FDW

FDW, or foreign data wrapper, is a component of PostgreSQL that enables a database server to query data stored outside of the local database as if it were a local table. FDWs are implemented as extensions that provide the logic for accessing external data sources, translating local SQL into operations understood by the remote system.

In PostgreSQL, the foreign data wrapper architecture consists of several objects. A wrapper defines how to interact with a particular type of external source, a server represents a specific external data source instance, and user mappings connect local roles to credentials used on the remote source. Foreign tables route queries to the appropriate wrapper and server. Schema can be imported from the remote source with IMPORT FOREIGN SCHEMA, allowing management similar to local tables.

FDWs can support a range of capabilities. Some wrappers, such as postgres_fdw, allow full read and write operations and enable server-to-server DML with pushdown of predicates and joins to the remote source. Others are limited to read access or to specialized data types. The effectiveness of a query depends on the wrapper’s ability to push operations to the remote source and on network considerations.

Common use cases include querying remote PostgreSQL databases, accessing non-PostgreSQL databases, and integrating data from files or other storage systems. Security is handled via user mappings and role-based access control, with authentication and permissions managed per foreign server and foreign table.

FDW is defined in PostgreSQL’s SQL/MED standard, introduced to enable federated data access and to provide a standardized approach for reading and, where supported, writing to external data sources.