Re: min_safe_lsn column in pg_replication_slots view

On Wed, Jun 24, 2020 at 8:45 PM Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
>
> On 2020-Jun-24, Fujii Masao wrote:
>
> > On 2020/06/24 8:39, Alvaro Herrera wrote:
>
> > > I think we should publish the value from wal_keep_segments separately
> > > from max_slot_wal_keep_size. ISTM that the user might decide to change
> > > or remove wal_keep_segments and be suddenly at risk of losing slots
> > > because of overlooking that it was wal_keep_segments, not
> > > max_slot_wal_keep_size, that was protecting them.
> >
> > You mean to have two functions that returns
> >
> > 1. "current WAL LSN - wal_keep_segments * 16MB"
> > 2. "current WAL LSN - max_slot_wal_keep_size"
>
> Hmm, but all the values there are easily findable. What would be the
> point in repeating it?
>
> Maybe we should disregard this line of thinking and go back to
> Horiguchi-san's original proposal, to wit use the "distance to
> breakage", as also supported now by Amit Kapila[1] (unless I
> misunderstand him).
>

+1. I also think let's drop the idea of exposing a function for this
value and revert the min_safe_lsn part of the work as proposed by
Michael above [1] excluding the function pg_wal_oldest_lsn() in that
patch. Then, we can expose this as a new stat for PG14. I feel it
would be better to display this stat in a new view (something like
pg_stat_replication_slots) as discussed in another thread [2]. Does
that make sense?

[1] - https://www.postgresql.org/message-id/20200622054950.GC50978%40paquier.xyz
[2] - https://www.postgresql.org/message-id/CA%2Bfd4k5_pPAYRTDrO2PbtTOe0eHQpBvuqmCr8ic39uTNmR49Eg%40mail.gmail.com

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com