diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 1cf71f8903a..bb36e803e19 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -22011,13 +22011,20 @@ SELECT * FROM pg_ls_dir('.') WITH ORDINALITY AS t(ls,n);
         One server process blocks another if it either holds a lock that
         conflicts with the blocked process's lock request (hard block), or is
         waiting for a lock that would conflict with the blocked process's lock
-        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
-        represented by a zero process ID.
+        request and is ahead of it in the wait queue (soft block).  A prepared
+        transaction that holds a conflicting lock is represented by a zero
+        process ID.
+       </para>
+       <para>
+        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.  Note
+        that the processes of a parallel query can block each other on
+        relation extension locks (lock type <literal>extend</literal> in
+        <link linkend="view-pg-locks"><structname>pg_locks</structname></link>),
+        in which case the result contains the blocked session's own process
+        ID.
        </para>
        <para>
         Frequent calls to this function could have some impact on database
diff --git a/src/backend/utils/adt/lockfuncs.c b/src/backend/utils/adt/lockfuncs.c
index 5dc0a5882cf..54355d97088 100644
--- a/src/backend/utils/adt/lockfuncs.c
+++ b/src/backend/utils/adt/lockfuncs.c
@@ -497,8 +497,14 @@ 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 don't block each other, either,
+			 * except on relation extension locks (see LockCheckConflicts()).
+			 */
+			if (instance->leaderPid == blocked_instance->leaderPid &&
+				blocked_instance->locktag.locktag_type !=
+				LOCKTAG_RELATION_EXTEND)
 				continue;
 
 			if (conflictMask & instance->holdMask)