How to check the replication delay in PostgreSQL?

Question

I would like to measure time between insert data into master-table and slave-table using streaming replication in PostgreSQL 9.3. For this I create table test_time

Answer 1

slightly different version of the correct answer:

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 important when "receive" not equal "replay". execute the query on the replica