Re: pg_stat_replication.*_lag sometimes shows NULL during active replication

On Mon, Mar 9, 2026 at 8:21 PM Fujii Masao <masao.fujii@gmail.com> 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