postgres queries to get replication log position deltas

On the slave (python string)

"select pg_last_xlog_receive_location(), pg_last_xlog_replay_location(), "
                         "EXTRACT (EPOCH FROM now() - pg_last_xact_replay_timestamp()) AS seconds_since_last_replay, "
                         "pg_xlog_location_diff(pg_last_xlog_receive_location(), pg_last_xlog_replay_location()) "
                         "AS replay_receive_xlog_pos_delta;
                         
On the master

use pg_xlog_location_diff with the extracted positions from the slave to do something similar

"select pg_current_xlog_location(), "
                          "pg_xlog_location_diff(pg_current_xlog_location(), '%s') AS slave_receive_xlog_pos_delta,"
                          "pg_xlog_location_diff(pg_current_xlog_location(), '%s') AS slave_replay_xlog_pos_delta;" %
                          ( raw_data['slave']['pg_last_xlog_receive_location'],
                            raw_data['slave']['pg_last_xlog_replay_location'],
                            )