum to a human-readable string
+ */
+static char *
+GetSlotSyncSkipReasonName(SlotSyncSkipReason reason)
+{
+	switch (reason)
+	{
+		case SS_SKIP_REMOTE_BEHIND:
+			return "remote_behind";
+		case SS_SKIP_WAL_NOT_FLUSHED:
+			return "wal_not_flushed";
+		case SS_SKIP_NO_CONSISTENT_SNAPSHOT:
+			return "no_consistent_snapshot";
+		case SS_SKIP_INVALID:
+			return "slot_invalidated";
+		default:
+			break;
+	}
+
+	Assert(false);
+	return NULL;
+}
+
 /*
  * pg_get_replication_slots - SQL SRF showing all replication slots
  * that currently exist on the database cluster.
@@ -235,7 +259,7 @@ 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 +467,11 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 
 		values[i++] = BoolGetDatum(slot_contents.data.synced);
 
+		if (slot_contents.slotsync_skip_reason == SS_SKIP_NONE)
+			nulls[i++] = true;
+		else
+			values[i++] = CStringGetTextDatum(GetSlotSyncSkipReasonName(slot_contents.slotsync_skip_reason));
+
 		Assert(i == PG_GET_REPLICATION_SLOTS_COLS);
 
 		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 66431940700..66af2d96d67 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -11519,9 +11519,9 @@
   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,slotsync_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/replication/slot.h b/src/include/replication/slot.h
index 09c69f83d57..667b36b8a25 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -71,6 +71,22 @@ 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_WAL_NOT_FLUSHED,	/* Standby did not flush the wal corresponding
+								 * to confirmed flush of 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 */
+	SS_SKIP_INVALID				/* Local slot is invalid */
+} SlotSyncSkipReason;
+
 /*
  * On-Disk data of a replication slot, preserved across restarts.
  */
@@ -249,6 +265,16 @@ typedef struct ReplicationSlot
 	 */
 	XLogRecPtr	last_saved_restart_lsn;
 
+	/*
+	 * The reason for last slot sync skip.
+	 *
+	 * A slotsync skip typically occurs only for temporary slots. For
+	 * persistent slots it is extremely rare (e.g., cases like
+	 * SS_SKIP_WAL_NOT_FLUSHED or SS_SKIP_REMOTE_BEHIND). Since, temporary
+	 * slots are dropped after server restart, so there is no value in
+	 * persisting the slotsync_skip_reason.
+	 */
+	SlotSyncSkipReason slotsync_skip_reason;
 } ReplicationSlot;
 
 #define SlotIsPhysical(slot) ((slot)->data.database == InvalidOid)
diff --git a/src/test/recovery/t/040_standby_failover_slots_sync.pl b/src/test/recovery/t/040_standby_failover_slots_sync.pl
index 7d3c82e0a29..da1c3aee65d 100644
--- a/src/test/recovery/t/040_standby_failover_slots_sync.pl
+++ b/src/test/recovery/t/040_standby_failover_slots_sync.pl
@@ -1049,6 +1049,12 @@ $standby2->wait_for_log(qr/slot sync worker started/, $log_offset);
 $standby2->wait_for_log(
 	qr/could not synchronize replication slot \"lsub1_slot\"/, $log_offset);
 
+# Confirm that the slotsync skip reason is updated
+$result = $standby2->safe_psql('postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots WHERE slot_name = 'lsub1_slot'"
+);
+is($result, 'remote_behind', "check slot sync skip reason");
+
 # Confirm that the slotsync skip statistics is updated
 $result = $standby2->safe_psql('postgres',
 	"SELECT slotsync_skip_count > 0 FROM pg_stat_replication_slots WHERE slot_name = 'lsub1_slot'"
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index c337f0bc30d..94e45dd4d57 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.slotsync_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, slotsync_skip_reason)
      LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
 pg_roles| SELECT pg_authid.rolname,
     pg_authid.rolsuper,
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index dfcd619bfee..c7e52cc2191 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2807,6 +2807,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

From b0837f71b447db4977fbcba99f066170e025cfcc Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Wed, 26 Nov 2025 08:52:29 +0530
Subject: [PATCH v14] Add slotsync_skip_reason to pg_replication_slots

This patch introduces a new column slotsync_skip_reason to
pg_replication_slots view. This indicates the reason for last slot
synchronization skip.
---
 doc/src/sgml/monitoring.sgml                  |  4 +-
 doc/src/sgml/system-views.sgml                | 39 +++++++++++++++++
 src/backend/catalog/system_views.sql          |  3 +-
 src/backend/replication/logical/slotsync.c    | 43 ++++++++++++++++---
 src/backend/replication/slot.c                |  1 +
 src/backend/replication/slotfuncs.c           | 16 ++++++-
 src/include/catalog/pg_proc.dat               |  6 +--
 src/include/replication/slot.h                | 26 +++++++++++
 .../t/040_standby_failover_slots_sync.pl      |  6 +++
 src/test/regress/expected/rules.out           |  5 ++-
 src/tools/pgindent/typedefs.list              |  1 +
 11 files changed, 136 insertions(+), 14 deletions(-)

