Explain PostgreSQL foreign data wrappers (FDW).

FDW allows PostgreSQL to access external data sources as if they were local tables.

Popular FDWs:
• postgres_fdw: Other PostgreSQL databases
• file_fdw: CSV files
• mysql_fdw: MySQL databases
• oracle_fdw: Oracle databases

Example:

CREATE EXTENSION postgres_fdw;
CREATE SERVER remote_server
    FOREIGN DATA WRAPPER postgres_fdw
    OPTIONS (host 'remote.example.com', dbname 'remote_db');

CREATE FOREIGN TABLE remote_users (
    id INT,
    name TEXT
) SERVER remote_server;