see https://www.cybertec-postgresql.com/en/joining-data-from-multiple-postgres-databases/ [published 2017]

These days you also have the option to use https://prestodb.io/

You can run SQL on that PrestoDB node and it will distribute the SQL query as required. It can connect to the same node twice for different databases, or it might be connecting to different nodes on different hosts.

It does not support:

DELETE
ALTER TABLE
CREATE TABLE (CREATE TABLE AS is supported)
GRANT
REVOKE
SHOW GRANTS
SHOW ROLES
SHOW ROLE GRANTS

So you should only use it for SELECT and JOIN needs. Connect directly to each database for the above needs. (It looks like you can also INSERT or UPDATE which is nice)

Client applications connect to PrestoDB primarily using JDBC, but other types of connection are possible including a Tableu compatible web API

This is an open source tool governed by the Linux Foundation and Presto Foundation.

The founding members of the Presto Foundation are: Facebook, Uber,
Twitter, and Alibaba.

The current members are: Facebook, Uber, Twitter, Alibaba, Alluxio,
Ahana, Upsolver, and Intel.