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index dcc8474a7f7..e0556b6baac 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1665,7 +1665,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para>
        <para>
         Number of times the slot synchronization is skipped. Slot
-        synchronization occur only on standby servers and thus this column has
+        synchronization occurs only on standby servers and thus this column has
         no meaning on the primary server.
        </para>
       </entry>
@@ -1677,7 +1677,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para>
        <para>
         Time at which last slot synchronization was skipped. Slot
-        synchronization occur only on standby servers and thus this column has
+        synchronization occurs only on standby servers and thus this column has
         no meaning on the primary server.
        </para>
       </entry>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 0e623e7fb86..e37ca7b2da7 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3102,6 +3102,45 @@ 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>slotsync_skip_reason</structfield><type>text</type>
+      </para>
+      <para>
+       The reason for the last slot synchronization skip. Slot
+       synchronization occurs only on standby servers and thus this column
+       has no meaning on the primary server. It is relevant mainly for
+       logical slots on standby servers whose <structfield>synced</structfield>
+       field is <literal>true</literal>. <literal>NULL</literal> if
+       slot synchronization is successful. Possible values are:
+       <itemizedlist spacing="compact">
+        <listitem>
+         <para>
+          <literal>wal_or_rows_removed</literal> means that the required WALs or
+          catalog rows have already been removed from the standby.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>wal_not_flushed</literal> means that the standby had not
+          flushed the WAL corresponding to the position reserved on the failover
+          slot.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>no_consistent_snapshot</literal> means that the standby could
+          not build a consistent snapshot.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>slot_invalidated</literal> means that the slot is invalidated.
+         </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 6fffdb9398e..086c4c8fb6f 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.slotsync_skip_reason
     FROM pg_get_replication_slots() AS L
             LEFT JOIN pg_database D ON (L.datoid = D.oid);
 
diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 1f4f06d467b..53c7d629239 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -148,6 +148,35 @@ 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. This function requires the caller to acquire
+ * the slot.
+ */
+static void
+update_slotsync_skip_stats(SlotSyncSkipReason skip_reason)
+{
+	ReplicationSlot *slot;
+
+	Assert(MyReplicationSlot);
+
+	slot = MyReplicationSlot;
+
+	/*
+	 * 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_replslotsync(slot);
+
+	/* Update the slot sync skip reason */
+	if (slot->slotsync_skip_reason != skip_reason)
+	{
+		SpinLockAcquire(&slot->mutex);
+		slot->slotsync_skip_reason = skip_reason;
+		SpinLockRelease(&slot->mutex);
+	}
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -170,6 +199,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 	ReplicationSlot *slot = MyReplicationSlot;
 	bool		updated_xmin_or_lsn = false;
 	bool		updated_config = false;
+	SlotSyncSkipReason skip_reason = SS_SKIP_NONE;
 
 	Assert(slot->data.invalidated == RS_INVAL_NONE);
 
@@ -188,7 +218,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 							  slot->data.catalog_xmin))
 	{
 		/* Update slot sync skip stats */
-		pgstat_report_replslotsync(slot);
+		update_slotsync_skip_stats(SS_SKIP_WAL_OR_ROWS_REMOVED);
 
 		/*
 		 * This can happen in following situations:
@@ -286,12 +316,15 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 			 * persisted. See update_and_persist_local_synced_slot().
 			 */
 			if (found_consistent_snapshot && !(*found_consistent_snapshot))
-				pgstat_report_replslotsync(slot);
+				skip_reason = SS_SKIP_NO_CONSISTENT_SNAPSHOT;
 		}
 
 		updated_xmin_or_lsn = true;
 	}
 
+	/* Update slot sync skip stats */
+	update_slotsync_skip_stats(skip_reason);
+
 	if (remote_dbid != slot->data.database ||
 		remote_slot->two_phase != slot->data.two_phase ||
 		remote_slot->failover != slot->data.failover ||
@@ -696,7 +729,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		/* Skip the sync of an invalidated slot */
 		if (slot->data.invalidated != RS_INVAL_NONE)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_INVALID);
 
 			ReplicationSlotRelease();
 			return slot_updated;
@@ -711,7 +744,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		 */
 		if (remote_slot->confirmed_lsn > latestFlushPtr)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
 
 			/*
 			 * Can get here only if GUC 'synchronized_standby_slots' on the
@@ -812,7 +845,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		 */
 		if (remote_slot->confirmed_lsn > latestFlushPtr)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
 
 			/*
 			 * Can get here only if GUC 'synchronized_standby_slots' on the
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 1ec1e997b27..86ae99a3ca9 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -491,6 +491,7 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 	slot->last_saved_confirmed_flush = InvalidXLogRecPtr;
 	slot->last_saved_restart_lsn = InvalidXLogRecPtr;
 	slot->inactive_since = 0;
+	slot->slotsync_skip_reason = SS_SKIP_NONE;
 
 	/*
 	 * Create the slot on disk.  We haven't actually marked the slot allocated
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 0478fc9c977..8c1b92ffb69 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -24,6 +24,15 @@
 #include "utils/guc.h"
 #include "utils/pg_lsn.h"
 
