Check the replica lag for a logical replication in terms of bytes

postgresql-replica-lag-in-bytes.sql

/*
 * To be run in the source (publisher) database
 * It checks the replica lag in terms of bytes between the source and destination databases in a logical replication
 */

SELECT slot_name,
       confirmed_flush_lsn,
       pg_current_wal_lsn() as pg_current_wal_lsn,
       ((pg_current_wal_lsn() - confirmed_flush_lsn) / 1024 ) AS lsn_distance_kb
FROM pg_catalog.pg_replication_slots
WHERE slot_type = 'logical';

/*
 * Example output
 *
 *  slot_name | confirmed_flush_lsn | pg_current_wal_lsn | lsn_distance_kb
 *  ----------+---------------------+--------------------+----------------
 *  sub_all   | 1DEB/F261E100       | 1DEB/F261E100      |       0.0000000
 * (1 rows)
 */