Re: Null value returned by function pg_last_wal_receive_lsn() inLogical Replication

Hi Pavan ,

Please try below query , this should give you retention lag bytes in case of logical replication .

SELECT
slot_name, database, active,
pg_xlog_location_diff(pg_current_xlog_insert_location(), restart_lsn) AS retained_bytes
FROM pg_replication_slots;

Thanks ,
Soumitra
________________________________
From: pavan95 <pavan(dot)postgresdba(at)gmail(dot)com>
Sent: Tuesday, October 9, 2018 11:29 AM
To: pgsql-admin(at)postgresql(dot)org
Subject: Re: Null value returned by function pg_last_wal_receive_lsn() inLogical Replication

Hi Community,

I configured Logical Replication in Postgres 10.5 using 2 nodes(Publisher
and Subscriber). I'm in my way to find lag in logical replication.

From the documentation I found that, pg_current_wal_lsn() on the publisher
and pg_last_wal_receive_lsn() on the subscriber helps us to find the lag in
which the subscriber is falling behind.

But, the function pg_last_wal_receive_lsn() is continuously returning NULL
value. In fact, all the Recovery Information
Functions(pg_last_wal_receive_lsn(),pg_last_wal_replay_lsn(),pg_last_xact_replay_timestamp())
are returning null.

From the Subscriber:

subscriber_db=# select pg_last_wal_receive_lsn();
pg_last_wal_receive_lsn
-------------------------

(1 row)

But according to documentation, the above 3 functions will return NULL only
if streaming replication is disabled or if it has not yet started. But in my
case, the logical replication is working fine. So I wanted to know if I
could get a value by enabling any settings recommended.

Also, is there a query to find the lag in bytes between publisher and
subscriber?

Thanks in Advance.

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