+/* Map SlotSyncSkipReason enum values to human-readable names. */
+static const char *SlotSyncSkipReasonNames[] = {
+	[SS_SKIP_NONE] = "none",
+	[SS_SKIP_WAL_NOT_FLUSHED] = "wal_not_flushed",
+	[SS_SKIP_WAL_OR_ROWS_REMOVED] = "wal_or_rows_removed",
+	[SS_SKIP_NO_CONSISTENT_SNAPSHOT] = "no_consistent_snapshot",
+	[SS_SKIP_INVALID] = "slot_invalidated"
+};
+
 /*
  * Helper function for creating a new physical replication slot with
  * given arguments. Note that this function doesn't release the created
@@ -235,7 +244,7 @@ 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 +452,11 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 
 		values[i++] = BoolGetDatum(slot_contents.data.synced);
 
+		if (slot_contents.slotsync_skip_reason == SS_SKIP_NONE)
+			nulls[i++] = true;
+		else
+			values[i++] = CStringGetTextDatum(SlotSyncSkipReasonNames[slot_contents.slotsync_skip_reason]);
+
 		Assert(i == PG_GET_REPLICATION_SLOTS_COLS);
 
 		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 66431940700..66af2d96d67 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -11519,9 +11519,9 @@
   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,slotsync_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/replication/slot.h b/src/include/replication/slot.h
index 09c69f83d57..5ade8e6f73e 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -71,6 +71,22 @@ 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_WAL_NOT_FLUSHED,	/* Standby did not flush the wal corresponding
+								 * to confirmed flush of remote slot */
+	SS_SKIP_WAL_OR_ROWS_REMOVED,	/* Remote slot is behind the local slot */
+	SS_SKIP_NO_CONSISTENT_SNAPSHOT, /* Standby could not build a consistent
+									 * snapshot */
+	SS_SKIP_INVALID				/* Local slot is invalid */
+} SlotSyncSkipReason;
+
 /*
  * On-Disk data of a replication slot, preserved across restarts.
  */
@@ -249,6 +265,16 @@ typedef struct ReplicationSlot
 	 */
 	XLogRecPtr	last_saved_restart_lsn;
 
+	/*
+	 * The reason for last slot sync skip.
+	 *
+	 * A slotsync skip typically occurs only for temporary slots. For
+	 * persistent slots it is extremely rare (e.g., cases like
+	 * SS_SKIP_WAL_NOT_FLUSHED or SS_SKIP_WAL_OR_ROWS_REMOVED). Since,
+	 * temporary slots are dropped after server restart, so there is no value
+	 * in persisting the slotsync_skip_reason.
+	 */
+	SlotSyncSkipReason slotsync_skip_reason;
 } ReplicationSlot;
 
 #define SlotIsPhysical(slot) ((slot)->data.database == InvalidOid)
diff --git a/src/test/recovery/t/040_standby_failover_slots_sync.pl b/src/test/recovery/t/040_standby_failover_slots_sync.pl
index 7d3c82e0a29..25777fa188c 100644
--- a/src/test/recovery/t/040_standby_failover_slots_sync.pl
+++ b/src/test/recovery/t/040_standby_failover_slots_sync.pl
@@ -1049,6 +1049,12 @@ $standby2->wait_for_log(qr/slot sync worker started/, $log_offset);
 $standby2->wait_for_log(
 	qr/could not synchronize replication slot \"lsub1_slot\"/, $log_offset);
 
+# Confirm that the slotsync skip reason is updated
+$result = $standby2->safe_psql('postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots WHERE slot_name = 'lsub1_slot'"
+);
+is($result, 'wal_or_rows_removed', "check slot sync skip reason");
+
 # Confirm that the slotsync skip statistics is updated
 $result = $standby2->safe_psql('postgres',
 	"SELECT slotsync_skip_count > 0 FROM pg_stat_replication_slots WHERE slot_name = 'lsub1_slot'"
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index c337f0bc30d..94e45dd4d57 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.slotsync_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, slotsync_skip_reason)
      LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
 pg_roles| SELECT pg_authid.rolname,
     pg_authid.rolsuper,
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index dfcd619bfee..c7e52cc2191 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2807,6 +2807,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

From 20e0694b056f8981987f31af0922e79e295b01a5 Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Wed, 26 Nov 2025 08:52:29 +0530
Subject: [PATCH v15] Add slotsync_skip_reason to pg_replication_slots

This patch introduces a new column slotsync_skip_reason to
pg_replication_slots view. This indicates the reason for last slot
synchronization skip.
---
 doc/src/sgml/monitoring.sgml                  |  4 +-
 doc/src/sgml/system-views.sgml                | 41 ++++++++++++++++++
 src/backend/catalog/system_views.sql          |  3 +-
 src/backend/replication/logical/slotsync.c    | 43 ++++++++++++++++---
 src/backend/replication/slot.c                |  1 +
 src/backend/replication/slotfuncs.c           | 18 +++++++-
 src/include/catalog/pg_proc.dat               |  6 +--
 src/include/replication/slot.h                | 27 ++++++++++++
 .../t/040_standby_failover_slots_sync.pl      |  6 +++
 src/test/regress/expected/rules.out           |  5 ++-
 src/tools/pgindent/typedefs.list              |  1 +
 11 files changed, 141 insertions(+), 14 deletions(-)

