Re: min_safe_lsn column in pg_replication_slots view

On 2020-Jun-25, Amit Kapila wrote:

> +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?

I don't understand the proposal. Michael posted a patch that adds
pg_wal_oldest_lsn(), and you say we should apply the patch except the
part that adds that function -- so what part would be applying?

If the proposal is to apply just the hunk in pg_get_replication_slots
that removes min_safe_lsn, and do nothing else in pg13, then I don't like
it. The feature exposes a way to monitor slots w.r.t. the maximum slot
size; I'm okay if you prefer to express that in a different way, but I
don't like the idea of shipping pg13 without any way to monitor it.

As reported by Masao-san, the current min_safe_lsn has a definitional
problem when used with pg_walfile_name(), but we've established that
that's because pg_walfile_name() has a special-case definition, not
because min_safe_lsn itself is bogus. If we're looking for a minimal
change that can fix this problem, let's increment one byte, which should
fix that issue, no?

I also see that some people complain that all slots return the same
value and therefore this column is redundant. To that argument I say
that it's not unreasonable that we'll add a slot-specific size limit;
and if we do, we'll be happy we had slot-specific min safe LSN; see e.g.
https://postgr.es/m/20170301160610.wc7ez3vihmialntd@alap3.anarazel.de

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Álvaro Herrera https://www.2ndQuadrant.com/
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