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

On Tue, Mar 10, 2026 at 10:54 AM Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> Even with your latest patch, if we remove fullyAppliedLastTime, and set
> clearLagTimes to true when applyPtr == sentPtr && noLagSamples &&
> positionsUnchanged,
> wouldn't the time for the lag to become NULL be almost the same as
> wal_receiver_status_interval?
>
> The documentation doesn't clearly specify how long it should take for
> the lag to become NULL, so doubling that time might be acceptable.
> However, if we can keep it roughly the same without much complexity,
> I think that would be preferable.
>
> Thought?

Thank you for the suggestion. I tested this by removing
fullyAppliedLastTime, but even with synchronous replication, NULL
still appears. Here is why:

- Reply 1 (flush notification): positions = X. Lag samples are
consumed with real values, so noLagSamples = false. clearLagTimes is
not set, and prevPtrs = X is saved.

- Reply 2 (force_reply): positions = X again. Here, noLagSamples =
true and positionsUnchanged = true. Since applyPtr == sentPtr,
clearLagTimes is set to true, resulting in a NULL value.

Therefore, I believe fullyAppliedLastTime is still necessary to ensure
that the previous reply also contained no lag samples.

BTW I noticed an incorrect comment in walreceiver.c and have included
a fix for it. Patch 0001 remains unchanged.

--
Best regards,
Shinya Kato
NTT OSS Center