PostgreSQL: Find Lagged Replication

gistfile1.txt

SELECT slot_name,
       pg_wal_lsn_diff(
          pg_current_wal_lsn(),
          restart_lsn
       ) AS bytes_behind,
       active,
       wal_status
FROM pg_replication_slots
WHERE wal_status <> 'lost'
ORDER BY restart_lsn;