Display the leader apply worker's PID for parallel apply workers.

Add leader_pid to pg_stat_subscription. leader_pid is the process ID of
the leader apply worker if this process is a parallel apply worker. If
this field is NULL, it indicates that the process is a leader apply
worker or a synchronization worker. The new column makes it easier to
distinguish parallel apply workers from other kinds of workers and helps
to identify the leader for the parallel workers corresponding to a
particular subscription.

Additionally, update the leader_pid column in pg_stat_activity as well to
display the PID of the leader apply worker for parallel apply workers.

Author: Hou Zhijie
Reviewed-by: Peter Smith, Sawada Masahiko, Amit Kapila, Shveta Mallik
Discussion: https://postgr.es/m/CAA4eK1+wyN6zpaHUkCLorEWNx75MG0xhMwcFhvjqm2KURZEAGw@mail.gmail.com

2 files changed, 28 insertions, 11 deletions

diff --git a/doc/src/sgml/logical-replication.sgml b/doc/src/sgml/logical-replication.sgml
index 54f48be87f3..f4b4e641beb 100644
--- a/doc/src/sgml/logical-replication.sgml
+++ b/doc/src/sgml/logical-replication.sgml
@@ -1692,7 +1692,8 @@ CONTEXT:  processing remote data for replication origin "pg_16395" during "INSER
    subscription.  A disabled subscription or a crashed subscription will have
    zero rows in this view.  If the initial data synchronization of any
    table is in progress, there will be additional workers for the tables
-   being synchronized.
+   being synchronized. Moreover, if the streaming transaction is applied in
+   parallel, there may be additional parallel apply workers.
   </para>
  </sect1>
 
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 358d2ff90f8..e3a783abd0f 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -743,9 +743,11 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
        <structfield>leader_pid</structfield> <type>integer</type>
       </para>
       <para>
-       Process ID of the parallel group leader, if this process is a
-       parallel query worker.  <literal>NULL</literal> if this process is a
-       parallel group leader or does not participate in parallel query.
+       Process ID of the parallel group leader if this process is a parallel
+       query worker, or process ID of the leader apply worker if this process
+       is a parallel apply worker.  <literal>NULL</literal> indicates that this
+       process is a parallel group leader or leader apply worker, or does not
+       participate in any parallel operation.
       </para></entry>
      </row>
 
@@ -3208,11 +3210,22 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
 
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>leader_pid</structfield> <type>integer</type>
+      </para>
+      <para>
+       Process ID of the leader apply worker if this process is a parallel
+       apply worker; NULL if this process is a leader apply worker or a
+       synchronization worker
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>relid</structfield> <type>oid</type>
       </para>
       <para>
-       OID of the relation that the worker is synchronizing; null for the
-       main apply worker
+       OID of the relation that the worker is synchronizing; NULL for the
+       leader apply worker and parallel apply workers
       </para></entry>
      </row>
 
@@ -3222,7 +3235,7 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
       </para>
       <para>
        Last write-ahead log location received, the initial value of
-       this field being 0
+       this field being 0; NULL for parallel apply workers
       </para></entry>
      </row>
 
@@ -3231,7 +3244,8 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
        <structfield>last_msg_send_time</structfield> <type>timestamp with time zone</type>
       </para>
       <para>
-       Send time of last message received from origin WAL sender
+       Send time of last message received from origin WAL sender; NULL for
+       parallel apply workers
       </para></entry>
      </row>
 
@@ -3240,7 +3254,8 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
        <structfield>last_msg_receipt_time</structfield> <type>timestamp with time zone</type>
       </para>
       <para>
-       Receipt time of last message received from origin WAL sender
+       Receipt time of last message received from origin WAL sender; NULL for
+       parallel apply workers
       </para></entry>
      </row>
 
@@ -3249,7 +3264,8 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
        <structfield>latest_end_lsn</structfield> <type>pg_lsn</type>
       </para>
       <para>
-       Last write-ahead log location reported to origin WAL sender
+       Last write-ahead log location reported to origin WAL sender; NULL for
+       parallel apply workers
       </para></entry>
      </row>
 
@@ -3259,7 +3275,7 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
       </para>
       <para>
        Time of last write-ahead log location reported to origin WAL
-       sender
+       sender; NULL for parallel apply workers
       </para></entry>
      </row>
     </tbody>