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From: Shinya Kato <shinya11.kato@gmail.com>
Date: Wed, 11 Mar 2026 11:38:23 +0900
Message-ID: 
 <CAOzEurRF+OWcMZpfE=NV_Wcm6CFFGOnuxC6L9WWCjOMN0_eZMQ@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 10, 2026 at 10:54=E2=80=AFAM Fujii Masao <masao.fujii@gmail.com=
> wrote:
> Even with your latest patch, if we remove fullyAppliedLastTime, and set
> clearLagTimes to true when applyPtr =3D=3D sentPtr && noLagSamples &&
> positionsUnchanged,
> wouldn't the time for the lag to become NULL be almost the same as
> wal_receiver_status_interval?
>
> The documentation doesn't clearly specify how long it should take for
> the lag to become NULL, so doubling that time might be acceptable.
> However, if we can keep it roughly the same without much complexity,
> I think that would be preferable.
>
> Thought?

Thank you for the suggestion. I tested this by removing
fullyAppliedLastTime, but even with synchronous replication, NULL
still appears. Here is why:

- Reply 1 (flush notification): positions =3D X. Lag samples are
consumed with real values, so noLagSamples =3D false. clearLagTimes is
not set, and prevPtrs =3D X is saved.

- Reply 2 (force_reply): positions =3D X again. Here, noLagSamples =3D
true and positionsUnchanged =3D true. Since applyPtr =3D=3D sentPtr,
clearLagTimes is set to true, resulting in a NULL value.

Therefore, I believe fullyAppliedLastTime is still necessary to ensure
that the previous reply also contained no lag samples.

BTW I noticed an incorrect comment in walreceiver.c and have included
a fix for it. Patch 0001 remains unchanged.


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

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