Re: Fix pg_stat_get_backend_wait_event() for aux processes

> On Feb 2, 2026, at 22:38, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>
> pg_stat_get_backend_wait_event() and pg_stat_get_backend_wait_event_type() functions don't work for aux processes:
>
>> postgres=# select pid, backend_type, wait_event, wait_event_type from pg_stat_activity ;
>> pid | backend_type | wait_event | wait_event_type ---------+------------------------------+---------------------+-----------------
>> 3665058 | client backend | | 3665051 | autovacuum launcher | AutovacuumMain | Activity
>> 3665052 | logical replication launcher | LogicalLauncherMain | Activity
>> 3665044 | io worker | IoWorkerMain | Activity
>> 3665045 | io worker | IoWorkerMain | Activity
>> 3665046 | io worker | IoWorkerMain | Activity
>> 3665047 | checkpointer | CheckpointerMain | Activity
>> 3665048 | background writer | BgwriterMain | Activity
>> 3665050 | walwriter | WalWriterMain | Activity
>> (9 rows)
>> postgres=# SELECT pg_stat_get_backend_pid(backendid) AS pid,
>> pg_stat_get_backend_wait_event_type(backendid) as wait_event_type,
>> pg_stat_get_backend_wait_event(backendid) as wait_event
>> FROM pg_stat_get_backend_idset() AS backendid;
>> pid | wait_event_type | wait_event ---------+-----------------+---------------------
>> 3665058 | | 3665051 | Activity | AutovacuumMain
>> 3665052 | Activity | LogicalLauncherMain
>> 3665044 | | 3665045 | | 3665046 | | 3665047 | | 3665048 | | 3665050 | | (9 rows)
>
> We added aux processes to pg_stat_activity in commit fc70a4b0df, but apparently forgot to do the same for those functions.
>
> With the attached fix:
>
>> postgres=# SELECT pg_stat_get_backend_pid(backendid) AS pid,
>> pg_stat_get_backend_wait_event_type(backendid) as wait_event_type,
>> pg_stat_get_backend_wait_event(backendid) as wait_event
>> FROM pg_stat_get_backend_idset() AS backendid;
>> pid | wait_event_type | wait_event ---------+-----------------+---------------------
>> 3667552 | | 3667545 | Activity | AutovacuumMain
>> 3667546 | Activity | LogicalLauncherMain
>> 3667538 | Activity | IoWorkerMain
>> 3667539 | Activity | IoWorkerMain
>> 3667540 | Activity | IoWorkerMain
>> 3667541 | Activity | CheckpointerMain
>> 3667542 | Activity | BgwriterMain
>> 3667544 | Activity | WalWriterMain
>> (9 rows)
>
> While looking at this, I noticed that pg_stat_activity has a "backend_type" field, but there's no corresponding "pg_stat_get_backend_type(backend_id)" function, similar to "pg_stat_get_backend_wait_event(backend_id)" et al. I wonder if that was on purpose, or we just forgot to add it when we added it to pg_stat_activity?
>
> Another thing I didn't do in this patch yet: I feel we should replace BackendPidGetProc() with a function like "PGPROC *PidGetPGProc(pid_t)", that would work for backends and aux processes alike. It's a common pattern to call BackendPidGetProc() followed by AuxiliaryPidGetProc() currently. Even for the callers that specifically want to only check backend processes, I think it would be more natural to call PidGetPGProc(), and then check the process type.
>
> - Heikki
> <0001-Fix-pg_stat_get_backend_wait_event-for-aux-processes.patch>

Hi Heikki,

I reviewed and tested the patch, it works well, and the code change looks solid to me.

I only have one small comment. In the following case:
```
if ((beentry = pgstat_get_beentry_by_proc_number(procNumber)) == NULL)
wait_event_type = "<backend information not available>";
```

With this patch, aux processes are now supported as well. Do we want to update this message?

For example, in my test system max_connections = 100, so procNumber >= 100 corresponds to aux processes. If I run:
```
evantest=# select pg_stat_get_backend_wait_event(188);
pg_stat_get_backend_wait_event
-------------------------------------
<backend information not available>
(1 row)
```

Here 188 refers to an aux process, but the message still says “backend information”, which feels a bit misleading. Would it make sense to change this to something like “process information not available”?

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/