SELECT
   pg_wal_lsn_diff(A .c1, replay_lsn) /(1024 * 1024) AS slave_latency_MB
  FROM
   pg_stat_replication,
   pg_current_wal_lsn() AS A(c1)
  WHERE client_addr='%s' and application_name = '%s'
  ORDER BY
   slave_latency_MB
  LIMIT 1;

postgresql.conf
synchronous_standby_names = 'test1,test2,test3'

primary_conninfo = 'application_name=test1 host=127.0.0.1 port=1999 user=postgres keepalives_idle=60'

primary_conninfo = 'application_name=test2 host=127.0.0.1 port=1999 user=postgres keepalives_idle=60'

primary_conninfo = 'application_name=test3 host=127.0.0.1 port=1999 user=postgres keepalives_idle=60'

digoal=# select pid,application_name,client_addr,sync_state from pg_stat_replication;
 pid | application_name | client_addr | sync_state 
------+------------------+-------------+------------
 6311 | test1   | 127.0.0.1 | sync
 6321 | test2   | 127.0.0.1 | potential
 6391 | test3   | 127.0.0.1 | potential
(3 rows)

pg_ctl stop -m fast -D /pgdata11999
digoal=# select pid,application_name,client_addr,sync_state from pg_stat_replication;
 pid | application_name | client_addr | sync_state 
------+------------------+-------------+------------
 6564 | test2   | 127.0.0.1 | sync
 6568 | test3   | 127.0.0.1 | potential
(2 rows)

pg93@db-172-16-3-33-> pg_ctl start -D /pgdata11999
server starting
digoal=# select pid,application_name,client_addr,sync_state from pg_stat_replication;
 pid | application_name | client_addr | sync_state 
------+------------------+-------------+------------
 6564 | test2   | 127.0.0.1 | potential
 6605 | test1   | 127.0.0.1 | sync
 6568 | test3   | 127.0.0.1 | potential
(3 rows)

synchronous_standby_names = 'test1,test2'

digoal=# select pid,application_name,client_addr,sync_state from pg_stat_replication;
 pid | application_name | client_addr | sync_state 
------+------------------+-------------+------------
 6470 | test1   | 127.0.0.1 | sync
 6472 | test3   | 127.0.0.1 | async
 6474 | test2   | 127.0.0.1 | potential
(3 rows)

digoal=# select pid,application_name,client_addr,sync_state from pg_stat_replication;
 pid | application_name | client_addr | sync_state 
------+------------------+-------------+------------
 6519 | test2   | 127.0.0.1 | async
 6521 | test3   | 127.0.0.1 | async
 6523 | test1   | 127.0.0.1 | sync
(3 rows)

/*
 * Returns activity of walsenders, including pids and xlog locations sent to
 * standby servers.
 */
Datum
pg_stat_get_wal_senders(PG_FUNCTION_ARGS)
{
...略
   /*
    * More easily understood version of standby state. This is purely
    * informational, not different from priority.
    */
   if (sync_priority[i] == 0)
    values[7] = CStringGetTextDatum("async");
   else if (i == sync_standby)
    values[7] = CStringGetTextDatum("sync");
   else
    values[7] = CStringGetTextDatum("potential");
...略