Message-ID: <0e00fcc0062b59b88f98dba4900ce49d5a9a3fea.camel@cybertec.at>
Subject: Re: AW: AW: AW: Replication Testing- How to introduce a Lag
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: "Subramanian,Ramachandran" <ramachandran.subramanian@alte-leipziger.de>,
  "pgsql-novice@lists.postgresql.org"	 <pgsql-novice@lists.postgresql.org>
Date: Mon, 23 Mar 2026 15:33:29 +0100
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On Mon, 2026-03-23 at 05:55 +0000, Subramanian,Ramachandran wrote:
> Thank you so much for your guidance,=C2=A0 Today morning I noticed the co=
lumns being null .=C2=A0=20
>=20
> > streaming | 0/491179E8 | 0/491179E8 | 0/491179E8 | 0/491179E8 |=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |
>=20
> Does it make sense to use the below SQL as a primary health check ?
>=20
> [postgres@Source_server~]$ psql -p 5432 -c " select=C2=A0 pg_wal_lsn_diff=
(sent_lsn,replay_lsn) from pg_stat_replication"
> =C2=A0pg_wal_lsn_diff
> -----------------
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0 0
> (1 row)

No; you want to measure the lag between the primary's current WAL position =
and
what was already replayed on the standby:

  SELECT pg_wal_lsn_diff(pg_current_wal_lsn(), replay_lsn)
  FROM pg_stat_replication;

Yours,
Laurenz Albe