How to check replication backlog in PostgreSQL?

Question

PostgreSQL

Good afternoon!
I would like to see how much the write to the slave table lags behind the write to the master table with streaming replication in PostgreSQL 9.3. To do this, I made a test_time table of 2 fields: id (integer), t (text). Added trigger:

my_time:=to_char(current_timestamp, 'HH12:MI:SS:MS:US');
update test_time set t=my_time where id=new.id;

And put this trigger on after update. As a result, both on the master and on the slave, the time is the same.
Tell me, please, where could be the mistake or how to do it differently?

Answer 1

Problem solved. Calculating the difference between what the trigger wrote to the database and the value of pg_last_xact_replay_timestamp () on the slave, I just get the desired delay time.