Create a query with multiple databases Let say I have DB1, a database in PostgreSQL, and DB2, a database in Oracle.

How can I make a query joining these databases?

Is this even possible?
You can use a foreign data wrappers in Postgres to make Oracle tables appear like local tables in Postgres. 

Then you can use the Oracle tables in a query running in Postgres and join them to the "real"  Postgres tables in a single query. 

A list of foreign data wrappers (not included with Postgres itself) is available in the Postgres Wiki