Use HTAB for replication slot statistics.

Previously, we used to use the array of size max_replication_slots to
store stats for replication slots. But that had two problems in the cases
where a message for dropping a slot gets lost: 1) the stats for the new
slot are not recorded if the array is full and 2) writing beyond the end
of the array if the user reduces the max_replication_slots.

This commit uses HTAB for replication slot statistics, resolving both
problems. Now, pgstat_vacuum_stat() search for all the dead replication
slots in stats hashtable and tell the collector to remove them. To avoid
showing the stats for the already-dropped slots, pg_stat_replication_slots
view searches slot stats by the slot name taken from pg_replication_slots.

Also, we send a message for creating a slot at slot creation, initializing
the stats. This reduces the possibility that the stats are accumulated
into the old slot stats when a message for dropping a slot gets lost.

Reported-by: Andres Freund
Author: Sawada Masahiko, test case by Vignesh C
Reviewed-by: Amit Kapila, Vignesh C, Dilip Kumar
Discussion: https://postgr.es/m/20210319185247.ldebgpdaxsowiflw@alap3.anarazel.de

contrib/test_decoding/t/001_repl_stats.pl

@@ -2,9 +2,10 @@
 # drop replication slot and restart.
 use strict;
 use warnings;
+use File::Path qw(rmtree);
 use PostgresNode;
 use TestLib;
-use Test::More tests => 1;
+use Test::More tests => 2;
 
 # Test set-up
 my $node = get_new_node('test');
@@ -12,9 +13,22 @@ $node->init(allows_streaming => 'logical');
 $node->append_conf('postgresql.conf', 'synchronous_commit = on');
 $node->start;
 
+# Check that replication slot stats are expected.
+sub test_slot_stats
+{
+	my ($node, $expected, $msg) = @_;
+
+	my $result = $node->safe_psql(
+		'postgres', qq[
+		SELECT slot_name, total_txns > 0 AS total_txn,
+			   total_bytes > 0 AS total_bytes
+			   FROM pg_stat_replication_slots
+			   ORDER BY slot_name]);
+	is($result, $expected, $msg);
+}
+
 # Create table.
-$node->safe_psql('postgres',
-        "CREATE TABLE test_repl_stat(col1 int)");
+$node->safe_psql('postgres', "CREATE TABLE test_repl_stat(col1 int)");
 
 # Create replication slots.
 $node->safe_psql(
@@ -26,7 +40,8 @@ $node->safe_psql(
 ]);
 
 # Insert some data.
-$node->safe_psql('postgres', "INSERT INTO test_repl_stat values(generate_series(1, 5));");
+$node->safe_psql('postgres',
+	"INSERT INTO test_repl_stat values(generate_series(1, 5));");
 
 $node->safe_psql(
 	'postgres', qq[
@@ -50,27 +65,51 @@ $node->poll_query_until(
 
 # Test to drop one of the replication slot and verify replication statistics data is
 # fine after restart.
-$node->safe_psql('postgres', "SELECT pg_drop_replication_slot('regression_slot4')");
+$node->safe_psql('postgres',
+	"SELECT pg_drop_replication_slot('regression_slot4')");
 
 $node->stop;
 $node->start;
 
 # Verify statistics data present in pg_stat_replication_slots are sane after
 # restart.
-my $result = $node->safe_psql('postgres',
-	"SELECT slot_name, total_txns > 0 AS total_txn,
-	total_bytes > 0 AS total_bytes FROM pg_stat_replication_slots
-	ORDER BY slot_name"
-);
-is($result, qq(regression_slot1|t|t
+test_slot_stats(
+	$node,
+	qq(regression_slot1|t|t
 regression_slot2|t|t
-regression_slot3|t|t), 'check replication statistics are updated');
+regression_slot3|t|t),
+	'check replication statistics are updated');
+
+# Test to remove one of the replication slots and adjust
+# max_replication_slots accordingly to the number of slots. This leads
+# to a mismatch between the number of slots present in the stats file and the
+# number of stats present in the shared memory, simulating the scenario for
+# drop slot message lost by the statistics collector process. We verify
+# replication statistics data is fine after restart.
+
+$node->stop;
+my $datadir           = $node->data_dir;
+my $slot3_replslotdir = "$datadir/pg_replslot/regression_slot3";
+
+rmtree($slot3_replslotdir);
+
+$node->append_conf('postgresql.conf', 'max_replication_slots = 2');
+$node->start;
+
+# Verify statistics data present in pg_stat_replication_slots are sane after
+# restart.
+test_slot_stats(
+	$node,
+	qq(regression_slot1|t|t
+regression_slot2|t|t),
+	'check replication statistics after removing the slot file');
 
 # cleanup
 $node->safe_psql('postgres', "DROP TABLE test_repl_stat");
-$node->safe_psql('postgres', "SELECT pg_drop_replication_slot('regression_slot1')");
-$node->safe_psql('postgres', "SELECT pg_drop_replication_slot('regression_slot2')");
-$node->safe_psql('postgres', "SELECT pg_drop_replication_slot('regression_slot3')");
+$node->safe_psql('postgres',
+	"SELECT pg_drop_replication_slot('regression_slot1')");
+$node->safe_psql('postgres',
+	"SELECT pg_drop_replication_slot('regression_slot2')");
 
 # shutdown
 $node->stop;

src/backend/catalog/system_views.sql

@@ -866,20 +866,6 @@ CREATE VIEW pg_stat_replication AS
         JOIN pg_stat_get_wal_senders() AS W ON (S.pid = W.pid)
         LEFT JOIN pg_authid AS U ON (S.usesysid = U.oid);
 
