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help / color / mirror / Atom feedFrom: Shinya Kato <[email protected]>
To: Fujii Masao <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: pg_stat_replication.*_lag sometimes shows NULL during active replication
Date: Mon, 16 Mar 2026 09:25:52 +0900
Message-ID: <CAOzEurSDzFsRXjofhq7mbNgoL8HaVbNeEhWBm7m9_K2ZNQnaBw@mail.gmail.com> (raw)
In-Reply-To: <CAHGQGwGPg20Rw2PydBDXiKHgn55s--E14-qRy7t3M+DHreCJww@mail.gmail.com>
References: <CAOzEurTzcUrEzrH97DD7+Yz=HGPU81kzWQonKZvqBwYhx2G9_A@mail.gmail.com>
<CAHGQGwE=kyQ+YnGPn8zpZ959+3ywg8OR_Nu__uXxxuE0E+Y_Zg@mail.gmail.com>
<CAOzEurRGiGE2Dfe+ySpb=+93ku=7ZC6RgAbHtLC6Xsq3g2XexA@mail.gmail.com>
<CAHGQGwH2h_R7FWPvEs3+NWLwHZoj9r96tUyRKi5haqxMc6FXiQ@mail.gmail.com>
<CAOzEurQiP3uebd1GMiC1Dzf5VJwF4ZBEpJ6QYQFE6Y+rVjxqNA@mail.gmail.com>
<CAHGQGwEmMBBAE0RG-R3_LacfT4fbB55qGE6n9O5mNwrqvbNBtw@mail.gmail.com>
<CAOzEurRF+OWcMZpfE=NV_Wcm6CFFGOnuxC6L9WWCjOMN0_eZMQ@mail.gmail.com>
<CAHGQGwGPg20Rw2PydBDXiKHgn55s--E14-qRy7t3M+DHreCJww@mail.gmail.com>
On Fri, Mar 13, 2026 at 12:27 AM Fujii Masao <[email protected]> wrote:
> Thanks for testing and for the clarification! You're right.
>
> However, if we apply this change, the time required for the lag information to
> be reset would effectively double. I start wondering if that's really
> acceptable, especially for back branches. Although the docs doesn't clearly
> specify this timing, doubling it could affect systems that monitor
> replication lag, for example. It might still be reasonable to apply
> such a change in master, though.
Yes, I agree. Doubling the lag reset time should be avoided in back
branches if possible.
> On further thought, the root cause seems to be that walreceiver can send
> two consecutive status reply messages with identical WAL locations even
> when wal_receiver_status_interval has not yet elapsed. Addressing that
> behavior directly might resolve the issue you reported. I've attached a PoC
> patch that does this. Thought?
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.
+ * previsou update, i.e., when 'replyApply' is true.
One minor thing: there is a typo "previsou". It should be "previous".
--
Best regards,
Shinya Kato
NTT OSS Center
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Subject: Re: pg_stat_replication.*_lag sometimes shows NULL during active replication
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