Add mem_exceeded_count column to pg_stat_replication_slots.

This commit introduces a new column mem_exceeded_count to the
pg_stat_replication_slots view. This counter tracks how often the
memory used by logical decoding exceeds the logical_decoding_work_mem
limit. The new statistic helps users determine whether exceeding the
logical_decoding_work_mem limit is a rare occurrences or a frequent
issue, information that wasn't available through existing statistics.

Bumps catversion.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/978D21E8-9D3B-40EA-A4B1-F87BABE7868C@yesql.se

10 files changed, 57 insertions, 21 deletions

diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index c94f1f05f52..fcc86fd43be 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1063,6 +1063,7 @@ CREATE VIEW pg_stat_replication_slots AS
             s.stream_txns,
             s.stream_count,
             s.stream_bytes,
+            s.mem_exceeded_count,
             s.total_txns,
             s.total_bytes,
             s.stats_reset
diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index c68c0481f42..93ed2eb368e 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -1955,10 +1955,11 @@ UpdateDecodingStats(LogicalDecodingContext *ctx)
 	PgStat_StatReplSlotEntry repSlotStat;
 
 	/* Nothing to do if we don't have any replication stats to be sent. */
-	if (rb->spillBytes <= 0 && rb->streamBytes <= 0 && rb->totalBytes <= 0)
+	if (rb->spillBytes <= 0 && rb->streamBytes <= 0 && rb->totalBytes <= 0 &&
+		rb->memExceededCount <= 0)
 		return;
 
-	elog(DEBUG2, "UpdateDecodingStats: updating stats %p %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64,
+	elog(DEBUG2, "UpdateDecodingStats: updating stats %p %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64,
 		 rb,
 		 rb->spillTxns,
 		 rb->spillCount,
@@ -1966,6 +1967,7 @@ UpdateDecodingStats(LogicalDecodingContext *ctx)
 		 rb->streamTxns,
 		 rb->streamCount,
 		 rb->streamBytes,
+		 rb->memExceededCount,
 		 rb->totalTxns,
 		 rb->totalBytes);
 
@@ -1975,6 +1977,7 @@ UpdateDecodingStats(LogicalDecodingContext *ctx)
 	repSlotStat.stream_txns = rb->streamTxns;
 	repSlotStat.stream_count = rb->streamCount;
 	repSlotStat.stream_bytes = rb->streamBytes;
+	repSlotStat.mem_exceeded_count = rb->memExceededCount;
 	repSlotStat.total_txns = rb->totalTxns;
 	repSlotStat.total_bytes = rb->totalBytes;
 
@@ -1986,6 +1989,7 @@ UpdateDecodingStats(LogicalDecodingContext *ctx)
 	rb->streamTxns = 0;
 	rb->streamCount = 0;
 	rb->streamBytes = 0;
+	rb->memExceededCount = 0;
 	rb->totalTxns = 0;
 	rb->totalBytes = 0;
 }
diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
index a5e165fb123..a54434151de 100644
--- a/src/backend/replication/logical/reorderbuffer.c
+++ b/src/backend/replication/logical/reorderbuffer.c
@@ -390,6 +390,7 @@ ReorderBufferAllocate(void)
 	buffer->streamTxns = 0;
 	buffer->streamCount = 0;
 	buffer->streamBytes = 0;
+	buffer->memExceededCount = 0;
 	buffer->totalTxns = 0;
 	buffer->totalBytes = 0;
 
@@ -3898,14 +3899,26 @@ static void
 ReorderBufferCheckMemoryLimit(ReorderBuffer *rb)
 {
 	ReorderBufferTXN *txn;
+	bool		update_stats = true;
 
-	/*
-	 * Bail out if debug_logical_replication_streaming is buffered and we
-	 * haven't exceeded the memory limit.
-	 */
-	if (debug_logical_replication_streaming == DEBUG_LOGICAL_REP_STREAMING_BUFFERED &&
-		rb->size < logical_decoding_work_mem * (Size) 1024)
+	if (rb->size >= logical_decoding_work_mem * (Size) 1024)
+	{
+		/*
+		 * Update the statistics as the memory usage has reached the limit. We
+		 * report the statistics update later in this function since we can
+		 * update the slot statistics altogether while streaming or
+		 * serializing transactions in most cases.
+		 */
+		rb->memExceededCount += 1;
+	}
+	else if (debug_logical_replication_streaming == DEBUG_LOGICAL_REP_STREAMING_BUFFERED)
+	{
+		/*
+		 * Bail out if debug_logical_replication_streaming is buffered and we
+		 * haven't exceeded the memory limit.
+		 */
 		return;
+	}
 
 	/*
 	 * If debug_logical_replication_streaming is immediate, loop until there's
@@ -3965,8 +3978,17 @@ ReorderBufferCheckMemoryLimit(ReorderBuffer *rb)
 		 */
 		Assert(txn->size == 0);
 		Assert(txn->nentries_mem == 0);
+
+		/*
+		 * We've reported the memExceededCount update while streaming or
+		 * serializing the transaction.
+		 */
+		update_stats = false;
 	}
 