-CREATE VIEW pg_stat_replication_slots AS
-    SELECT
-            s.slot_name,
-            s.spill_txns,
-            s.spill_count,
-            s.spill_bytes,
-            s.stream_txns,
-            s.stream_count,
-            s.stream_bytes,
-            s.total_txns,
-            s.total_bytes,
-            s.stats_reset
-    FROM pg_stat_get_replication_slots() AS s;
-
 CREATE VIEW pg_stat_slru AS
     SELECT
             s.name,
@@ -984,6 +970,22 @@ CREATE VIEW pg_replication_slots AS
     FROM pg_get_replication_slots() AS L
             LEFT JOIN pg_database D ON (L.datoid = D.oid);
 
+CREATE VIEW pg_stat_replication_slots AS
+    SELECT
+            s.slot_name,
+            s.spill_txns,
+            s.spill_count,
+            s.spill_bytes,
+            s.stream_txns,
+            s.stream_count,
+            s.stream_bytes,
+            s.total_txns,
+            s.total_bytes,
+            s.stats_reset
+    FROM pg_replication_slots as r,
+        LATERAL pg_stat_get_replication_slot(slot_name) as s
+    WHERE r.datoid IS NOT NULL; -- excluding physical slots
+
 CREATE VIEW pg_stat_database AS
     SELECT
             D.oid AS datid,

src/backend/postmaster/pgstat.c

@@ -106,6 +106,7 @@
 #define PGSTAT_DB_HASH_SIZE		16
 #define PGSTAT_TAB_HASH_SIZE	512
 #define PGSTAT_FUNCTION_HASH_SIZE	512
+#define PGSTAT_REPLSLOT_HASH_SIZE	32
 
 
 /* ----------
@@ -278,8 +279,7 @@ static PgStat_ArchiverStats archiverStats;
 static PgStat_GlobalStats globalStats;
 static PgStat_WalStats walStats;
 static PgStat_SLRUStats slruStats[SLRU_NUM_ELEMENTS];
-static PgStat_ReplSlotStats *replSlotStats;
-static int	nReplSlotStats;
+static HTAB *replSlotStatHash = NULL;
 static PgStat_RecoveryPrefetchStats recoveryPrefetchStats;
 
 /*
@@ -319,8 +319,8 @@ static void backend_read_statsfile(void);
 static bool pgstat_write_statsfile_needed(void);
 static bool pgstat_db_requested(Oid databaseid);
 
-static int	pgstat_replslot_index(const char *name, bool create_it);
-static void pgstat_reset_replslot(int i, TimestampTz ts);
+static PgStat_StatReplSlotEntry *pgstat_get_replslot_entry(NameData name, bool create_it);
+static void pgstat_reset_replslot(PgStat_StatReplSlotEntry *slotstats, TimestampTz ts);
 
 static void pgstat_send_tabstat(PgStat_MsgTabstat *tsmsg);
 static void pgstat_send_funcstats(void);
@@ -1109,6 +1109,24 @@ pgstat_vacuum_stat(void)
 	/* Clean up */
 	hash_destroy(htab);
 
+	/*
+	 * Search for all the dead replication slots in stats hashtable and tell
+	 * the stats collector to drop them.
+	 */
+	if (replSlotStatHash)
+	{
+		PgStat_StatReplSlotEntry *slotentry;
+
+		hash_seq_init(&hstat, replSlotStatHash);
+		while ((slotentry = (PgStat_StatReplSlotEntry *) hash_seq_search(&hstat)) != NULL)
+		{
+			CHECK_FOR_INTERRUPTS();
+
+			if (SearchNamedReplicationSlot(NameStr(slotentry->slotname), true) == NULL)
+				pgstat_report_replslot_drop(NameStr(slotentry->slotname));
+		}
+	}
+
 	/*
 	 * Lookup our own database entry; if not found, nothing more to do.
 	 */
@@ -1516,30 +1534,6 @@ pgstat_reset_replslot_counter(const char *name)
 
 	if (name)
 	{
-		ReplicationSlot *slot;
-
-		/*
-		 * Check if the slot exists with the given name. It is possible that by
-		 * the time this message is executed the slot is dropped but at least
-		 * this check will ensure that the given name is for a valid slot.
-		 */
-		LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
-		slot = SearchNamedReplicationSlot(name);
-		LWLockRelease(ReplicationSlotControlLock);
-
-		if (!slot)
-			ereport(ERROR,
-					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-					 errmsg("replication slot \"%s\" does not exist",
-							name)));
-
-		/*
-		 * Nothing to do for physical slots as we collect stats only for
-		 * logical slots.
-		 */
-		if (SlotIsPhysical(slot))
-			return;
-
 		namestrcpy(&msg.m_slotname, name);
 		msg.clearall = false;
 	}
@@ -1813,7 +1807,7 @@ pgstat_report_tempfile(size_t filesize)
  * ----------
  */
 void
-pgstat_report_replslot(const PgStat_ReplSlotStats *repSlotStat)
+pgstat_report_replslot(const PgStat_StatReplSlotEntry *repSlotStat)
 {
 	PgStat_MsgReplSlot msg;
 
@@ -1822,6 +1816,7 @@ pgstat_report_replslot(const PgStat_ReplSlotStats *repSlotStat)
 	 */
 	pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_REPLSLOT);
 	namestrcpy(&msg.m_slotname, NameStr(repSlotStat->slotname));
+	msg.m_create = false;
 	msg.m_drop = false;
 	msg.m_spill_txns = repSlotStat->spill_txns;
 	msg.m_spill_count = repSlotStat->spill_count;
@@ -1834,6 +1829,24 @@ pgstat_report_replslot(const PgStat_ReplSlotStats *repSlotStat)
 	pgstat_send(&msg, sizeof(PgStat_MsgReplSlot));
 }
 
