Equivalent of Oracle database link

One can use a combination of foreign server (defining the host, port and database name), user mapping (defining user and password on an already created foreign server) and one of the extensions postgres_fdw (foreign data wrapper for external PostgreSQL servers) and dblink (older, less transparent and standards-compliant syntax but more flexible than postgres_fdw) and foreign table (defining tables not in the very database instance).

It might even be possible to db link to other database systems by other yet to write foreign data wrappers. Files can be sourced as tables already.