BufferPin

Mechanism

WAIT_EVENT_BUFFER_PIN at src/backend/storage/buffer/bufmgr.c:5819 is the grep-verified reporting path. The public identity/resource is mapped at src/backend/utils/activity/wait_event_names.txt:286. The instrumented operation is: Waiting to acquire an exclusive pin on a buffer. The buffer manager needs an exclusive pin, but another backend still holds a pin on the same shared buffer. It sleeps until the last conflicting pin is released.

Expected behavior

An isolated sub-second sample can occur during scans or index maintenance.

When to investigate

Treat a wait beyond one second as suspicious, especially when a cursor or idle transaction may keep the page pinned.

Diagnostic actions

• Preserve the waiting query and relation context.
• Find long cursors and idle-in-transaction sessions touching the same relation.
• Cancel only after identifying the safest holder or maintenance victim.

Diagnostic queries

Sessions waiting on BufferPin/BufferPin

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 = 'BufferPin'
  AND wait_event = 'BufferPin'
ORDER BY query_age DESC NULLS LAST;

Current BufferPin 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 = 'BufferPin'
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 = 'BufferPin'
  AND a.wait_event = 'BufferPin'
ORDER BY a.pid, l.granted, l.locktype, l.mode;