Currently Ambari (as of 126.96.36.199) does not support Postgres HA.
Does Ambari supports MySQL HA, I think Ambari HA is not officially documented or supported currently. But it can be achieved by having two different Ambari Server instances pointing to Active Active DB replication setup for MySQL/Postgres.
Pardeep, we do not currently support MySQL HA, and it is my understanding going forward that MySQL is going away (from the distros of linux), so you would have to add it manually.
If you want a Postgresql HA for your Ambari and the other components like Ranger, Hive, Oozie, you have to use an external postgresql database, not the embeded.
In front of your Postgresql HA, use a connection pooler like pgbouncer or pgpool, so you will reduce the impact of opening a client connection to the database by keeping sessions open between the connection pooler and the database.