Sometimes, you may want to access data in other PostgreSQL databases. The reasons may be as follows:
You have more than one database server, and you need to extract data (such as reference) from one server and load it into the other.
You want to access data that is in a different database on the same database server, which was split for administrative purposes.
You want to perform some changes that you do not wish to rollback in the event of an error or transaction abort. These are known as function side-effects or autonomous transactions.
You might also be considering this because you are exploring the scale out, sharding, or load balancing approaches. If so, read the last part of this recipe, the See also section, and then skip to Chapter 12, Replication and Upgrades.