In the previous recipe, you saw how to use objects from a different PostgreSQL database, either with dblink or by using the Foreign Data Wrapper infrastructure. Here, we will explore another variant of the latter—using Foreign Data Wrappers to access databases other than PostgreSQL.
There are many Foreign Data Wrappers for other database systems, all of which are maintained as extensions independently from the PostgreSQL project. The PostgreSQL Extension Network (PGXN) we mentioned in Chapter 3, Configuration, is a good place where you can see which extensions are available.
Just note this so that you don't get confused: while you can find Foreign Data Wrappers to access several database systems, there are also other wrappers for different types of data sources, such as text files, web services, and so on. There is even postgres_fdw, a backport of the contrib module that we covered in the previous recipe, for users of older PostgreSQL versions...