Track statistics for streaming of changes from ReorderBuffer.

This adds the statistics about transactions streamed to the decoding
output plugin from ReorderBuffer. Users can query the
pg_stat_replication_slots view to check these stats and call
pg_stat_reset_replication_slot to reset the stats of a particular slot.
Users can pass NULL in pg_stat_reset_replication_slot to reset stats of
all the slots.

Commit 9868167500 has added the basic infrastructure to capture the stats
of slot and this commit extends the statistics collector to track
additional information about slots.

Bump the catversion as we have added new columns in the catalog entry.

Author: Ajin Cherian and Amit Kapila
Reviewed-by: Sawada Masahiko and Dilip Kumar
Discussion: https://postgr.es/m/CAA4eK1+chpEomLzgSoky-D31qev19AmECNiEAietPQUGEFhtVA@mail.gmail.com

doc/src/sgml/monitoring.sgml

@@ -2632,6 +2632,44 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
       </para></entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>stream_txns</structfield> <type>bigint</type>
+       </para>
+       <para>
+        Number of in-progress transactions streamed to the decoding output plugin
+        after the memory used by logical decoding of changes from WAL for this
+        slot exceeds <literal>logical_decoding_work_mem</literal>. Streaming only
+        works with toplevel transactions (subtransactions can't be streamed
+        independently), so the counter does not get incremented for subtransactions.
+       </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>stream_count</structfield><type>bigint</type>
+       </para>
+       <para>
+        Number of times in-progress transactions were streamed to the decoding
+        output plugin while decoding changes from WAL for this slot. Transactions
+        may get streamed repeatedly, and this counter gets incremented on every
+        such invocation.
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>stream_bytes</structfield><type>bigint</type>
+       </para>
+       <para>
+        Amount of decoded in-progress transaction data streamed to the decoding
+        output plugin while decoding changes from WAL for this slot. This and other
+        streaming counters for this slot can be used to gauge the network I/O which
+        occurred during logical decoding and allow tuning <literal>logical_decoding_work_mem</literal>.
+       </para>
+      </entry>
+     </row>
+
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
         <structfield>stats_reset</structfield> <type>timestamp with time zone</type>

src/backend/catalog/system_views.sql

@@ -802,6 +802,9 @@ CREATE VIEW pg_stat_replication_slots AS
             s.spill_txns,
             s.spill_count,
             s.spill_bytes,
+            s.stream_txns,
+            s.stream_count,
+            s.stream_bytes,
             s.stats_reset
     FROM pg_stat_get_replication_slots() AS s;
 

src/backend/postmaster/pgstat.c

@@ -1708,7 +1708,7 @@ pgstat_report_tempfile(size_t filesize)
  */
 void
 pgstat_report_replslot(const char *slotname, int spilltxns, int spillcount,
-					   int spillbytes)
+					   int spillbytes, int streamtxns, int streamcount, int streambytes)
 {
 	PgStat_MsgReplSlot msg;
 
@@ -1721,6 +1721,9 @@ pgstat_report_replslot(const char *slotname, int spilltxns, int spillcount,
 	msg.m_spill_txns = spilltxns;
 	msg.m_spill_count = spillcount;
 	msg.m_spill_bytes = spillbytes;
+	msg.m_stream_txns = streamtxns;
+	msg.m_stream_count = streamcount;
+	msg.m_stream_bytes = streambytes;
 	pgstat_send(&msg, sizeof(PgStat_MsgReplSlot));
 }
 
@@ -6892,6 +6895,9 @@ pgstat_recv_replslot(PgStat_MsgReplSlot *msg, int len)
 		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;
 	}
 }
 
@@ -7125,6 +7131,9 @@ pgstat_reset_replslot(int i, TimestampTz ts)
 	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].stat_reset_timestamp = ts;
 }
 

src/backend/replication/logical/logical.c

@@ -1471,21 +1471,28 @@ UpdateDecodingStats(LogicalDecodingContext *ctx)
 	ReorderBuffer *rb = ctx->reorder;
 
