Linked servers in Postgres can be implemented using foreign data wrappers (part of the SQL/MED implementation in Postgres - MED = management of external data which is actually in the SQL spec):
https://wiki.postgresql.org/wiki/Foreign_data_wrappers
but stuff like "Does not compile with PostgreSQL >= 9.2!", "A patched, but completely untested version", and "The current version does not yet support WHERE clause push-down, column push-down, JOIN push-down, or write operations."[0] are riskier (and a harder sell to management) than having the functionality backed in the core engine and admin interfaces.
