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* [PATCH v1] Report relation extension blockers within parallel lock groups in pg_blocking_pids()
@ 2026-08-29 04:24 Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
0 siblings, 0 replies; 1+ messages in thread
From: Bertrand Drouvot @ 2026-08-29 04:24 UTC (permalink / raw)
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.patch
diff --git a/doc/src/sgml/func/func-info.sgml b/doc/src/sgml/func/func-info.sgml
index 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.c
index 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
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2026-08-29 04:24 [PATCH v1] Report relation extension blockers within parallel lock groups in pg_blocking_pids() Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
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