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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