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From: Shinya Kato <shinya11.kato@gmail.com>
Date: Thu, 19 Mar 2026 22:58:18 +0900
Message-ID: 
 <CAOzEurRV117N2neo1N_O+xWPv=-R7qou+i7k-h79JjTP9sO1Fg@mail.gmail.com>
Subject: Re: pg_stat_replication.*_lag sometimes shows NULL during active
 replication
To: Fujii Masao <masao.fujii@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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On Tue, Mar 17, 2026 at 11:00=E2=80=AFAM Fujii Masao <masao.fujii@gmail.com=
> wrote:
>
> On Mon, Mar 16, 2026 at 9:26=E2=80=AFAM Shinya Kato <shinya11.kato@gmail.=
com> wrote:
> > Thank you for the v4 patch. I think this approach is better than mine.
> > I tested the patch and confirmed that the issue no longer reproduces
> > with physical replication. However, with logical replication, the lag
> > columns in pg_stat_replication still show NULL periodically at
> > wal_receiver_status_interval, since send_feedback() in worker.c can
> > still send duplicate positions.
>
> I was thinking that if a feedback message triggered by
> wal_receiver_status_interval has the same LSNs as the previous message,
> it's expected for the lag columns to become NULL. But you see it differen=
tly,
> don't you? Sorry, I failed to understand your point...

Sorry for the confusion. I ran a script inserting one row every 0.5
seconds under logical replication and confirmed that NULL still
appears in the lag columns even while replication is actively running.
I was initially mistaken that this was tied to
wal_receiver_status_interval timing =E2=80=94 that turned out to be unrelat=
ed.

I haven't had time to investigate further, but my current impression
is that the existing approach may not be sufficient for logical
replication.


--=20
Best regards,
Shinya Kato
NTT OSS Center