LogicalSubxactWrite

Mechanism

The catalog identity is present, but source audit found no live reporter in 14. Known active ranges: none. The definition location below is retained as negative evidence; this exact release cannot emit the event from a core code path. Evidence: pgsql-hackers confirmation.

Expected behavior

No live core occurrence is expected on the audited release.

When to investigate

If telemetry shows it, verify the exact patch version, extension origin, and whether the sample is stale or came from a different server.

Diagnostic actions

• Confirm the workload phase should touch this file class.
• Correlate with pg_stat_io where available and per-device latency.
• Fix the access path, burst shape, or affected storage tier before tuning unrelated memory settings.

Diagnostic queries

Sessions waiting on IO/LogicalSubxactWrite

SELECT pid, backend_type, usename, datname, application_name,
       state, now() - query_start AS query_age,
       now() - xact_start AS xact_age,
       wait_event_type, wait_event,
       pg_blocking_pids(pid) AS blocking_pids,
       left(query, 160) AS query
FROM pg_stat_activity
WHERE wait_event_type = 'IO'
  AND wait_event = 'LogicalSubxactWrite'
ORDER BY query_age DESC NULLS LAST;

Current IO cohort

SELECT wait_event, count(*) AS waiting_sessions,
       count(*) FILTER (WHERE state = 'active') AS active_waiters,
       max(now() - query_start) AS oldest_query
FROM pg_stat_activity
WHERE wait_event_type = 'IO'
GROUP BY wait_event
ORDER BY waiting_sessions DESC, wait_event;

Lock and relation context for these sessions

SELECT a.pid, l.locktype, l.mode, l.granted, l.fastpath,
       d.datname, n.nspname, c.relname,
       l.page, l.tuple, l.virtualxid, l.transactionid,
       l.classid, l.objid, l.objsubid
FROM pg_stat_activity AS a
LEFT JOIN pg_locks AS l ON l.pid = a.pid
LEFT JOIN pg_database AS d ON d.oid = l.database
LEFT JOIN pg_class AS c
  ON c.oid = l.relation
 AND l.database = (
       SELECT oid FROM pg_database WHERE datname = current_database()
     )
LEFT JOIN pg_namespace AS n ON n.oid = c.relnamespace
WHERE a.wait_event_type = 'IO'
  AND a.wait_event = 'LogicalSubxactWrite'
ORDER BY a.pid, l.granted, l.locktype, l.mode;