There _is_ a Foreign Data Wrapper that wraps MySQL[1]. Not exactly the same thing though, and no idea how well it works.

[1] https://github.com/EnterpriseDB/mysql_fdw

I can tell you it doesn't work very well for wrapping remote postgres tables... it doesn't even send the where clause to the remote server, instead does a table scan across the network and matches locally.

Depends which Postgres version you are using: 9.1 added read only FDW support, 9.3 made it writable, 9.5 also has join pushdown into remote table.

Obviously, a lot depends on the FDW extension you are using - it doesn't help if there is support in postgres, if your specific extension does not utilize it.