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From: Shinya Kato <shinya11.kato@gmail.com>
Date: Wed, 25 Mar 2026 16:02:53 +0900
Message-ID: 
 <CAOzEurRHuBYBt4BXyFnD8DL9dDgbAexqSC9HOT2=6swiyLS1Kw@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 24, 2026, 00:31 Fujii Masao <masao.fujii@gmail.com> wrote:

> On Sat, Mar 21, 2026 at 11:05=E2=80=AFAM Shinya Kato <shinya11.kato@gmail=
.com>
> wrote:
> >
> > 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 poin=
t
> to
> > > the same LSN (e.g., 0/030D5230), and the second message reports the
> same but
> > > larger LSN (e.g., 0/030D52E0).
> >
> > Thanks for the explanation!
> >
> > > 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.
> > >
> > > Patch attached. Could you test and review this updated patch?
> >
> > The patch works properly. I think it looks nice to me, except for the
> > typo I sent in the previous message.
>
> Thanks for the review!
>
> 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.
>
> As for backpatching, I'm hesitant to backpatch the full patch since it ma=
y
> reduce the number of replication feedback messages, which feels too
> invasive
> for stable branches.
>
> 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.
>

Thanks for the updated patch. LGTM.

Regarding the backpatch, I'd personally appreciate it if the walsender.c
changes were backpatched to stable branches. As you noted, it don't fully
solve the reported issue, but they do help reduce the cases where lag
columns in pg_stat_replication unexpectedly become NULL.

Even a partial mitigation in the back branches would be valuable for users
running stable releases.

--
Best regards,
Shinya Kato

>

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<div dir=3D"auto"><div><br><br><div class=3D"gmail_quote gmail_quote_contai=
ner"><div dir=3D"ltr" class=3D"gmail_attr">On Tue, Mar 24, 2026, 00:31 Fuji=
i Masao &lt;<a href=3D"mailto:masao.fujii@gmail.com">masao.fujii@gmail.com<=
/a>&gt; wrote:<br></div><blockquote class=3D"gmail_quote" style=3D"margin:0=
 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">On Sat, Mar 21, 2026=
 at 11:05=E2=80=AFAM Shinya Kato &lt;<a href=3D"mailto:shinya11.kato@gmail.=
com" target=3D"_blank" rel=3D"noreferrer">shinya11.kato@gmail.com</a>&gt; w=
rote:<br>
&gt;<br>
&gt; On Fri, Mar 20, 2026 at 2:13=E2=80=AFAM Fujii Masao &lt;<a href=3D"mai=
lto:masao.fujii@gmail.com" target=3D"_blank" rel=3D"noreferrer">masao.fujii=
@gmail.com</a>&gt; wrote:<br>
&gt; &gt; I think the issue occurs when the positions in the first message =
point to<br>
&gt; &gt; the same LSN (e.g., 0/030D5230), and the second message reports t=
he same but<br>
&gt; &gt; larger LSN (e.g., 0/030D52E0).<br>
&gt;<br>
&gt; Thanks for the explanation!<br>
&gt;<br>
&gt; &gt; I&#39;ve updated the patch to address this. It removes fullyAppli=
edLastTime,<br>
&gt; &gt; tracks the positions from the previous reply, and clears the lag =
values only<br>
&gt; &gt; when the positions remain unchanged across two consecutive messag=
es.<br>
&gt; &gt;<br>
&gt; &gt; Patch attached. Could you test and review this updated patch?<br>
&gt;<br>
&gt; The patch works properly. I think it looks nice to me, except for the<=
br>
&gt; typo I sent in the previous message.<br>
<br>
Thanks for the review!<br>
<br>
I&#39;ve fixed the typo and attached an updated patch. I also incorporated<=
br>
Chao&#39;s comments from upthread. I&#39;m planning to commit this to maste=
r.<br>
<br>
As for backpatching, I&#39;m hesitant to backpatch the full patch since it =
may<br>
reduce the number of replication feedback messages, which feels too invasiv=
e<br>
for stable branches.<br>
<br>
That said, the patch&#39;s changes in walsender.c could be backpatched.<br>
As discussed earlier, they don&#39;t fully address the reported issue,<br>
but they do help mitigate cases where lag becomes NULL unexpectedly<br>
in logical replication. So it might be worth considering those changes<br>
for stable branches.<br></blockquote></div></div><div dir=3D"auto"><br></di=
v><div dir=3D"auto">Thanks for the updated patch. LGTM.</div><div dir=3D"au=
to"><br></div><div dir=3D"auto">Regarding the backpatch, I&#39;d personally=
 appreciate it if the walsender.c changes were backpatched to stable branch=
es. As you noted, it don&#39;t fully solve the reported issue, but they do =
help reduce the cases where lag columns in pg_stat_replication unexpectedly=
 become NULL.</div><div dir=3D"auto"><br></div><div dir=3D"auto">Even a par=
tial mitigation in the back branches would be valuable for users running st=
able releases.</div><div dir=3D"auto"><br></div><div dir=3D"auto">--</div><=
div dir=3D"auto">Best regards,</div><div dir=3D"auto">Shinya Kato</div><div=
 dir=3D"auto"><div class=3D"gmail_quote gmail_quote_container"><blockquote =
class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid=
;padding-left:1ex">
</blockquote></div></div></div>

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