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index dcc8474a7f7..e0556b6baac 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1665,7 +1665,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para>
        <para>
         Number of times the slot synchronization is skipped. Slot
-        synchronization occur only on standby servers and thus this column has
+        synchronization occurs only on standby servers and thus this column has
         no meaning on the primary server.
        </para>
       </entry>
@@ -1677,7 +1677,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para>
        <para>
         Time at which last slot synchronization was skipped. Slot
-        synchronization occur only on standby servers and thus this column has
+        synchronization occurs only on standby servers and thus this column has
         no meaning on the primary server.
        </para>
       </entry>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 0e623e7fb86..a7db2291df5 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3102,6 +3102,47 @@ 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>slotsync_skip_reason</structfield><type>text</type>
+      </para>
+      <para>
+       The reason for the last slot synchronization skip. Slot
+       synchronization occurs only on standby servers and thus this column
+       has no meaning on the primary server. It is relevant mainly for
+       logical slots on standby servers whose <structfield>synced</structfield>
+       field is <literal>true</literal>. <literal>NULL</literal> if
+       slot synchronization is successful. Possible values are:
+       <itemizedlist spacing="compact">
+        <listitem>
+         <para>
+          <literal>wal_or_rows_removed</literal> means that the required WALs or
+          catalog rows have already been removed or are at the risk of removal
+          from the standby.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>wal_not_flushed</literal> means that the standby had not
+          flushed the WAL corresponding to the position reserved on the failover
+          slot.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>no_consistent_snapshot</literal> means that the standby could
+          not build a consistent snapshot to decode WALs from
+          <structfield>restart_lsn</structfield>.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>slot_invalidated</literal> means that the slot is invalidated.
+         </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 6fffdb9398e..086c4c8fb6f 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.slotsync_skip_reason
     FROM pg_get_replication_slots() AS L
             LEFT JOIN pg_database D ON (L.datoid = D.oid);
 
diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 1f4f06d467b..53c7d629239 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -148,6 +148,35 @@ 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. This function requires the caller to acquire
+ * the slot.
+ */
+static void
+update_slotsync_skip_stats(SlotSyncSkipReason skip_reason)
+{
+	ReplicationSlot *slot;
+
+	Assert(MyReplicationSlot);
+
+	slot = MyReplicationSlot;
+
+	/*
+	 * 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_replslotsync(slot);
+
+	/* Update the slot sync skip reason */
+	if (slot->slotsync_skip_reason != skip_reason)
+	{
+		SpinLockAcquire(&slot->mutex);
+		slot->slotsync_skip_reason = skip_reason;
+		SpinLockRelease(&slot->mutex);
+	}
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -170,6 +199,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 	ReplicationSlot *slot = MyReplicationSlot;
 	bool		updated_xmin_or_lsn = false;
 	bool		updated_config = false;
+	SlotSyncSkipReason skip_reason = SS_SKIP_NONE;
 
 	Assert(slot->data.invalidated == RS_INVAL_NONE);
 
@@ -188,7 +218,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 							  slot->data.catalog_xmin))
 	{
 		/* Update slot sync skip stats */
-		pgstat_report_replslotsync(slot);
+		update_slotsync_skip_stats(SS_SKIP_WAL_OR_ROWS_REMOVED);
 
 		/*
 		 * This can happen in following situations:
@@ -286,12 +316,15 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 			 * persisted. See update_and_persist_local_synced_slot().
 			 */
 			if (found_consistent_snapshot && !(*found_consistent_snapshot))
-				pgstat_report_replslotsync(slot);
+				skip_reason = SS_SKIP_NO_CONSISTENT_SNAPSHOT;
 		}
 
 		updated_xmin_or_lsn = true;
 	}
 
+	/* Update slot sync skip stats */
+	update_slotsync_skip_stats(skip_reason);
+
 	if (remote_dbid != slot->data.database ||
 		remote_slot->two_phase != slot->data.two_phase ||
 		remote_slot->failover != slot->data.failover ||
@@ -696,7 +729,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		/* Skip the sync of an invalidated slot */
 		if (slot->data.invalidated != RS_INVAL_NONE)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_INVALID);
 
 			ReplicationSlotRelease();
 			return slot_updated;
@@ -711,7 +744,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		 */
 		if (remote_slot->confirmed_lsn > latestFlushPtr)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
 
 			/*
 			 * Can get here only if GUC 'synchronized_standby_slots' on the
@@ -812,7 +845,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		 */
 		if (remote_slot->confirmed_lsn > latestFlushPtr)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
 
 			/*
 			 * Can get here only if GUC 'synchronized_standby_slots' on the
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 1ec1e997b27..86ae99a3ca9 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -491,6 +491,7 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 	slot->last_saved_confirmed_flush = InvalidXLogRecPtr;
 	slot->last_saved_restart_lsn = InvalidXLogRecPtr;
 	slot->inactive_since = 0;
+	slot->slotsync_skip_reason = SS_SKIP_NONE;
 
 	/*
 	 * Create the slot on disk.  We haven't actually marked the slot allocated
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 0478fc9c977..7647f051581 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -24,6 +24,17 @@
 #include "utils/guc.h"
 #include "utils/pg_lsn.h"
 
+/*
+ * Map SlotSyncSkipReason enum values to human-readable names.
+ */
+static const char *SlotSyncSkipReasonNames[] = {
+	[SS_SKIP_NONE] = "none",
+	[SS_SKIP_WAL_NOT_FLUSHED] = "wal_not_flushed",
+	[SS_SKIP_WAL_OR_ROWS_REMOVED] = "wal_or_rows_removed",
+	[SS_SKIP_NO_CONSISTENT_SNAPSHOT] = "no_consistent_snapshot",
+	[SS_SKIP_INVALID] = "slot_invalidated"
+};
+
 /*
  * Helper function for creating a new physical replication slot with
  * given arguments. Note that this function doesn't release the created
@@ -235,7 +246,7 @@ 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 +454,11 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 
 		values[i++] = BoolGetDatum(slot_contents.data.synced);
 
