Subject: [PATCH v3 2/3] Adding XID generation count per backend

From: Bertrand Drouvot <[email-masked]>
Date: Fri, 8 Aug 2025 15:58:05 +0000

This commit adds a new counter to record the number of XIDs generated per
backend. It will help to detect if a backend is consuming XIDs at a high rate.

Virtual transactions are not taken into account on purpose, we do want to track
only the XID where there is a risk of wraparound.

The counter is not part of PgStat_BackendPending, because we want to avoid
an extra function call in this code path to increment the counter in
PendingBackendStats. The counter increment here behaves more or less the same as
we do for WAL statistics.
---
 src/backend/access/transam/varsup.c         | 2 ++
 src/backend/utils/activity/pgstat_backend.c | 6 +++++-
 src/include/pgstat.h                        | 3 +++
 3 files changed, 10 insertions(+), 1 deletion(-)
  10.1% src/backend/access/transam/
  69.3% src/backend/utils/activity/
  20.5% src/include/

diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c
index fe895787cb7..26c72cbe091 100644
--- a/src/backend/access/transam/varsup.c
+++ b/src/backend/access/transam/varsup.c
@@ -21,6 +21,7 @@
 #include "access/xlogutils.h"
 #include "commands/dbcommands.h"
 #include "miscadmin.h"
+#include "pgstat.h"
 #include "postmaster/autovacuum.h"
 #include "storage/pmsignal.h"
 #include "storage/proc.h"
@@ -257,6 +258,7 @@ GetNewTransactionId(bool isSubXact)
 		/* LWLockRelease acts as barrier */
 		MyProc->xid = xid;
 		ProcGlobal->xids[MyProc->pgxactoff] = xid;
+		XidGenCount++;
 	}
 	else
 	{
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index bf164854c4b..8b530c510b9 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -52,6 +52,8 @@ static WalUsage prevBackendWalUsage;
  */
 static bool backend_has_xactstats = false;
 
+PgStat_Counter XidGenCount = 0;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -285,9 +287,11 @@ pgstat_flush_backend_entry_xact(PgStat_EntryRef *entry_ref)
 
 	shbackendent->stats.xact_commit += PendingBackendStats.pending_xact_commit;
 	shbackendent->stats.xact_rollback += PendingBackendStats.pending_xact_rollback;
+	shbackendent->stats.xid_count += XidGenCount;
 
 	PendingBackendStats.pending_xact_commit = 0;
 	PendingBackendStats.pending_xact_rollback = 0;
+	XidGenCount = 0;
 
 	backend_has_xactstats = false;
 }
@@ -317,7 +321,7 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 		has_pending_data = true;
 
 	/* Some transaction data pending? */
-	if ((flags & PGSTAT_BACKEND_FLUSH_XACT) && backend_has_xactstats)
+	if ((flags & PGSTAT_BACKEND_FLUSH_XACT) && (backend_has_xactstats || XidGenCount > 0))
 		has_pending_data = true;
 
 	if (!has_pending_data)
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 9efc0e10ebf..94dd24b3eac 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -492,6 +492,7 @@ typedef struct PgStat_Backend
 	PgStat_WalCounters wal_counters;
 	PgStat_Counter xact_commit;
 	PgStat_Counter xact_rollback;
+	PgStat_Counter xid_count;
 } PgStat_Backend;
 
 /* ---------
@@ -571,6 +572,8 @@ extern PgStat_Backend *pgstat_fetch_stat_backend_by_pid(int pid,
 extern bool pgstat_tracks_backend_bktype(BackendType bktype);
 extern void pgstat_create_backend(ProcNumber procnum);
 
+extern PGDLLIMPORT PgStat_Counter XidGenCount;
+
 /*
  * Functions in pgstat_bgwriter.c
  */
-- 
2.34.1

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