 	/*
-	 * Nothing to do if we haven't spilled anything since the last time the
-	 * stats has been sent.
+	 * Nothing to do if we haven't spilled or streamed anything since the last
+	 * time the stats has been sent.
 	 */
-	if (rb->spillBytes <= 0)
+	if (rb->spillBytes <= 0 && rb->streamBytes <= 0)
 		return;
 
-	elog(DEBUG2, "UpdateDecodingStats: updating stats %p %lld %lld %lld",
+	elog(DEBUG2, "UpdateDecodingStats: updating stats %p %lld %lld %lld %lld %lld %lld",
 		 rb,
 		 (long long) rb->spillTxns,
 		 (long long) rb->spillCount,
-		 (long long) rb->spillBytes);
+		 (long long) rb->spillBytes,
+		 (long long) rb->streamTxns,
+		 (long long) rb->streamCount,
+		 (long long) rb->streamBytes);
 
 	pgstat_report_replslot(NameStr(ctx->slot->data.name),
-						   rb->spillTxns, rb->spillCount, rb->spillBytes);
+						   rb->spillTxns, rb->spillCount, rb->spillBytes,
+						   rb->streamTxns, rb->streamCount, rb->streamBytes);
 	rb->spillTxns = 0;
 	rb->spillCount = 0;
 	rb->spillBytes = 0;
+	rb->streamTxns = 0;
+	rb->streamCount = 0;
+	rb->streamBytes = 0;
 }

src/backend/replication/logical/reorderbuffer.c

@@ -346,6 +346,9 @@ ReorderBufferAllocate(void)
 	buffer->spillTxns = 0;
 	buffer->spillCount = 0;
 	buffer->spillBytes = 0;
+	buffer->streamTxns = 0;
+	buffer->streamCount = 0;
+	buffer->streamBytes = 0;
 
 	buffer->current_restart_decoding_lsn = InvalidXLogRecPtr;
 
@@ -3482,6 +3485,8 @@ ReorderBufferStreamTXN(ReorderBuffer *rb, ReorderBufferTXN *txn)
 {
 	Snapshot	snapshot_now;
 	CommandId	command_id;
+	Size		stream_bytes;
+	bool		txn_is_streamed;
 
 	/* We can never reach here for a subtransaction. */
 	Assert(txn->toptxn == NULL);
@@ -3562,10 +3567,25 @@ ReorderBufferStreamTXN(ReorderBuffer *rb, ReorderBufferTXN *txn)
 		txn->snapshot_now = NULL;
 	}
 
+	/*
+	 * Remember this information to be used later to update stats. We can't
+	 * update the stats here as an error while processing the changes would
+	 * lead to the accumulation of stats even though we haven't streamed all
+	 * the changes.
+	 */
+	txn_is_streamed = rbtxn_is_streamed(txn);
+	stream_bytes = txn->total_size;
+
 	/* Process and send the changes to output plugin. */
 	ReorderBufferProcessTXN(rb, txn, InvalidXLogRecPtr, snapshot_now,
 							command_id, true);
 
+	rb->streamCount += 1;
+	rb->streamBytes += stream_bytes;
+
+	/* Don't consider already streamed transaction. */
+	rb->streamTxns += (txn_is_streamed) ? 0 : 1;
+
 	Assert(dlist_is_empty(&txn->changes));
 	Assert(txn->nentries == 0);
 	Assert(txn->nentries_mem == 0);

src/backend/replication/slot.c

@@ -320,7 +320,7 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 	 * ReplicationSlotAllocationLock.
 	 */
 	if (SlotIsLogical(slot))
-		pgstat_report_replslot(NameStr(slot->data.name), 0, 0, 0);
+		pgstat_report_replslot(NameStr(slot->data.name), 0, 0, 0, 0, 0, 0);
 
 	/*
 	 * Now that the slot has been marked as in_use and active, it's safe to

src/backend/utils/adt/pgstatfuncs.c

@@ -2153,7 +2153,7 @@ pg_stat_get_archiver(PG_FUNCTION_ARGS)
 Datum
 pg_stat_get_replication_slots(PG_FUNCTION_ARGS)
 {
-#define PG_STAT_GET_REPLICATION_SLOT_COLS 5
+#define PG_STAT_GET_REPLICATION_SLOT_COLS 8
 	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
 	TupleDesc	tupdesc;
 	Tuplestorestate *tupstore;
@@ -2201,11 +2201,14 @@ pg_stat_get_replication_slots(PG_FUNCTION_ARGS)
 		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);
 
 		if (s->stat_reset_timestamp == 0)
-			nulls[4] = true;
+			nulls[7] = true;
 		else
-			values[4] = TimestampTzGetDatum(s->stat_reset_timestamp);
+			values[7] = TimestampTzGetDatum(s->stat_reset_timestamp);
 
