From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Subject: [PATCH v1] Report relation extension blockers within parallel lock groups in pg_blocking_pids()
Date: Sat, 29 Aug 2026 04:24:42 +0000
Commit 85f6b49c2c53 made relation extension locks conflict between members of
the same parallel lock group, but did not update the same group filtering in
pg_blocking_pids(). So, pg_blocking_pids() can omit the process that is actually
blocking a relation extension request.
Add the relation extension exception in pg_blocking_pids().
pg_blocking_pids() reports parallel workers using their lock group leader PID.
Therefore, when one member of a parallel lock group blocks another, the PID
supplied to pg_blocking_pids() can appear in the result. This does not mean
that a process blocks itself. Rather, a lock held by one member of its parallel
lock group blocks a lock request made by another member of that group. The
commit also documents this behavior.
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by:
Discussion: https://postgr.es/m/...
---
doc/src/sgml/func/func-info.sgml | 14 +++++++++++---
src/backend/utils/adt/lockfuncs.c | 10 ++++++++--
2 files changed, 19 insertions(+), 5 deletions(-)
68.7% doc/src/sgml/func/
31.2% src/backend/utils/adt/
inline patch 1: doc/src/sgml/func/func-info.sgml, src/backend/utils/adt/lockfuncs.c (+19/-5 lines) download inline-1.patchdiff --git a/doc/src/sgml/func/func-info.sgml b/doc/src/sgml/func/func-info.sgmlindex 2f03766b67a..226a815916d 100644--- a/doc/src/sgml/func/func-info.sgml+++ b/doc/src/sgml/func/func-info.sgml@@ -246,9 +246,17 @@
request and is ahead of it in the wait queue (soft block). When using
parallel queries the result always lists client-visible process IDs
(that is, <function>pg_backend_pid</function> results) even if the
- actual lock is held or awaited by a child worker process. As a result- of that, there may be duplicated PIDs in the result. Also note that- when a prepared transaction holds a conflicting lock, it will be+ actual lock is held or awaited by a child worker process. As a result+ of that, there may be duplicated PIDs in the result. When members of+ the same parallel lock group block each other on a relation extension+ lock (shown as lock type <literal>extend</literal> in <link+ linkend="view-pg-locks"><structname>pg_locks</structname></link>), the+ specified process ID can also appear in the result. This does not mean+ that a process blocks itself. Rather, a lock held by one member of its+ parallel lock group blocks a lock request made by another member of that+ group.+ Also note that when a prepared transaction holds a conflicting lock,+ it will be
represented by a zero process ID.
</para>
<para>
diff --git a/src/backend/utils/adt/lockfuncs.c b/src/backend/utils/adt/lockfuncs.cindex 3eb9a5b4215..1f0cc2b9bcb 100644--- a/src/backend/utils/adt/lockfuncs.c+++ b/src/backend/utils/adt/lockfuncs.c@@ -518,8 +518,14 @@ pg_blocking_pids(PG_FUNCTION_ARGS)
/* A proc never blocks itself, so ignore that entry */
if (instance == blocked_instance)
continue;
- /* Members of same lock group never block each other, either */- if (instance->leaderPid == blocked_instance->leaderPid)++ /*+ * Members of the same lock group do not block each other, except+ * when extending a relation.+ */+ if (instance->leaderPid == blocked_instance->leaderPid &&+ blocked_instance->locktag.locktag_type !=+ LOCKTAG_RELATION_EXTEND)
continue;
if (conflictMask & instance->holdMask)
--
2.34.1
--DQnhTvUE7k3IbPB4--
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Cc: bertranddrouvot.pg@gmail.com
Subject: Re: [PATCH v1] Report relation extension blockers within parallel lock groups in pg_blocking_pids()
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