You can get the delay in bytes from the master side quite easily using pg_xlog_location_diff to compare the master’s pg_current_xlog_insert_location with the replay_location for that backend’s pg_stat_replication entry.
postgres=# SELECT
pg_last_xlog_receive_location() receive,
pg_last_xlog_replay_location() replay,
(
extract(epoch FROM now()) –
extract(epoch FROM pg_last_xact_replay_timestamp())
)::int lag;
receive | replay | lag
————+————+——-
1/AB861728 | 1/AB861728 | 2027
the lag is only only important when receive is different than replay. execute the query on the slave
postgres 