 		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
 	}

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202010281
+#define CATALOG_VERSION_NO	202010291
 
 #endif

src/include/catalog/pg_proc.dat

@@ -5260,9 +5260,9 @@
   proname => 'pg_stat_get_replication_slots', prorows => '10', proisstrict => 'f',
   proretset => 't', provolatile => 's', proparallel => 'r',
   prorettype => 'record', proargtypes => '',
-  proallargtypes => '{text,int8,int8,int8,timestamptz}',
-  proargmodes => '{o,o,o,o,o}',
-  proargnames => '{slot_name,spill_txns,spill_count,spill_bytes,stats_reset}',
+  proallargtypes => '{text,int8,int8,int8,int8,int8,int8,timestamptz}',
+  proargmodes => '{o,o,o,o,o,o,o,o}',
+  proargnames => '{slot_name,spill_txns,spill_count,spill_bytes,stream_txns,stream_count,stream_bytes,stats_reset}',
   prosrc => 'pg_stat_get_replication_slots' },
 { oid => '6118', descr => 'statistics: information about subscription',
   proname => 'pg_stat_get_subscription', proisstrict => 'f', provolatile => 's',

src/include/pgstat.h

@@ -492,6 +492,9 @@ typedef struct PgStat_MsgReplSlot
 	PgStat_Counter m_spill_txns;
 	PgStat_Counter m_spill_count;
 	PgStat_Counter m_spill_bytes;
+	PgStat_Counter m_stream_txns;
+	PgStat_Counter m_stream_count;
+	PgStat_Counter m_stream_bytes;
 } PgStat_MsgReplSlot;
 
 
@@ -823,6 +826,9 @@ typedef struct PgStat_ReplSlotStats
 	PgStat_Counter spill_txns;
 	PgStat_Counter spill_count;
 	PgStat_Counter spill_bytes;
+	PgStat_Counter stream_txns;
+	PgStat_Counter stream_count;
+	PgStat_Counter stream_bytes;
 	TimestampTz stat_reset_timestamp;
 } PgStat_ReplSlotStats;
 
@@ -1387,7 +1393,7 @@ 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 char *slotname, int spilltxns, int spillcount,
-								   int spillbytes);
+								   int spillbytes, int streamtxns, int streamcount, int streambytes);
 extern void pgstat_report_replslot_drop(const char *slotname);
 
 extern void pgstat_initialize(void);

src/include/replication/reorderbuffer.h

@@ -551,6 +551,11 @@ struct ReorderBuffer
 	int64		spillTxns;		/* number of transactions spilled to disk */
 	int64		spillCount;		/* spill-to-disk invocation counter */
 	int64		spillBytes;		/* amount of data spilled to disk */
+
+	/* Statistics about transactions streamed to the decoding output plugin */
+	int64		streamTxns;		/* number of transactions streamed */
+	int64		streamCount;	/* streaming invocation counter */
+	int64		streamBytes;	/* amount of data streamed */
 };
 
 

src/test/regress/expected/rules.out

@@ -2022,8 +2022,11 @@ pg_stat_replication_slots| SELECT s.slot_name,
     s.spill_txns,
     s.spill_count,
     s.spill_bytes,
+    s.stream_txns,
+    s.stream_count,
+    s.stream_bytes,
     s.stats_reset
-   FROM pg_stat_get_replication_slots() s(slot_name, spill_txns, spill_count, spill_bytes, stats_reset);
+   FROM pg_stat_get_replication_slots() s(slot_name, spill_txns, spill_count, spill_bytes, stream_txns, stream_count, stream_bytes, stats_reset);
 pg_stat_slru| SELECT s.name,
     s.blks_zeroed,
     s.blks_hit,