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

as of 10 release:

https://www.postgresql.org/docs/10/static/monitoring-stats.html#pg-stat-replication-view

write_lag interval Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written it (but not yet flushed it or applied it). This can be used to gauge the delay that synchronous_commit level remote_write incurred while committing if this server was configured as a synchronous standby.

flush_lag interval Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written and flushed it (but not yet applied it). This can be used to gauge the delay that synchronous_commit level remote_flush incurred while committing if this server was configured as a synchronous standby.

replay_lag interval Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. This can be used to gauge the delay that synchronous_commit level remote_apply incurred while committing if this server was configured as a synchronous standby.

(formatting mine)

Alas new columns seem to suit only synchronous replication (otherwise master would not know exact lag) thus async replication delay chack seem to remain now()-pg_last_xact_replay_timestamp()...