I need to measure how far in the past a hot standby is, async streaming replication.
On the Hot Standby, "select age(current_timestamp,pg_last_xact_replay_timestamp())" gets me this (or close enough for my purposes - I understand that if there are no updates, there are no logs to replay and the lag time will increase).
Is there some way to get this same information on the master? pg_stat_replication contains the log information, but I can't see how to map this to a timestamp.
Is there a better way of measuring this?
Comparing "pg_controldata" output on prod and standby might help you with this.