+		if (slot_contents.slotsync_skip_reason == SS_SKIP_NONE)
+			nulls[i++] = true;
+		else
+			values[i++] = CStringGetTextDatum(SlotSyncSkipReasonNames[slot_contents.slotsync_skip_reason]);
+
 		Assert(i == PG_GET_REPLICATION_SLOTS_COLS);
 
 		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 66431940700..66af2d96d67 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -11519,9 +11519,9 @@
   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,slotsync_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/replication/slot.h b/src/include/replication/slot.h
index 09c69f83d57..01d949bb3c1 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -71,6 +71,23 @@ 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_WAL_NOT_FLUSHED,	/* Standby did not flush the wal corresponding
+								 * to confirmed flush of remote slot */
+	SS_SKIP_WAL_OR_ROWS_REMOVED,	/* Remote slot is behind; required WAL or
+									 * rows may be removed or at risk */
+	SS_SKIP_NO_CONSISTENT_SNAPSHOT, /* Standby could not build a consistent
+									 * snapshot */
+	SS_SKIP_INVALID				/* Local slot is invalid */
+} SlotSyncSkipReason;
+
 /*
  * On-Disk data of a replication slot, preserved across restarts.
  */
@@ -249,6 +266,16 @@ typedef struct ReplicationSlot
 	 */
 	XLogRecPtr	last_saved_restart_lsn;
 
+	/*
+	 * The reason for last slot sync skip.
+	 *
+	 * A slotsync skip typically occurs only for temporary slots. For
+	 * persistent slots it is extremely rare (e.g., cases like
+	 * SS_SKIP_WAL_NOT_FLUSHED or SS_SKIP_WAL_OR_ROWS_REMOVED). Since,
+	 * temporary slots are dropped after server restart, so there is no value
+	 * in persisting the slotsync_skip_reason.
+	 */
+	SlotSyncSkipReason slotsync_skip_reason;
 } ReplicationSlot;
 
 #define SlotIsPhysical(slot) ((slot)->data.database == InvalidOid)
diff --git a/src/test/recovery/t/040_standby_failover_slots_sync.pl b/src/test/recovery/t/040_standby_failover_slots_sync.pl
index 7d3c82e0a29..25777fa188c 100644
--- a/src/test/recovery/t/040_standby_failover_slots_sync.pl
+++ b/src/test/recovery/t/040_standby_failover_slots_sync.pl
@@ -1049,6 +1049,12 @@ $standby2->wait_for_log(qr/slot sync worker started/, $log_offset);
 $standby2->wait_for_log(
 	qr/could not synchronize replication slot \"lsub1_slot\"/, $log_offset);
 
+# Confirm that the slotsync skip reason is updated
+$result = $standby2->safe_psql('postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots WHERE slot_name = 'lsub1_slot'"
+);
+is($result, 'wal_or_rows_removed', "check slot sync skip reason");
+
 # Confirm that the slotsync skip statistics is updated
 $result = $standby2->safe_psql('postgres',
 	"SELECT slotsync_skip_count > 0 FROM pg_stat_replication_slots WHERE slot_name = 'lsub1_slot'"
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index c337f0bc30d..94e45dd4d57 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.slotsync_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, slotsync_skip_reason)
      LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
 pg_roles| SELECT pg_authid.rolname,
     pg_authid.rolsuper,
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index e3c3523b5b2..cf3f6a7dafd 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2807,6 +2807,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

From d85cc22825c52acd6c5ae281350357584a948aef Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Wed, 26 Nov 2025 08:52:29 +0530
Subject: [PATCH v16] Add slotsync_skip_reason to pg_replication_slots

This patch introduces a new column slotsync_skip_reason to
pg_replication_slots view. This indicates the reason for last slot
synchronization skip.
---
 doc/src/sgml/monitoring.sgml                  |  4 +-
 doc/src/sgml/system-views.sgml                | 42 ++++++++++++++++++
 src/backend/catalog/system_views.sql          |  3 +-
 src/backend/replication/logical/slotsync.c    | 43 ++++++++++++++++---
 src/backend/replication/slot.c                |  1 +
 src/backend/replication/slotfuncs.c           | 18 +++++++-
 src/backend/utils/activity/pgstat_replslot.c  |  4 +-
 src/include/catalog/pg_proc.dat               |  6 +--
 src/include/replication/slot.h                | 27 ++++++++++++
 .../t/040_standby_failover_slots_sync.pl      |  6 +++
 src/test/regress/expected/rules.out           |  5 ++-
 src/tools/pgindent/typedefs.list              |  1 +
 12 files changed, 144 insertions(+), 16 deletions(-)