+/* ----------
+ * pgstat_report_replslot_create() -
+ *
+ *	Tell the collector about creating the replication slot.
+ * ----------
+ */
+void
+pgstat_report_replslot_create(const char *slotname)
+{
+	PgStat_MsgReplSlot msg;
+
+	pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_REPLSLOT);
+	namestrcpy(&msg.m_slotname, slotname);
+	msg.m_create = true;
+	msg.m_drop = false;
+	pgstat_send(&msg, sizeof(PgStat_MsgReplSlot));
+}
+
 /* ----------
  * pgstat_report_replslot_drop() -
  *
@@ -1847,6 +1860,7 @@ pgstat_report_replslot_drop(const char *slotname)
 
 	pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_REPLSLOT);
 	namestrcpy(&msg.m_slotname, slotname);
+	msg.m_create = false;
 	msg.m_drop = true;
 	pgstat_send(&msg, sizeof(PgStat_MsgReplSlot));
 }
@@ -2872,17 +2886,15 @@ pgstat_fetch_slru(void)
  * pgstat_fetch_replslot() -
  *
  *	Support function for the SQL-callable pgstat* functions. Returns
- *	a pointer to the replication slot statistics struct and sets the
- *	number of entries in nslots_p.
+ *	a pointer to the replication slot statistics struct.
  * ---------
  */
-PgStat_ReplSlotStats *
-pgstat_fetch_replslot(int *nslots_p)
+PgStat_StatReplSlotEntry *
+pgstat_fetch_replslot(NameData slotname)
 {
 	backend_read_statsfile();
 
-	*nslots_p = nReplSlotStats;
-	return replSlotStats;
+	return pgstat_get_replslot_entry(slotname, false);
 }
 
 /*
@@ -3654,7 +3666,6 @@ pgstat_write_statsfiles(bool permanent, bool allDbs)
 	const char *tmpfile = permanent ? PGSTAT_STAT_PERMANENT_TMPFILE : pgstat_stat_tmpname;
 	const char *statfile = permanent ? PGSTAT_STAT_PERMANENT_FILENAME : pgstat_stat_filename;
 	int			rc;
-	int			i;
 
 	elog(DEBUG2, "writing stats file \"%s\"", statfile);
 
@@ -3744,11 +3755,17 @@ pgstat_write_statsfiles(bool permanent, bool allDbs)
 	/*
 	 * Write replication slot stats struct
 	 */
-	for (i = 0; i < nReplSlotStats; i++)
+	if (replSlotStatHash)
 	{
-		fputc('R', fpout);
-		rc = fwrite(&replSlotStats[i], sizeof(PgStat_ReplSlotStats), 1, fpout);
-		(void) rc;				/* we'll check for error with ferror */
+		PgStat_StatReplSlotEntry *slotent;
+
+		hash_seq_init(&hstat, replSlotStatHash);
+		while ((slotent = (PgStat_StatReplSlotEntry *) hash_seq_search(&hstat)) != NULL)
+		{
+			fputc('R', fpout);
+			rc = fwrite(slotent, sizeof(PgStat_StatReplSlotEntry), 1, fpout);
+			(void) rc;				/* we'll check for error with ferror */
+		}
 	}
 
 	/*
@@ -3975,12 +3992,6 @@ pgstat_read_statsfiles(Oid onlydb, bool permanent, bool deep)
 	dbhash = hash_create("Databases hash", PGSTAT_DB_HASH_SIZE, &hash_ctl,
 						 HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
 
-	/* Allocate the space for replication slot statistics */
-	replSlotStats = MemoryContextAllocZero(pgStatLocalContext,
-										   max_replication_slots
-										   * sizeof(PgStat_ReplSlotStats));
-	nReplSlotStats = 0;
-
 	/*
 	 * Clear out global, archiver, WAL and SLRU statistics so they start from
 	 * zero in case we can't load an existing statsfile.
@@ -4005,12 +4016,6 @@ pgstat_read_statsfiles(Oid onlydb, bool permanent, bool deep)
 	for (i = 0; i < SLRU_NUM_ELEMENTS; i++)
 		slruStats[i].stat_reset_timestamp = globalStats.stat_reset_timestamp;
 
-	/*
-	 * Set the same reset timestamp for all replication slots too.
-	 */
-	for (i = 0; i < max_replication_slots; i++)
-		replSlotStats[i].stat_reset_timestamp = globalStats.stat_reset_timestamp;
-
 	/*
 	 * Try to open the stats file. If it doesn't exist, the backends simply
 	 * return zero for anything and the collector simply starts from scratch
@@ -4197,21 +4202,43 @@ pgstat_read_statsfiles(Oid onlydb, bool permanent, bool deep)
 				break;
 
