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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: Tue, 10 Mar 2026 10:01:45 +0900
Message-ID: <CAOzEurQiP3uebd1GMiC1Dzf5VJwF4ZBEpJ6QYQFE6Y+rVjxqNA@mail.gmail.com> (raw)
In-Reply-To: <CAHGQGwH2h_R7FWPvEs3+NWLwHZoj9r96tUyRKi5haqxMc6FXiQ@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>
On Mon, Mar 9, 2026 at 8:21 PM Fujii Masao <[email protected]> wrote:
> > 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—if any position has advanced, new
> > WAL activity must have occurred, so we should not clear the lag values
> > even if the lag tracker is empty.
>
> This approach looks good to me.
Thank you for looking into this.
> One comment: currently, the lag becomes NULL basically after about one
> wal_receiver_status_interval during periods of no activity. OTOH, with this
> approach, it seems it would take about twice wal_receiver_status_interval.
> Is this understanding correct?
Exactly. With this patch, it takes about two
wal_receiver_status_interval cycles to show NULL instead of one. I
think this is an acceptable trade-off because it is better to take a
bit longer to detect inactivity than to incorrectly show NULL during
active replication.
--
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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