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index dcc8474a7f7..e0556b6baac 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1665,7 +1665,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para>
        <para>
         Number of times the slot synchronization is skipped. Slot
-        synchronization occur only on standby servers and thus this column has
+        synchronization occurs only on standby servers and thus this column has
         no meaning on the primary server.
        </para>
       </entry>
@@ -1677,7 +1677,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para>
        <para>
         Time at which last slot synchronization was skipped. Slot
-        synchronization occur only on standby servers and thus this column has
+        synchronization occurs only on standby servers and thus this column has
         no meaning on the primary server.
        </para>
       </entry>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 0e623e7fb86..1fbf83079f0 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -3102,6 +3102,48 @@ 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>slotsync_skip_reason</structfield><type>text</type>
+      </para>
+      <para>
+       The reason for the last slot synchronization skip. Slot
+       synchronization occurs only on standby servers and thus this column has
+       no meaning on the primary server. It is relevant mainly for logical slots
+       on standby servers whose <structfield>synced</structfield> field is
+       <literal>true</literal>. It is <literal>NULL</literal> if slot
+       synchronization is successful.
+       Possible values are:
+       <itemizedlist spacing="compact">
+        <listitem>
+         <para>
+          <literal>wal_or_rows_removed</literal> means that the required WALs or
+          catalog rows have already been removed or are at the risk of removal
+          from the standby.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>wal_not_flushed</literal> means that the standby had not
+          flushed the WAL corresponding to the position reserved on the failover
+          slot.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>no_consistent_snapshot</literal> means that the standby could
+          not build a consistent snapshot to decode WALs from
+          <structfield>restart_lsn</structfield>.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>slot_invalidated</literal> means that the slot is invalidated.
+         </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 6fffdb9398e..086c4c8fb6f 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.slotsync_skip_reason
     FROM pg_get_replication_slots() AS L
             LEFT JOIN pg_database D ON (L.datoid = D.oid);
 
diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 1f4f06d467b..53c7d629239 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -148,6 +148,35 @@ 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. This function requires the caller to acquire
+ * the slot.
+ */
+static void
+update_slotsync_skip_stats(SlotSyncSkipReason skip_reason)
+{
+	ReplicationSlot *slot;
+
+	Assert(MyReplicationSlot);
+
+	slot = MyReplicationSlot;
+
+	/*
+	 * 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_replslotsync(slot);
+
+	/* Update the slot sync skip reason */
+	if (slot->slotsync_skip_reason != skip_reason)
+	{
+		SpinLockAcquire(&slot->mutex);
+		slot->slotsync_skip_reason = skip_reason;
+		SpinLockRelease(&slot->mutex);
+	}
+}
+
 /*
  * If necessary, update the local synced slot's metadata based on the data
  * from the remote slot.
@@ -170,6 +199,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 	ReplicationSlot *slot = MyReplicationSlot;
 	bool		updated_xmin_or_lsn = false;
 	bool		updated_config = false;
+	SlotSyncSkipReason skip_reason = SS_SKIP_NONE;
 
 	Assert(slot->data.invalidated == RS_INVAL_NONE);
 
@@ -188,7 +218,7 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 							  slot->data.catalog_xmin))
 	{
 		/* Update slot sync skip stats */
-		pgstat_report_replslotsync(slot);
+		update_slotsync_skip_stats(SS_SKIP_WAL_OR_ROWS_REMOVED);
 
 		/*
 		 * This can happen in following situations:
@@ -286,12 +316,15 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 			 * persisted. See update_and_persist_local_synced_slot().
 			 */
 			if (found_consistent_snapshot && !(*found_consistent_snapshot))
-				pgstat_report_replslotsync(slot);
+				skip_reason = SS_SKIP_NO_CONSISTENT_SNAPSHOT;
 		}
 
 		updated_xmin_or_lsn = true;
 	}
 
+	/* Update slot sync skip stats */
+	update_slotsync_skip_stats(skip_reason);
+
 	if (remote_dbid != slot->data.database ||
 		remote_slot->two_phase != slot->data.two_phase ||
 		remote_slot->failover != slot->data.failover ||
@@ -696,7 +729,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		/* Skip the sync of an invalidated slot */
 		if (slot->data.invalidated != RS_INVAL_NONE)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_INVALID);
 
 			ReplicationSlotRelease();
 			return slot_updated;
@@ -711,7 +744,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		 */
 		if (remote_slot->confirmed_lsn > latestFlushPtr)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
 
 			/*
 			 * Can get here only if GUC 'synchronized_standby_slots' on the
@@ -812,7 +845,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		 */
 		if (remote_slot->confirmed_lsn > latestFlushPtr)
 		{
-			pgstat_report_replslotsync(slot);
+			update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
 
 			/*
 			 * Can get here only if GUC 'synchronized_standby_slots' on the
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 1ec1e997b27..86ae99a3ca9 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -491,6 +491,7 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 	slot->last_saved_confirmed_flush = InvalidXLogRecPtr;
 	slot->last_saved_restart_lsn = InvalidXLogRecPtr;
 	slot->inactive_since = 0;
+	slot->slotsync_skip_reason = SS_SKIP_NONE;
 
 	/*
 	 * Create the slot on disk.  We haven't actually marked the slot allocated
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 0478fc9c977..7647f051581 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -24,6 +24,17 @@
 #include "utils/guc.h"
 #include "utils/pg_lsn.h"
 