+	if (update_stats)
+		UpdateDecodingStats((LogicalDecodingContext *) rb->private_data);
+
 	/* We must be under the memory limit now. */
 	Assert(rb->size < logical_decoding_work_mem * (Size) 1024);
 }
diff --git a/src/backend/utils/activity/pgstat_replslot.c b/src/backend/utils/activity/pgstat_replslot.c
index ccfb11c49bf..d210c261ac6 100644
--- a/src/backend/utils/activity/pgstat_replslot.c
+++ b/src/backend/utils/activity/pgstat_replslot.c
@@ -94,6 +94,7 @@ pgstat_report_replslot(ReplicationSlot *slot, const PgStat_StatReplSlotEntry *re
 	REPLSLOT_ACC(stream_txns);
 	REPLSLOT_ACC(stream_count);
 	REPLSLOT_ACC(stream_bytes);
+	REPLSLOT_ACC(mem_exceeded_count);
 	REPLSLOT_ACC(total_txns);
 	REPLSLOT_ACC(total_bytes);
 #undef REPLSLOT_ACC
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 3802a4cb888..1fe33df2756 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 10
+#define PG_STAT_GET_REPLICATION_SLOT_COLS 11
 	text	   *slotname_text = PG_GETARG_TEXT_P(0);
 	NameData	slotname;
 	TupleDesc	tupdesc;
@@ -2146,11 +2146,13 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 7, "stream_bytes",
 					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 8, "total_txns",
+	TupleDescInitEntry(tupdesc, (AttrNumber) 8, "mem_exceeded_count",
 					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 9, "total_bytes",
+	TupleDescInitEntry(tupdesc, (AttrNumber) 9, "total_txns",
 					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "stats_reset",
+	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "total_bytes",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 11, "stats_reset",
 					   TIMESTAMPTZOID, -1, 0);
 	BlessTupleDesc(tupdesc);
 
@@ -2173,13 +2175,14 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 	values[4] = Int64GetDatum(slotent->stream_txns);
 	values[5] = Int64GetDatum(slotent->stream_count);
 	values[6] = Int64GetDatum(slotent->stream_bytes);
-	values[7] = Int64GetDatum(slotent->total_txns);
-	values[8] = Int64GetDatum(slotent->total_bytes);
+	values[7] = Int64GetDatum(slotent->mem_exceeded_count);
+	values[8] = Int64GetDatum(slotent->total_txns);
+	values[9] = Int64GetDatum(slotent->total_bytes);
 
 	if (slotent->stat_reset_timestamp == 0)
-		nulls[9] = true;
+		nulls[10] = true;
 	else
-		values[9] = TimestampTzGetDatum(slotent->stat_reset_timestamp);
+		values[10] = 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/catversion.h b/src/include/catalog/catversion.h
index af54211f330..a98c6d6d820 100644
--- a/src/include/catalog/catversion.h
+++ b/src/include/catalog/catversion.h
@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202510082
+#define CATALOG_VERSION_NO	202510083
 
 #endif
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 6bb31892d1d..25687eaecea 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,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}',
+  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}',
   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 d24bf864a22..bc8077cbae6 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -395,6 +395,7 @@ typedef struct PgStat_StatReplSlotEntry
 	PgStat_Counter stream_txns;
 	PgStat_Counter stream_count;
 	PgStat_Counter stream_bytes;
+	PgStat_Counter mem_exceeded_count;
 	PgStat_Counter total_txns;
 	PgStat_Counter total_bytes;
 	TimestampTz stat_reset_timestamp;
diff --git a/src/include/replication/reorderbuffer.h b/src/include/replication/reorderbuffer.h
index 91dc7e5e448..3cbe106a3c7 100644
--- a/src/include/replication/reorderbuffer.h
+++ b/src/include/replication/reorderbuffer.h
@@ -690,6 +690,9 @@ struct ReorderBuffer
 	int64		streamCount;	/* streaming invocation counter */
 	int64		streamBytes;	/* amount of data decoded */
 
+	/* Number of times the logical_decoding_work_mem limit has been reached */
+	int64		memExceededCount;
+
 	/*
 	 * Statistics about all the transactions sent to the decoding output
 	 * plugin
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 8859a5a885f..d67af144d69 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -2140,11 +2140,12 @@ pg_stat_replication_slots| SELECT s.slot_name,
     s.stream_txns,
     s.stream_count,
     s.stream_bytes,
+    s.mem_exceeded_count,
     s.total_txns,
     s.total_bytes,
     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, mem_exceeded_count, total_txns, total_bytes, stats_reset)
   WHERE (r.datoid IS NOT NULL);
 pg_stat_slru| SELECT name,
     blks_zeroed,