PostgreSQL's foreign data wrapper API allows users to create foreign tables, which are almost identical to regular tables in PostgreSQL. The main difference is that the data isn't stored in PostgreSQL. Instead, PostgreSQL fetches the data from the foreign source whenever the table is queried.

tds_fdw is a foreign data wrapper for PostgreSQL that allows users to fetch table data from Microsoft SQL Server, Microsoft Azure Database, Sybase ASE, Sybase IQ, and possibly other databases from Microsoft and Sybase. Many users find tds_fdw useful for migrating to PostgreSQL from Microsoft/Sybase databases. However, that is not the only possible use.