Re: Report bytes and transactions actually sent downtream

Hi,

On Mon, Sep 29, 2025 at 12:54:24PM +0530, Ashutosh Bapat wrote:
> On Fri, Sep 26, 2025 at 10:28 PM Bertrand Drouvot
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> >
> > > >
> > >
> > > I don't think this is an issue. There is no way for the core to tell
> > > whether the plugin will provide stats or not, unless it sets that
> > > ctx->stats which happens in the startup callback. Till then it is
> > > rightly providing the values accumulated so far. Once the decoding
> > > starts, we know that the plugin is not providing any stats and we
> > > don't display anything.
> >
> > Yeah, I got the technical reasons, but I think there's a valid user experience
> > concern here: seeing statistics for a plugin that doesn't actually support
> > statistics is misleading.
> >
>
> 3. If the plugin starts supporting statistics and midway discontinues
> its support, it already has a problem with backward compatibility.
>
> Practically it would 1 or 2, which are working fine.
>
> I don't think we will encounter case 3 practically. Do you have a
> practical use case where a plugin would discontinue supporting stats?

Not that I can think of currently. That looks unlikely but wanted to raise
the point though. Maybe others see a use case and/or have a different point
of view.

> > What we need is a call to pgstat_report_replslot() to display stats that reflect
> > the current plugin behavior. We can't just call pgstat_report_replslot()
> > in say RestoreSlotFromDisk() because we really need the decoding to start.
> >
> > So one idea could be to set a flag (per slot) when pgstat_report_replslot()
> > has been called (for good reasons) and check for this flag in
> > pg_stat_get_replication_slot().
> >
> > If the flag is not set, then set the plugin fields to NULL.
> > If the flag is set, then display their values (like now).
>
> This approach will have the same problem. Till
> pgstat_report_replslot() is called, the old statistics will continue
> to be shown.

I don't think so because the flag would not be set.

> > And we should document that the plugin stats are not available (i.e are NULL)
> > until the decoding has valid stats to report after startup.
>
> The current documentation is " It is NULL when statistics is not
> initialized or immediately after a reset or when not maintained by the
> output plugin.". I think that covers all the cases.

Do you think the doc covers the case we discussed above? i.e when a plugin
discontinue supporting stats, it would display stats until the decoding actually
starts.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com