Translate PostgreSQL Lag in bytes

pg_streaming_lag

-- Calculate From : http://munin-monitoring.org/browser/munin/plugins/node.d/postgres_streaming_.in
-- with help : http://www.postgresql.org/docs/9.3/static/monitoring-stats.html#PG-STAT-REPLICATION-VIEW
-- http://eulerto.blogspot.fr/2011/11/understanding-wal-nomenclature.html

create or replace function CalculateNumericalOffset(text)
returns bigint
language sql 
as $$
select ('x'||lpad( 'ff000000', 16, '0'))::bit(64)::bigint 
     * ('x'||lpad( split_part( $1 ,'/',1), 16, '0'))::bit(64)::bigint
     + ('x'||lpad( split_part( $1 ,'/',2), 16, '0'))::bit(64)::bigint  ; 
$$
;

create or replace function pg_streaming_lag( text, text ) 
returns bigint
language sql
as $$
select CalculateNumericalOffset( $1 )  - CalculateNumericalOffset( $2 ) ;
$$
;

select pid, backend_start, sync_state,
  pg_size_pretty(pg_streaming_lag(sent_location,write_location)) as write_lag, 
  pg_size_pretty(pg_streaming_lag(sent_location,flush_location)) as flush_lag,   
  pg_size_pretty(pg_streaming_lag(sent_location,replay_location) ) as replay_lag 
from pg_stat_replication ;