Обсуждение: [BeginnerQuestion]Why these is 6 rows in my SELECT statement?

2022年9月8日(木) 17:37 BeginnerC <chuxuec@outlook.com>:
>
> Hello community,
> I am checking my pg_stat_activity view,but something confused me.
> Just like this:
>
> postgres=# SELECT wait_event_type, wait_event FROM pg_stat_activity;
>  wait_event_type |     wait_event
> -----------------+---------------------
>  Activity        | AutoVacuumMain
>  Activity        | LogicalLauncherMain
>                  |
>  Activity        | BgWriterHibernate
>  Activity        | CheckpointerMain
>  Activity        | WalWriterMain
> (6 rows)
>
> As you can see, the third row is empty,but I don't know why.
> Can anyone provide some solution to me?

It's empty because that's your own client session and it's not waiting
on anything :).

     postgres=# SELECT pg_backend_pid();
     pg_backend_pid
    ----------------
              24941
    (1 row)

   postgres=# SELECT pid, backend_type, wait_event_type, wait_event
FROM pg_stat_activity;
      pid  |         backend_type         | wait_event_type |
wait_event
    -------+------------------------------+-----------------+---------------------
     20859 | autovacuum launcher          | Activity        | AutoVacuumMain
     20860 | logical replication launcher | Activity        |
LogicalLauncherMain
     21554 | walsender                    | Activity        | WalSenderMain
     24941 | client backend               |                 |
     20856 | background writer            | Activity        | BgWriterHibernate
     20855 | checkpointer                 | Activity        | CheckpointerMain
     20858 | walwriter                    | Activity        | WalWriterMain
    (7 rows)

Regards

Ian Barwick

On Thu, Sep 08, 2022 at 04:37:00PM +0800, BeginnerC wrote:
> Hello community,
> I am checking my pg_stat_activity view,but something confused me.
> Just like this:
> 
> postgres=# SELECT wait_event_type, wait_event FROM pg_stat_activity;
> wait_event_type |     wait_event
> -----------------+---------------------
> Activity        | AutoVacuumMain
> Activity        | LogicalLauncherMain
>                 |
> Activity        | BgWriterHibernate
> Activity        | CheckpointerMain
> Activity        | WalWriterMain
> (6 rows)
> 
> As you can see, the third row is empty,but I don't know why.
> Can anyone provide some solution to me?
> Thanks in advance!

Please do:

\pset null '-- null --'
SELECT wait_event_type, wait_event FROM pg_stat_activity;

it will most likely show -- null -- in these fields. which means values
in these columns are null.SELECT wait_event_type, wait_event FROM
pg_stat_activity;

You can find more about this/these backends by doing:

select * from pg_stat_activity where wait_event_type is null \gx

depesz