Re: is pg_log_standby_snapshot() really needed?

On Wed, Jun 7, 2023 at 5:19 AM Drouvot, Bertrand
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> Hi,
>
> On 6/7/23 7:32 AM, Jaime Casanova wrote:
> >
> > So, I wonder if that function is really needed because as I said I
> > solved it with already existing functionality. Or if it is really
> > needed maybe it is a bug that a CHECKPOINT and pg_switch_wal() have
> > the same effect?
> >
>
> Even if CHECKPOINT and pg_switch_wal() do produce the same effect, I think
> they are expensive (as compare to pg_log_standby_snapshot() which does nothing but
> emit a xl_running_xacts).
>
> For this reason, I think pg_log_standby_snapshot() is worth to have/keep.
>

CHECKPOINT could be expensive in a busy system, but the problem
pg_log_standby_snapshot() is solving is about a no-activity system,
and in a no-activity system CHECKPOINT is very fast.
Even with very low activity SUBSCRIPTION flows fine. As an example I
put an INSERT happening every 10s and SUBSCRIPTION never stuck no
CHECKPOINT nor pg_log_standby_snapshot() needed.

--
Jaime Casanova
Director de Servicios Profesionales
SYSTEMGUARDS - Consultores de PostgreSQL