PostgreSQL foreign data wrappers (FDW) are an implementation of SQL Management of External Data (SQL/MED), which is a standard added to SQL in 2013.

FDWs are drivers that allow PostgreSQL database users to read and write data to other external data sources, such as other relational databases, NoSQL data sources, files, JSON, LDAP, and even Twitter.

You can query the foreign data sources using SQL and create joins across different systems or even across different data sources.

There are several different types of data wrappers developed by different developers and not all of them are production quality. You can see a select list of wrappers on the PostgreSQL wiki at http://wiki.postgresql.org/wiki/Foreign_data_wrappers.

Another list of FDWs can be found on PGXN at http://pgxn.org/tag/fdw/.

Let's take look at a small example of using file_fdw to access data in a CSV file.

First, you need to install the file_fdw extension. If you compiled PostgreSQL from the source, you...