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

If your database has frequent writes, then the below query is a close approximation to get the slave lag

select now() - pg_last_xact_replay_timestamp() AS replication_delay;

Below is a more accurate query for calculating replication lag for databases with very few writes. If the master doesnt sent down any write to the slave, then pg_last_xact_replay_timestamp() can be constant and hence may not accurately determine the slave lag using the above query.

SELECT CASE WHEN pg_last_xlog_receive_location() =
pg_last_xlog_replay_location() THEN 0 ELSE EXTRACT (EPOCH FROM now() -
pg_last_xact_replay_timestamp()) END AS log_delay;