Database management systems are meant to house data but, occasionally, they may need to talk with another DBMS. For example, to access an external server which may be hosting a different DBMS. With heterogeneous environments becoming more and more common, a bridge between the servers is established. We call this bridge a “Foreign Data Wrapper” [&]

https://www.percona.com/blog/2019/03/29/postgresql-access-clickhouse-one-of-the-fastest-column-dbmss-with-clickhousedb_fdw/