Re: pg_stat_replication.*_lag sometimes shows NULL during active replication

On Tue, Mar 17, 2026 at 11:00 AM Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>
> On Mon, Mar 16, 2026 at 9:26 AM Shinya Kato <shinya11(dot)kato(at)gmail(dot)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 differently,
> 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 — that turned out to be unrelated.

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

--
Best regards,
Shinya Kato
NTT OSS Center