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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/

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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  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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