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.wc7ez3vihmial...@alap3.anarazel.de -- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