+/*
+ * Map SlotSyncSkipReason enum values to human-readable names.
+ */
+static const char *SlotSyncSkipReasonNames[] = {
+	[SS_SKIP_NONE] = "none",
+	[SS_SKIP_WAL_NOT_FLUSHED] = "wal_not_flushed",
+	[SS_SKIP_WAL_OR_ROWS_REMOVED] = "wal_or_rows_removed",
+	[SS_SKIP_NO_CONSISTENT_SNAPSHOT] = "no_consistent_snapshot",
+	[SS_SKIP_INVALID] = "slot_invalidated"
+};
+
 /*
  * Helper function for creating a new physical replication slot with
  * given arguments. Note that this function doesn't release the created
@@ -235,7 +246,7 @@ 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 +454,11 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 
 		values[i++] = BoolGetDatum(slot_contents.data.synced);
 
+		if (slot_contents.slotsync_skip_reason == SS_SKIP_NONE)
+			nulls[i++] = true;
+		else
+			values[i++] = CStringGetTextDatum(SlotSyncSkipReasonNames[slot_contents.slotsync_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 f93179146c2..e08d33e8b4c 100644
--- a/src/backend/utils/activity/pgstat_replslot.c
+++ b/src/backend/utils/activity/pgstat_replslot.c
@@ -115,8 +115,8 @@ pgstat_report_replslotsync(ReplicationSlot *slot)
 	PgStatShared_ReplSlot *shstatent;
 	PgStat_StatReplSlotEntry *statent;
 
-	/* Slot sync stats are valid only for logical slots on standby. */
-	Assert(SlotIsLogical(slot));
+	/* Slot sync stats are valid only for synced logical slots on standby. */
+	Assert(slot->data.synced);
 	Assert(RecoveryInProgress());
 
 	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_REPLSLOT, InvalidOid,
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 66431940700..66af2d96d67 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -11519,9 +11519,9 @@
   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,slotsync_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/replication/slot.h b/src/include/replication/slot.h
index 09c69f83d57..01d949bb3c1 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -71,6 +71,23 @@ 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_WAL_NOT_FLUSHED,	/* Standby did not flush the wal corresponding
+								 * to confirmed flush of remote slot */
+	SS_SKIP_WAL_OR_ROWS_REMOVED,	/* Remote slot is behind; required WAL or
+									 * rows may be removed or at risk */
+	SS_SKIP_NO_CONSISTENT_SNAPSHOT, /* Standby could not build a consistent
+									 * snapshot */
+	SS_SKIP_INVALID				/* Local slot is invalid */
+} SlotSyncSkipReason;
+
 /*
  * On-Disk data of a replication slot, preserved across restarts.
  */
@@ -249,6 +266,16 @@ typedef struct ReplicationSlot
 	 */
 	XLogRecPtr	last_saved_restart_lsn;
 
+	/*
+	 * The reason for last slot sync skip.
+	 *
+	 * A slotsync skip typically occurs only for temporary slots. For
+	 * persistent slots it is extremely rare (e.g., cases like
+	 * SS_SKIP_WAL_NOT_FLUSHED or SS_SKIP_WAL_OR_ROWS_REMOVED). Since,
+	 * temporary slots are dropped after server restart, so there is no value
+	 * in persisting the slotsync_skip_reason.
+	 */
+	SlotSyncSkipReason slotsync_skip_reason;
 } ReplicationSlot;
 
 #define SlotIsPhysical(slot) ((slot)->data.database == InvalidOid)
diff --git a/src/test/recovery/t/040_standby_failover_slots_sync.pl b/src/test/recovery/t/040_standby_failover_slots_sync.pl
index 7d3c82e0a29..25777fa188c 100644
--- a/src/test/recovery/t/040_standby_failover_slots_sync.pl
+++ b/src/test/recovery/t/040_standby_failover_slots_sync.pl
@@ -1049,6 +1049,12 @@ $standby2->wait_for_log(qr/slot sync worker started/, $log_offset);
 $standby2->wait_for_log(
 	qr/could not synchronize replication slot \"lsub1_slot\"/, $log_offset);
 
+# Confirm that the slotsync skip reason is updated
+$result = $standby2->safe_psql('postgres',
+	"SELECT slotsync_skip_reason FROM pg_replication_slots WHERE slot_name = 'lsub1_slot'"
+);
+is($result, 'wal_or_rows_removed', "check slot sync skip reason");
+
 # Confirm that the slotsync skip statistics is updated
 $result = $standby2->safe_psql('postgres',
 	"SELECT slotsync_skip_count > 0 FROM pg_stat_replication_slots WHERE slot_name = 'lsub1_slot'"
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index c337f0bc30d..94e45dd4d57 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.slotsync_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, slotsync_skip_reason)
      LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
 pg_roles| SELECT pg_authid.rolname,
     pg_authid.rolsuper,
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index e3c3523b5b2..cf3f6a7dafd 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2807,6 +2807,7 @@ SlabSlot
 SlotInvalidationCauseMap
 SlotNumber
 SlotSyncCtxStruct
+SlotSyncSkipReason
 SlruCtl
 SlruCtlData
 SlruErrorCause
-- 
2.34.1

From d6d5a31a8d1d6d2bf25ffd79bb351206da64bf2a Mon Sep 17 00:00:00 2001
From: Shlok Kyal <shlok.kyal.oss@gmail.com>
Date: Wed, 3 Dec 2025 14:27:51 +0530
Subject: [PATCH v1] Rename column slotsync_skip_at to slotsync_last_skip