 				/*
-				 * 'R'	A PgStat_ReplSlotStats struct describing a replication
-				 * slot follows.
+				 * 'R'	A PgStat_StatReplSlotEntry struct describing a
+				 * replication slot follows.
 				 */
 			case 'R':
-				if (fread(&replSlotStats[nReplSlotStats], 1, sizeof(PgStat_ReplSlotStats), fpin)
-					!= sizeof(PgStat_ReplSlotStats))
 				{
-					ereport(pgStatRunningInCollector ? LOG : WARNING,
-							(errmsg("corrupted statistics file \"%s\"",
-									statfile)));
-					memset(&replSlotStats[nReplSlotStats], 0, sizeof(PgStat_ReplSlotStats));
-					goto done;
+					PgStat_StatReplSlotEntry slotbuf;
+					PgStat_StatReplSlotEntry *slotent;
+
+					if (fread(&slotbuf, 1, sizeof(PgStat_StatReplSlotEntry), fpin)
+						!= sizeof(PgStat_StatReplSlotEntry))
+					{
+						ereport(pgStatRunningInCollector ? LOG : WARNING,
+								(errmsg("corrupted statistics file \"%s\"",
+										statfile)));
+						goto done;
+					}
+
+					/* Create hash table if we don't have it already. */
+					if (replSlotStatHash == NULL)
+					{
+						HASHCTL		hash_ctl;
+
+						hash_ctl.keysize = sizeof(NameData);
+						hash_ctl.entrysize = sizeof(PgStat_StatReplSlotEntry);
+						hash_ctl.hcxt = pgStatLocalContext;
+						replSlotStatHash = hash_create("Replication slots hash",
+													   PGSTAT_REPLSLOT_HASH_SIZE,
+													   &hash_ctl,
+													   HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
+					}
+
+					slotent = (PgStat_StatReplSlotEntry *) hash_search(replSlotStatHash,
+																	   (void *) &slotbuf.slotname,
+																	   HASH_ENTER, NULL);
+					memcpy(slotent, &slotbuf, sizeof(PgStat_StatReplSlotEntry));
+					break;
 				}
-				nReplSlotStats++;
-				break;
 
 			case 'E':
 				goto done;
@@ -4424,7 +4451,7 @@ pgstat_read_db_statsfile_timestamp(Oid databaseid, bool permanent,
 	PgStat_ArchiverStats myArchiverStats;
 	PgStat_WalStats myWalStats;
 	PgStat_SLRUStats mySLRUStats[SLRU_NUM_ELEMENTS];
-	PgStat_ReplSlotStats myReplSlotStats;
+	PgStat_StatReplSlotEntry myReplSlotStats;
 	PgStat_RecoveryPrefetchStats myRecoveryPrefetchStats;
 	FILE	   *fpin;
 	int32		format_id;
@@ -4553,12 +4580,12 @@ pgstat_read_db_statsfile_timestamp(Oid databaseid, bool permanent,
 				break;
 
 				/*
-				 * 'R'	A PgStat_ReplSlotStats struct describing a replication
-				 * slot follows.
+				 * 'R'	A PgStat_StatReplSlotEntry struct describing a
+				 * replication slot follows.
 				 */
 			case 'R':
-				if (fread(&myReplSlotStats, 1, sizeof(PgStat_ReplSlotStats), fpin)
-					!= sizeof(PgStat_ReplSlotStats))
+				if (fread(&myReplSlotStats, 1, sizeof(PgStat_StatReplSlotEntry), fpin)
+					!= sizeof(PgStat_StatReplSlotEntry))
 				{
 					ereport(pgStatRunningInCollector ? LOG : WARNING,
 							(errmsg("corrupted statistics file \"%s\"",
@@ -4764,8 +4791,7 @@ pgstat_clear_snapshot(void)
 	/* Reset variables */
 	pgStatLocalContext = NULL;
 	pgStatDBHash = NULL;
-	replSlotStats = NULL;
-	nReplSlotStats = 0;
+	replSlotStatHash = NULL;
 
 	/*
 	 * Historically the backend_status.c facilities lived in this file, and
@@ -5189,20 +5215,26 @@ static void
 pgstat_recv_resetreplslotcounter(PgStat_MsgResetreplslotcounter *msg,
 								 int len)
 {
-	int			i;
-	int			idx = -1;
+	PgStat_StatReplSlotEntry *slotent;
 	TimestampTz ts;
 
+	/* Return if we don't have replication slot statistics */
+	if (replSlotStatHash == NULL)
+		return;
+
 	ts = GetCurrentTimestamp();
 	if (msg->clearall)
 	{
-		for (i = 0; i < nReplSlotStats; i++)
-			pgstat_reset_replslot(i, ts);
+		HASH_SEQ_STATUS sstat;
+
+		hash_seq_init(&sstat, replSlotStatHash);
+		while ((slotent = (PgStat_StatReplSlotEntry *) hash_seq_search(&sstat)) != NULL)
+			pgstat_reset_replslot(slotent, ts);
 	}
 	else
 	{
-		/* Get the index of replication slot statistics to reset */
-		idx = pgstat_replslot_index(NameStr(msg->m_slotname), false);
+		/* Get the slot statistics to reset */
+		slotent = pgstat_get_replslot_entry(msg->m_slotname, false);
 
 		/*
 		 * Nothing to do if the given slot entry is not found.  This could
@@ -5210,11 +5242,11 @@ pgstat_recv_resetreplslotcounter(PgStat_MsgResetreplslotcounter *msg,
 		 * corresponding statistics entry is also removed before receiving the
 		 * reset message.
 		 */
-		if (idx < 0)
+		if (!slotent)
 			return;
 
 		/* Reset the stats for the requested replication slot */
-		pgstat_reset_replslot(idx, ts);
+		pgstat_reset_replslot(slotent, ts);
 	}
 }
 
