FDW
Foreign Data Wrapper — a PostgreSQL feature that lets SQL queries read from external sources (MySQL server, CSV file, REST API, MongoDB) as if they were local tables. Standard since 9.1 (2011).
Supports JOIN across heterogeneous sources: SELECT u.name, m.title FROM local_users u JOIN mysql_movies m ON u.id = m.user_id between PostgreSQL and MySQL. WHERE/JOIN pushdown to the source where possible. Wrappers: postgres_fdw (postgres-to-postgres), mysql_fdw, oracle_fdw, mongo_fdw, file_fdw (CSV). Popular for federated queries instead of ETL pipelines. Trade-off: query times can become unpredictable — pushdown is best-effort. Citus + Greenplum extend the FDW concept for distributed queries.