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Subject: Re: pg_stat_replication.*_lag sometimes shows NULL during active
 replication
From: Chao Li <li.evan.chao@gmail.com>
In-Reply-To: 
 <CAHGQGwGRLOYj95OfQ5T+mJYmyQ8YrvQ1SNOkiviV+5aWT1iCoA@mail.gmail.com>
Date: Tue, 24 Mar 2026 13:32:00 +0800
Cc: Shinya Kato <shinya11.kato@gmail.com>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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To: Fujii Masao <masao.fujii@gmail.com>
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> On Mar 23, 2026, at 23:31, Fujii Masao <masao.fujii@gmail.com> wrote:
>=20
> On Sat, Mar 21, 2026 at 11:05=E2=80=AFAM Shinya Kato =
<shinya11.kato@gmail.com> wrote:
>>=20
>> On Fri, Mar 20, 2026 at 2:13=E2=80=AFAM Fujii Masao =
<masao.fujii@gmail.com> wrote:
>>> I think the issue occurs when the positions in the first message =
point to
>>> the same LSN (e.g., 0/030D5230), and the second message reports the =
same but
>>> larger LSN (e.g., 0/030D52E0).
>>=20
>> Thanks for the explanation!
>>=20
>>> I've updated the patch to address this. It removes =
fullyAppliedLastTime,
>>> tracks the positions from the previous reply, and clears the lag =
values only
>>> when the positions remain unchanged across two consecutive messages.
>>>=20
>>> Patch attached. Could you test and review this updated patch?
>>=20
>> The patch works properly. I think it looks nice to me, except for the
>> typo I sent in the previous message.
>=20
> Thanks for the review!
>=20
> I've fixed the typo and attached an updated patch. I also incorporated
> Chao's comments from upthread. I'm planning to commit this to master.
>=20
> As for backpatching, I'm hesitant to backpatch the full patch since it =
may
> reduce the number of replication feedback messages, which feels too =
invasive
> for stable branches.
>=20
> That said, the patch's changes in walsender.c could be backpatched.
> As discussed earlier, they don't fully address the reported issue,
> but they do help mitigate cases where lag becomes NULL unexpectedly
> in logical replication. So it might be worth considering those changes
> for stable branches.
>=20
> Thoughts?
>=20
> Regards,
>=20
> --=20
> Fujii Masao
> <v6-0001-Avoid-sending-duplicate-WAL-locations-in-standby-.patch>

Thank you for updating the patch. I saw that the variable name and =
function name were changed to reflect my earlier comments.

v6 looks good to me.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/