Re: min_safe_lsn column in pg_replication_slots view

At Mon, 15 Jun 2020 13:44:31 +0900, Michael Paquier <michael(at)paquier(dot)xyz> wrote in
> On Mon, Jun 15, 2020 at 12:40:03PM +0900, Fujii Masao wrote:
> > BTW, I just wonder why each row in pg_replication_slots needs to have
> > min_safe_lsn column? Basically min_safe_lsn should be the same between
> > every replication slots.
>
> Indeed, that's confusing in its current shape. I would buy putting
> this value into pg_replication_slots if there were some consistency of
> the data to worry about because of locking issues, but here this data
> is controlled within info_lck, which is out of the the repslot LW
> lock. So I think that it is incorrect to put this data in this view
> and that we should remove it, and that instead we had better push for
> a system view that maps with the contents of XLogCtl.

It was once the difference from the safe_lsn to restart_lsn which is
distinct among slots. Then it was changed to the safe_lsn. I agree to
the discussion above, but it is needed anywhere since no one can know
the margin until the slot goes to the "lost" state without it. (Note
that currently even wal_status and min_safe_lsn can be inconsistent in
a line.)

Just for the need for table-consistency and in-line consistency, we
could just remember the result of XLogGetLastRemovedSegno() around
taking info_lock in the function. That doesn't make a practical
difference but makes the view look consistent.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center