@@ -5532,46 +5564,45 @@ pgstat_recv_checksum_failure(PgStat_MsgChecksumFailure *msg, int len)
 static void
 pgstat_recv_replslot(PgStat_MsgReplSlot *msg, int len)
 {
-	int			idx;
-
-	/*
-	 * Get the index of replication slot statistics.  On dropping, we don't
-	 * create the new statistics.
-	 */
-	idx = pgstat_replslot_index(NameStr(msg->m_slotname), !msg->m_drop);
-
-	/*
-	 * The slot entry is not found or there is no space to accommodate the new
-	 * entry.  This could happen when the message for the creation of a slot
-	 * reached before the drop message even though the actual operations
-	 * happen in reverse order.  In such a case, the next update of the
-	 * statistics for the same slot will create the required entry.
-	 */
-	if (idx < 0)
-		return;
-
-	/* it must be a valid replication slot index */
-	Assert(idx < nReplSlotStats);
-
 	if (msg->m_drop)
 	{
+		Assert(!msg->m_create);
+
 		/* Remove the replication slot statistics with the given name */
-		if (idx < nReplSlotStats - 1)
-			memcpy(&replSlotStats[idx], &replSlotStats[nReplSlotStats - 1],
-				   sizeof(PgStat_ReplSlotStats));
-		nReplSlotStats--;
+		if (replSlotStatHash != NULL)
+			(void) hash_search(replSlotStatHash,
+							   (void *) &(msg->m_slotname),
+							   HASH_REMOVE,
+							   NULL);
 	}
 	else
 	{
-		/* Update the replication slot statistics */
-		replSlotStats[idx].spill_txns += msg->m_spill_txns;
-		replSlotStats[idx].spill_count += msg->m_spill_count;
-		replSlotStats[idx].spill_bytes += msg->m_spill_bytes;
-		replSlotStats[idx].stream_txns += msg->m_stream_txns;
-		replSlotStats[idx].stream_count += msg->m_stream_count;
-		replSlotStats[idx].stream_bytes += msg->m_stream_bytes;
-		replSlotStats[idx].total_txns += msg->m_total_txns;
-		replSlotStats[idx].total_bytes += msg->m_total_bytes;
+		PgStat_StatReplSlotEntry *slotent;
+
+		slotent = pgstat_get_replslot_entry(msg->m_slotname, true);
+		Assert(slotent);
+
+		if (msg->m_create)
+		{
+			/*
+			 * If the message for dropping the slot with the same name gets
+			 * lost, slotent has stats for the old slot. So we initialize all
+			 * counters at slot creation.
+			 */
+			pgstat_reset_replslot(slotent, 0);
+		}
+		else
+		{
+			/* Update the replication slot statistics */
+			slotent->spill_txns += msg->m_spill_txns;
+			slotent->spill_count += msg->m_spill_count;
+			slotent->spill_bytes += msg->m_spill_bytes;
+			slotent->stream_txns += msg->m_stream_txns;
+			slotent->stream_count += msg->m_stream_count;
+			slotent->stream_bytes += msg->m_stream_bytes;
+			slotent->total_txns += msg->m_total_txns;
+			slotent->total_bytes += msg->m_total_bytes;
+		}
 	}
 }
 
@@ -5749,59 +5780,80 @@ pgstat_db_requested(Oid databaseid)
 }
 
 /* ----------
- * pgstat_replslot_index
+ * pgstat_replslot_entry
  *
- * Return the index of entry of a replication slot with the given name, or
- * -1 if the slot is not found.
+ * Return the entry of replication slot stats with the given name. Return
+ * NULL if not found and the caller didn't request to create it.
  *
- * create_it tells whether to create the new slot entry if it is not found.
+ * create tells whether to create the new slot entry if it is not found.
  * ----------
  */
-static int
-pgstat_replslot_index(const char *name, bool create_it)
+static PgStat_StatReplSlotEntry *
+pgstat_get_replslot_entry(NameData name, bool create)
 {
-	int			i;
+	PgStat_StatReplSlotEntry *slotent;
+	bool		found;
 
-	Assert(nReplSlotStats <= max_replication_slots);
-	for (i = 0; i < nReplSlotStats; i++)
+	if (replSlotStatHash == NULL)
 	{
-		if (namestrcmp(&replSlotStats[i].slotname, name) == 0)
-			return i;			/* found */
+		HASHCTL		hash_ctl;
+
+		/*
+		 * Quick return NULL if the hash table is empty and the caller didn't
+		 * request to create the entry.
+		 */
+		if (!create)
+			return NULL;
+
+		hash_ctl.keysize = sizeof(NameData);
+		hash_ctl.entrysize = sizeof(PgStat_StatReplSlotEntry);
+		replSlotStatHash = hash_create("Replication slots hash",
+									   PGSTAT_REPLSLOT_HASH_SIZE,
+									   &hash_ctl,
+									   HASH_ELEM | HASH_BLOBS);
 	}
 
-	/*
-	 * The slot is not found.  We don't want to register the new statistics if
-	 * the list is already full or the caller didn't request.
-	 */
-	if (i == max_replication_slots || !create_it)
-		return -1;
+	slotent = (PgStat_StatReplSlotEntry *) hash_search(replSlotStatHash,
+													   (void *) &name,
+													   create ? HASH_ENTER : HASH_FIND,
+													   &found);
 
-	/* Register new slot */
-	memset(&replSlotStats[nReplSlotStats], 0, sizeof(PgStat_ReplSlotStats));
-	namestrcpy(&replSlotStats[nReplSlotStats].slotname, name);
+	if (!slotent)
+	{
+		/* not found */
+		Assert(!create && !found);
+		return NULL;
+	}
+
+	/* initialize the entry */
+	if (create && !found)
+	{
+		namestrcpy(&(slotent->slotname), NameStr(name));
+		pgstat_reset_replslot(slotent, 0);
+	}
 
