Re: pgsql: Track last_inactive_time in pg_replication_slots.

On Mon, Mar 25, 2024 at 6:57 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Mon, Mar 25, 2024 at 7:11 AM Amit Kapila <akapila(at)postgresql(dot)org> wrote:
> > Track last_inactive_time in pg_replication_slots.
> >
> > This commit adds a new property called last_inactive_time for slots. It is
> > set to 0 whenever a slot is made active/acquired and set to the current
> > timestamp whenever the slot is inactive/released or restored from the disk.
> > Note that we don't set the last_inactive_time for the slots currently being
> > synced from the primary to the standby because such slots are typically
> > inactive as decoding is not allowed on those.
>
> So the field name is last_inactive_time, but if I'm reading this
> description right, it's actually the last time the slot was active,
> except for the weird exception for slots being synced. I'm wondering
> if this field needs to be renamed.
>

We considered the other two names as last_inactive_at and
last_active_time. For the first (last_inactive_at), there was an
argument that most other fields that display time ends with _time. For
the second (last_active_time), there was an argument that it could be
misleading as one could think that it should be updated each time WAL
record decoding is happening [1]. The other possibility is to name it
last_used_time but I think it won't be much different from
last_active_time.

> And I'm suspicious that having an exception for slots being synced is
> a bad idea. That makes too much of a judgement about how the user will
> use this field. It's usually better to just expose the data, and if
> the user needs helps to make sense of that data, then give them that
> help separately.

The reason we didn't set this for sync slots is that they won't be
usable (one can't use them to decode WAL) unless standby is promoted
[2]. But I see your point as well. So, I have copied the others
involved in this discussion to see what they think.

>
> In this case, that would mean removing the exception,
> but making it easy to tell the difference between slots are inactive
> because they're being synced and slots that are inactive for some
> other reason.
>

I think this can be differentiated with the help of 'synced' column in
pg_replication_slots.

[1] - https://www.postgresql.org/message-id/Zf1yx9QMbhgJ/Lfy%40ip-10-97-1-34.eu-west-3.compute.internal
[2] - https://www.postgresql.org/message-id/CAJpy0uBGv85dFiWMnNLm6NuEs3eTVicsJCyRvMGbR8H%2BfOVBnA%40mail.gmail.com

--
With Regards,
Amit Kapila.