SELECT pg_current_xlog_location(); command is applicable from 8.4 version and select pg_last_xlog_receive_location(); command is applicable from 9.0 version.
select now() - pg_last_xact_replay_timestamp(); command is applicable from 9.1 version.
Monitoring Streaming replication:
For monitoring purpose, you can use the pg_controldata command. pg_controldata command will give you the information checkpoint lagging status of Standby Server.
As per definition of Hot Standby, its a method of redundancy in which primary and secondary (Backup Server) runs simultaneously. The data is mirrored to secondary so that both should contain identical data in real time. With this user, would be able to execute the Query against Database while secondary is in archive recovery mode.
This is what introduce in PG9.0. In-built Hot Standby.
You can use below link where you can refer about Hot Standby.
Thanks very much for your valuable time, explaining me in detail.
Could you please tell me whether these commands applicable to 8.4, what are the versions that these commands are compatible. And one more thing would you please provide me some information regarding hot standby (what are the versions it is compatible, and useful views to check the status of hot standby)