-	return nReplSlotStats++;
+	return slotent;
 }
 
 /* ----------
  * pgstat_reset_replslot
  *
- * Reset the replication slot stats at index 'i'.
+ * Reset the given replication slot stats.
  * ----------
  */
 static void
-pgstat_reset_replslot(int i, TimestampTz ts)
+pgstat_reset_replslot(PgStat_StatReplSlotEntry *slotent, TimestampTz ts)
 {
 	/* reset only counters. Don't clear slot name */
-	replSlotStats[i].spill_txns = 0;
-	replSlotStats[i].spill_count = 0;
-	replSlotStats[i].spill_bytes = 0;
-	replSlotStats[i].stream_txns = 0;
-	replSlotStats[i].stream_count = 0;
-	replSlotStats[i].stream_bytes = 0;
-	replSlotStats[i].total_txns = 0;
-	replSlotStats[i].total_bytes = 0;
-	replSlotStats[i].stat_reset_timestamp = ts;
+	slotent->spill_txns = 0;
+	slotent->spill_count = 0;
+	slotent->spill_bytes = 0;
+	slotent->stream_txns = 0;
+	slotent->stream_count = 0;
+	slotent->stream_bytes = 0;
+	slotent->total_txns = 0;
+	slotent->total_bytes = 0;
+	slotent->stat_reset_timestamp = ts;
 }
 
 /*

src/backend/replication/logical/logical.c

@@ -1773,7 +1773,7 @@ void
 UpdateDecodingStats(LogicalDecodingContext *ctx)
 {
 	ReorderBuffer *rb = ctx->reorder;
-	PgStat_ReplSlotStats repSlotStat;
+	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)

src/backend/replication/slot.c

@@ -328,12 +328,7 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 	 * ReplicationSlotAllocationLock.
 	 */
 	if (SlotIsLogical(slot))
-	{
-		PgStat_ReplSlotStats repSlotStat;
-		MemSet(&repSlotStat, 0, sizeof(PgStat_ReplSlotStats));
-		namestrcpy(&repSlotStat.slotname, NameStr(slot->data.name));
-		pgstat_report_replslot(&repSlotStat);
-	}
+		pgstat_report_replslot_create(NameStr(slot->data.name));
 
 	/*
 	 * Now that the slot has been marked as in_use and active, it's safe to
@@ -349,17 +344,15 @@ ReplicationSlotCreate(const char *name, bool db_specific,
  * Search for the named replication slot.
  *
  * Return the replication slot if found, otherwise NULL.
- *
- * The caller must hold ReplicationSlotControlLock in shared mode.
  */
 ReplicationSlot *
-SearchNamedReplicationSlot(const char *name)
+SearchNamedReplicationSlot(const char *name, bool need_lock)
 {
 	int			i;
-	ReplicationSlot	*slot = NULL;
+	ReplicationSlot *slot = NULL;
 
-	Assert(LWLockHeldByMeInMode(ReplicationSlotControlLock,
-								LW_SHARED));
+	if (need_lock)
+		LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
 
 	for (i = 0; i < max_replication_slots; i++)
 	{
@@ -372,6 +365,9 @@ SearchNamedReplicationSlot(const char *name)
 		}
 	}
 
+	if (need_lock)
+		LWLockRelease(ReplicationSlotControlLock);
+
 	return slot;
 }
 
@@ -416,7 +412,7 @@ retry:
 	 * Search for the slot with the specified name if the slot to acquire is
 	 * not given. If the slot is not found, we either return -1 or error out.
 	 */
-	s = slot ? slot : SearchNamedReplicationSlot(name);
+	s = slot ? slot : SearchNamedReplicationSlot(name, false);
 	if (s == NULL || !s->in_use)
 	{
 		LWLockRelease(ReplicationSlotControlLock);
@@ -713,6 +709,12 @@ ReplicationSlotDropPtr(ReplicationSlot *slot)
 	 * reduce that possibility. If the messages reached in reverse, we would
 	 * lose one statistics update message. But the next update message will
 	 * create the statistics for the replication slot.
+	 *
+	 * XXX In case, the messages for creation and drop slot of the same name
+	 * get lost and create happens before (auto)vacuum cleans up the dead
+	 * slot, the stats will be accumulated into the old slot. One can imagine
+	 * having OIDs for each slot to avoid the accumulation of stats but that
+	 * doesn't seem worth doing as in practice this won't happen frequently.
 	 */
 	if (SlotIsLogical(slot))
 		pgstat_report_replslot_drop(NameStr(slot->data.name));

src/backend/utils/adt/pgstatfuncs.c

@@ -24,6 +24,7 @@
 #include "pgstat.h"
 #include "postmaster/bgworker_internals.h"
 #include "postmaster/postmaster.h"
+#include "replication/slot.h"
 #include "storage/proc.h"
 #include "storage/procarray.h"
 #include "utils/acl.h"
@@ -2207,8 +2208,33 @@ pg_stat_reset_replication_slot(PG_FUNCTION_ARGS)
 	char	   *target = NULL;
 
 	if (!PG_ARGISNULL(0))
+	{
+		ReplicationSlot *slot;
+
 		target = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+		/*
+		 * Check if the slot exists with the given name. It is possible that
+		 * by the time this message is executed the slot is dropped but at
+		 * least this check will ensure that the given name is for a valid
+		 * slot.
+		 */
+		slot = SearchNamedReplicationSlot(target, true);
+
+		if (!slot)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("replication slot \"%s\" does not exist",
+							target)));
+
+		/*
+		 * Nothing to do for physical slots as we collect stats only for
+		 * logical slots.
+		 */
+		if (SlotIsPhysical(slot))
+			PG_RETURN_VOID();
+	}
+
 	pgstat_reset_replslot_counter(target);
 
