Re: min_safe_lsn column in pg_replication_slots view

On Fri, Jun 26, 2020 at 4:54 AM Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
>
> On 2020-Jun-26, Michael Paquier wrote:
>
> > On Thu, Jun 25, 2020 at 11:24:27AM -0400, Alvaro Herrera wrote:
> > > 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?
> >
> > I have sent last week a patch about only the removal of min_safe_lsn:
> > https://www.postgresql.org/message-id/20200619121552.GH453547@paquier.xyz
> > So this applies to this part.
>
> Well, I oppose that because it leaves us with no way to monitor slot
> limits.  In his opening email, Masao-san proposed to simply change the
> value by adding 1.  How you go from adding 1 to a column to removing
> the column completely with no recourse, is beyond me.
>
> Let me summarize the situation and possible ways forward as I see them.
> If I'm mistaken, please correct me.
>
> Problems:
> i)  pg_replication_slot.min_safe_lsn has a weird definition in that all
>     replication slots show the same value
>

It is also not clear how the user can make use of that value?

> ii) min_safe_lsn cannot be used with pg_walfile_name, because it returns
>     the name of the previous segment.
>
> Proposed solutions:
>
> a) Do nothing -- keep the min_safe_lsn column as is.  Warn users that
>    pg_walfile_name should not be used with this column.
> b) Redefine min_safe_lsn to be lsn+1, so that pg_walfile_name can be used
>    and return a useful value.
> c) Remove min_safe_lsn; add functions that expose the same value
> d) Remove min_safe_lsn; add a new view that exposes the same value and
>    possibly others
>
> e) Replace min_safe_lsn with a "distance" column, which reports
>    restart_lsn - oldest valid LSN
>    (Note that you no longer have an LSN in this scenario, so you can't
>    call pg_walfile_name.)
>

Can we consider an option to "Remove min_safe_lsn; document how a user
can monitor the distance"?  We have a function to get current WAL
insert location and other things required are available either via
view or as guc variable values.  The reason I am thinking of this
option is that it might be better to get some more feedback on what is
the most appropriate value to display.  However, I am okay if we can
reach a consensus on one of the above options.

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