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From: Shinya Kato <shinya11.kato@gmail.com>
Date: Fri, 6 Mar 2026 16:12:48 +0900
Message-ID: 
 <CAOzEurRGiGE2Dfe+ySpb=+93ku=7ZC6RgAbHtLC6Xsq3g2XexA@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 Mon, Mar 2, 2026 at 11:44=E2=80=AFPM Fujii Masao <masao.fujii@gmail.com>=
 wrote:
> With the patch applied, I set up a logical replication and inserted a row=
 every
> second. Even with continuous inserts, NULL was shown in the lag columns o=
f
> pg_stat_replication. That makes me wonder whether the patch's approach is
> sufficient to address the issue.

Thank you for the review and testing! I had only considered the issue
in the context of physical replication, but as you pointed out, my
approach is insufficient for logical replication.

> Relying solely on replies from the standby or subscriber seems a bit frag=
ile to
> me. If the goal is to keep showing the last measured lag for some time,
> perhaps we should introduce a rate limit on when NULL is displayed in the=
 lag
> columns?

My primary goal was to ensure that the source code comments match the
actual behavior, as the comment stating "the second such message must
result from wal_receiver_status_interval expiring on the standby" is
inaccurate. However, as you noted, the patch alone is not sufficient
to fully address the issue.

> For example, if there has been no activity (i.e., sentPtr =3D=3D applyPtr=
 and
> applyPtr has not changed since the previous cycle) for, say, 10 seconds,
> then we could allow NULL to be shown. Thought?

I considered a time-based rate limit, but it is difficult to choose an
appropriate threshold. Furthermore, the walsender has no way of
knowing the standby's or subscriber's wal_receiver_status_interval
setting.

The attached v2 patch takes a different approach: it additionally
requires that all reported positions (write/flush/apply) remain
unchanged from the previous reply. This directly detects a truly idle
system without relying on timeouts=E2=80=94if any position has advanced, ne=
w
WAL activity must have occurred, so we should not clear the lag values
even if the lag tracker is empty.
--
Best regards,
Shinya Kato
NTT OSS Center

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