 	PG_RETURN_VOID();
@@ -2280,73 +2306,77 @@ pg_stat_get_archiver(PG_FUNCTION_ARGS)
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
-/* Get the statistics for the replication slots */
+/*
+ * Get the statistics for the replication slot. If the slot statistics is not
+ * available, return all-zeroes stats.
+ */
 Datum
-pg_stat_get_replication_slots(PG_FUNCTION_ARGS)
+pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 {
 #define PG_STAT_GET_REPLICATION_SLOT_COLS 10
-	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+	text	   *slotname_text = PG_GETARG_TEXT_P(0);
+	NameData	slotname;
 	TupleDesc	tupdesc;
-	Tuplestorestate *tupstore;
-	MemoryContext per_query_ctx;
-	MemoryContext oldcontext;
-	PgStat_ReplSlotStats *slotstats;
-	int			nstats;
-	int			i;
+	Datum		values[10];
+	bool		nulls[10];
+	PgStat_StatReplSlotEntry *slotent;
+	PgStat_StatReplSlotEntry allzero;
 
-	/* check to see if caller supports us returning a tuplestore */
-	if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
-		ereport(ERROR,
-				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-				 errmsg("set-valued function called in context that cannot accept a set")));
-	if (!(rsinfo->allowedModes & SFRM_Materialize))
-		ereport(ERROR,
-				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-				 errmsg("materialize mode required, but it is not allowed in this context")));
-
-	/* Build a tuple descriptor for our result type */
-	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
-		elog(ERROR, "return type must be a row type");
-
-	per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
-	oldcontext = MemoryContextSwitchTo(per_query_ctx);
-
-	tupstore = tuplestore_begin_heap(true, false, work_mem);
-	rsinfo->returnMode = SFRM_Materialize;
-	rsinfo->setResult = tupstore;
-	rsinfo->setDesc = tupdesc;
+	/* Initialise values and NULL flags arrays */
+	MemSet(values, 0, sizeof(values));
+	MemSet(nulls, 0, sizeof(nulls));
 
-	MemoryContextSwitchTo(oldcontext);
+	/* Initialise attributes information in the tuple descriptor */
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_REPLICATION_SLOT_COLS);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "slot_name",
+					   TEXTOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "spill_txns",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "spill_count",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 4, "spill_bytes",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 5, "stream_txns",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 6, "stream_count",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 7, "stream_bytes",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 8, "total_txns",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 9, "total_bytes",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "stats_reset",
+					   TIMESTAMPTZOID, -1, 0);
+	BlessTupleDesc(tupdesc);
 
-	slotstats = pgstat_fetch_replslot(&nstats);
-	for (i = 0; i < nstats; i++)
+	namestrcpy(&slotname, text_to_cstring(slotname_text));
+	slotent = pgstat_fetch_replslot(slotname);
+	if (!slotent)
 	{
-		Datum		values[PG_STAT_GET_REPLICATION_SLOT_COLS];
-		bool		nulls[PG_STAT_GET_REPLICATION_SLOT_COLS];
-		PgStat_ReplSlotStats *s = &(slotstats[i]);
-
-		MemSet(values, 0, sizeof(values));
-		MemSet(nulls, 0, sizeof(nulls));
-
-		values[0] = CStringGetTextDatum(NameStr(s->slotname));
-		values[1] = Int64GetDatum(s->spill_txns);
-		values[2] = Int64GetDatum(s->spill_count);
-		values[3] = Int64GetDatum(s->spill_bytes);
-		values[4] = Int64GetDatum(s->stream_txns);
-		values[5] = Int64GetDatum(s->stream_count);
-		values[6] = Int64GetDatum(s->stream_bytes);
-		values[7] = Int64GetDatum(s->total_txns);
-		values[8] = Int64GetDatum(s->total_bytes);
-
-		if (s->stat_reset_timestamp == 0)
-			nulls[9] = true;
-		else
-			values[9] = TimestampTzGetDatum(s->stat_reset_timestamp);
-
-		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
+		/*
+		 * If the slot is not found, initialise its stats. This is possible if
+		 * the create slot message is lost.
+		 */
+		memset(&allzero, 0, sizeof(PgStat_StatReplSlotEntry));
+		slotent = &allzero;
 	}
 
-	tuplestore_donestoring(tupstore);
+	values[0] = CStringGetTextDatum(NameStr(slotname));
+	values[1] = Int64GetDatum(slotent->spill_txns);
+	values[2] = Int64GetDatum(slotent->spill_count);
+	values[3] = Int64GetDatum(slotent->spill_bytes);
+	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);
 
-	return (Datum) 0;
+	if (slotent->stat_reset_timestamp == 0)
+		nulls[9] = true;
+	else
+		values[9] = TimestampTzGetDatum(slotent->stat_reset_timestamp);
+
+	/* Returns the record as Datum */
+	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202104231
+#define CATALOG_VERSION_NO	202104271
 
