pg_stat_get_backend_subxact() and backend IDs?

Hi

I was playing around with "pg_stat_get_backend_subxact()" (commit 10ea0f924)
and see it emits NULL values for some backends, e.g.:

postgres=# \pset null NULL
Null display is "NULL".

postgres=# SELECT id, pg_stat_get_backend_pid(id), s.*,
pg_stat_get_backend_activity (id)
FROM pg_stat_get_backend_idset() id
JOIN LATERAL pg_stat_get_backend_subxact(id) AS s ON TRUE;
id | pg_stat_get_backend_pid | subxact_count |
subxact_overflowed | pg_stat_get_backend_activity
-----+-------------------------+---------------+--------------------+------------------------------------------------------------
1 | 3175972 | 0 | f
| <command string not enabled>
2 | 3175973 | 0 | f
| <command string not enabled>
3 | 3177889 | 0 | f
| SELECT id, pg_stat_get_backend_pid(id), s.*, +
| | |
| pg_stat_get_backend_activity (id) +
| | |
| FROM pg_stat_get_backend_idset() id +
| | |
| JOIN LATERAL pg_stat_get_backend_subxact(id) AS s ON TRUE;
4 | 3176027 | 5 | f
| savepoint s4;
256 | 3175969 | NULL | NULL
| <command string not enabled>
258 | 3175968 | NULL | NULL
| <command string not enabled>
259 | 3175971 | NULL | NULL
| <command string not enabled>
(7 rows)

Reading through the thread [1], it looks like 0/false are intended to be
returned for non-backend processes too [2], so it seems odd that NULL/NULL is
getting returned in some cases, especially as that's what's returned if a
non-existent backend ID is provided.

[1] https://www.postgresql.org/message-id/flat/CAFiTN-uvYAofNRaGF4R%2Bu6_OrABdkqNRoX7V6%2BPP3H_0HuYMwg%40mail.gmail.com
[2] https://www.postgresql.org/message-id/flat/CAFiTN-ut0uwkRJDQJeDPXpVyTWD46m3gt3JDToE02hTfONEN%3DQ%40mail.gmail.com#821f6f40e91314066390efd06d71d5ac

Looking at the code, this is happening because
"pgstat_fetch_stat_local_beentry()"
expects to be passed the backend ID as an integer representing a 1-based index
referring to "localBackendStatusTable", but "pg_stat_get_backend_subxact()"
is presumably intended to take the actual BackendId , as per other
"pg_stat_get_XXX()"
functions.

Also, the comment for "pgstat_fetch_stat_local_beentry()" says:

Returns NULL if the argument is out of range (no current caller does that).

so the last part is currently incorrect.

Assuming I am not misunderstanding something here (always a
possibility, apologies
in advance if this is merely noise), what is actually needed is a function which
accepts a BackendId (as per "pgstat_fetch_stat_beentry()"), but returns a
LocalPgBackendStatus (as per "pgstat_fetch_stat_local_beentry()") like the
attached, clumsily named "pgstat_fetch_stat_backend_local_beentry()".

Regards

Ian Barwick