This patch renames column slotsync_skip_at of view
pg_stat_replication_slots to slotsync_last_skip to make it consistent
with similar columns names.
---
 contrib/test_decoding/expected/stats.out     | 12 ++++++------
 doc/src/sgml/monitoring.sgml                 |  2 +-
 src/backend/catalog/system_views.sql         |  2 +-
 src/backend/utils/activity/pgstat_replslot.c |  2 +-
 src/backend/utils/adt/pgstatfuncs.c          |  6 +++---
 src/include/catalog/pg_proc.dat              |  2 +-
 src/include/pgstat.h                         |  2 +-
 src/test/regress/expected/rules.out          |  4 ++--
 8 files changed, 16 insertions(+), 16 deletions(-)

diff --git a/contrib/test_decoding/expected/stats.out b/contrib/test_decoding/expected/stats.out
index e5117f88a14..a9ead3c41aa 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 | slotsync_skip_count | slotsync_skip_at | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+--------------------+------------+-------------+---------------------+------------------+-------------
- do-not-exist |          0 |           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 | slotsync_skip_count | slotsync_last_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 | slotsync_skip_count | slotsync_skip_at | stats_reset 
---------------+------------+-------------+-------------+-------------+--------------+--------------+--------------------+------------+-------------+---------------------+------------------+-------------
- do-not-exist |          0 |           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 | slotsync_skip_count | slotsync_last_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 039d73691be..d2dd5e28365 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1671,7 +1671,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
 
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
-        <structfield>slotsync_skip_at</structfield><type>timestamp with time zone</type>
+        <structfield>slotsync_last_skip</structfield><type>timestamp with time zone</type>
        </para>
        <para>
         Time at which last slot synchronization was skipped. Slot
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 086c4c8fb6f..48af8ee90a6 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1078,7 +1078,7 @@ CREATE VIEW pg_stat_replication_slots AS
             s.total_txns,
             s.total_bytes,
             s.slotsync_skip_count,
-            s.slotsync_skip_at,
+            s.slotsync_last_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/utils/activity/pgstat_replslot.c b/src/backend/utils/activity/pgstat_replslot.c
index e08d33e8b4c..d757e00eb54 100644
--- a/src/backend/utils/activity/pgstat_replslot.c
+++ b/src/backend/utils/activity/pgstat_replslot.c
@@ -127,7 +127,7 @@ pgstat_report_replslotsync(ReplicationSlot *slot)
 	statent = &shstatent->stats;
 
 	statent->slotsync_skip_count += 1;
-	statent->slotsync_skip_at = GetCurrentTimestamp();
+	statent->slotsync_last_skip = GetCurrentTimestamp();
 
 	pgstat_unlock_entry(entry_ref);
 }
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 7e2ed69138a..ef6fffe60b9 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -2162,7 +2162,7 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 11, "slotsync_skip_count",
 					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 12, "slotsync_skip_at",
+	TupleDescInitEntry(tupdesc, (AttrNumber) 12, "slotsync_last_skip",
 					   TIMESTAMPTZOID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 13, "stats_reset",
 					   TIMESTAMPTZOID, -1, 0);
@@ -2192,10 +2192,10 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 	values[9] = Int64GetDatum(slotent->total_bytes);
 	values[10] = Int64GetDatum(slotent->slotsync_skip_count);
 
-	if (slotent->slotsync_skip_at == 0)
+	if (slotent->slotsync_last_skip == 0)
 		nulls[11] = true;
 	else
-		values[11] = TimestampTzGetDatum(slotent->slotsync_skip_at);
+		values[11] = TimestampTzGetDatum(slotent->slotsync_last_skip);
 
 	if (slotent->stat_reset_timestamp == 0)
 		nulls[12] = true;
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 66af2d96d67..fd9448ec7b9 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5693,7 +5693,7 @@
   proparallel => 'r', prorettype => 'record', proargtypes => 'text',
   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,slotsync_skip_count,slotsync_skip_at,stats_reset}',
+  proargnames => '{slot_name,slot_name,spill_txns,spill_count,spill_bytes,stream_txns,stream_count,stream_bytes,mem_exceeded_count,total_txns,total_bytes,slotsync_skip_count,slotsync_last_skip,stats_reset}',
   prosrc => 'pg_stat_get_replication_slot' },
 
 { oid => '6230', descr => 'statistics: check if a stats object exists',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index ad85134f27a..f23dd5870da 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -401,7 +401,7 @@ typedef struct PgStat_StatReplSlotEntry
 	PgStat_Counter total_txns;
 	PgStat_Counter total_bytes;
 	PgStat_Counter slotsync_skip_count;
-	TimestampTz slotsync_skip_at;
+	TimestampTz slotsync_last_skip;
 	TimestampTz stat_reset_timestamp;
 } PgStat_StatReplSlotEntry;
 
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 94e45dd4d57..85d795dbd63 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -2153,10 +2153,10 @@ pg_stat_replication_slots| SELECT s.slot_name,
     s.total_txns,
     s.total_bytes,
     s.slotsync_skip_count,
-    s.slotsync_skip_at,
+    s.slotsync_last_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, slotsync_skip_count, slotsync_skip_at, 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, slotsync_skip_count, slotsync_last_skip, stats_reset)
   WHERE (r.datoid IS NOT NULL);
 pg_stat_slru| SELECT name,
     blks_zeroed,
-- 
2.34.1