 #endif

src/include/catalog/pg_proc.dat

@@ -5308,14 +5308,14 @@
   proargmodes => '{o,o,o,o,o,o,o,o,o,o,o,o,o,o,o}',
   proargnames => '{pid,status,receive_start_lsn,receive_start_tli,written_lsn,flushed_lsn,received_tli,last_msg_send_time,last_msg_receipt_time,latest_end_lsn,latest_end_time,slot_name,sender_host,sender_port,conninfo}',
   prosrc => 'pg_stat_get_wal_receiver' },
-{ oid => '8595', descr => 'statistics: information about replication slots',
-  proname => 'pg_stat_get_replication_slots', prorows => '10',
+{ oid => '8595', descr => 'statistics: information about replication slot',
+  proname => 'pg_stat_get_replication_slot', prorows => '1',
   proisstrict => 'f', proretset => 't', provolatile => 's', proparallel => 'r',
-  prorettype => 'record', proargtypes => '',
-  proallargtypes => '{text,int8,int8,int8,int8,int8,int8,int8,int8,timestamptz}',
-  proargmodes => '{o,o,o,o,o,o,o,o,o,o}',
-  proargnames => '{slot_name,spill_txns,spill_count,spill_bytes,stream_txns,stream_count,stream_bytes,total_txns,total_bytes,stats_reset}',
-  prosrc => 'pg_stat_get_replication_slots' },
+  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}',
+  prosrc => 'pg_stat_get_replication_slot' },
 { oid => '6118', descr => 'statistics: information about subscription',
   proname => 'pg_stat_get_subscription', prorows => '10', proisstrict => 'f',
   proretset => 't', provolatile => 's', proparallel => 'r',

src/include/pgstat.h

@@ -541,6 +541,7 @@ typedef struct PgStat_MsgReplSlot
 {
 	PgStat_MsgHdr m_hdr;
 	NameData	m_slotname;
+	bool		m_create;
 	bool		m_drop;
 	PgStat_Counter m_spill_txns;
 	PgStat_Counter m_spill_count;
@@ -917,7 +918,7 @@ typedef struct PgStat_SLRUStats
 /*
  * Replication slot statistics kept in the stats collector
  */
-typedef struct PgStat_ReplSlotStats
+typedef struct PgStat_StatReplSlotEntry
 {
 	NameData	slotname;
 	PgStat_Counter spill_txns;
@@ -929,7 +930,7 @@ typedef struct PgStat_ReplSlotStats
 	PgStat_Counter total_txns;
 	PgStat_Counter total_bytes;
 	TimestampTz stat_reset_timestamp;
-} PgStat_ReplSlotStats;
+} PgStat_StatReplSlotEntry;
 
 
 /*
@@ -1031,7 +1032,8 @@ extern void pgstat_report_recovery_conflict(int reason);
 extern void pgstat_report_deadlock(void);
 extern void pgstat_report_checksum_failures_in_db(Oid dboid, int failurecount);
 extern void pgstat_report_checksum_failure(void);
-extern void pgstat_report_replslot(const PgStat_ReplSlotStats *repSlotStat);
+extern void pgstat_report_replslot(const PgStat_StatReplSlotEntry *repSlotStat);
+extern void pgstat_report_replslot_create(const char *slotname);
 extern void pgstat_report_replslot_drop(const char *slotname);
 
 extern void pgstat_initialize(void);
@@ -1129,7 +1131,7 @@ extern PgStat_ArchiverStats *pgstat_fetch_stat_archiver(void);
 extern PgStat_GlobalStats *pgstat_fetch_global(void);
 extern PgStat_WalStats *pgstat_fetch_stat_wal(void);
 extern PgStat_SLRUStats *pgstat_fetch_slru(void);
-extern PgStat_ReplSlotStats *pgstat_fetch_replslot(int *nslots_p);
+extern PgStat_StatReplSlotEntry *pgstat_fetch_replslot(NameData slotname);
 extern PgStat_RecoveryPrefetchStats *pgstat_fetch_recoveryprefetch(void);
 
 extern void pgstat_count_slru_page_zeroed(int slru_idx);

src/include/replication/slot.h

@@ -223,7 +223,7 @@ extern XLogRecPtr ReplicationSlotsComputeLogicalRestartLSN(void);
 extern bool ReplicationSlotsCountDBSlots(Oid dboid, int *nslots, int *nactive);
 extern void ReplicationSlotsDropDBSlots(Oid dboid);
 extern void InvalidateObsoleteReplicationSlots(XLogSegNo oldestSegno);
-extern ReplicationSlot *SearchNamedReplicationSlot(const char *name);
+extern ReplicationSlot *SearchNamedReplicationSlot(const char *name, bool need_lock);
 extern void ReplicationSlotNameForTablesync(Oid suboid, Oid relid, char *syncslotname, int szslot);
 extern void ReplicationSlotDropAtPubNode(WalReceiverConn *wrconn, char *slotname, bool missing_ok);
 

src/test/regress/expected/rules.out

@@ -2071,7 +2071,9 @@ pg_stat_replication_slots| SELECT s.slot_name,
     s.total_txns,
     s.total_bytes,
     s.stats_reset
-   FROM pg_stat_get_replication_slots() s(slot_name, spill_txns, spill_count, spill_bytes, stream_txns, stream_count, stream_bytes, total_txns, total_bytes, 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)
+  WHERE (r.datoid IS NOT NULL);
 pg_stat_slru| SELECT s.name,
     s.blks_zeroed,
     s.blks_hit,

src/tools/pgindent/typedefs.list

@@ -1870,12 +1870,12 @@ PgStat_MsgTabstat
 PgStat_MsgTempFile
 PgStat_MsgVacuum
 PgStat_MsgWal
-PgStat_ReplSlotStats
 PgStat_SLRUStats
 PgStat_Shared_Reset_Target
 PgStat_Single_Reset_Type
 PgStat_StatDBEntry
 PgStat_StatFuncEntry
+PgStat_StatReplSlotEntry
 PgStat_StatTabEntry
 PgStat_SubXactStatus
 PgStat_TableCounts