Possible to perform cross-database queries with PostgreSQL?

Note: As the original asker implied, if you are setting up two databases on the same machine you probably want to make two schemas instead - in that case you don't need anything special to query across them.

postgres_fdw

Use postgres_fdw (foreign data wrapper) to connect to tables in any Postgres database - local or remote.

Note that there are foreign data wrappers for other popular data sources. At this time, only postgres_fdw and file_fdw are part of the official Postgres distribution.

For Postgres versions before 9.3

Versions this old are no longer supported, but if you need to do this in a pre-2013 Postgres installation, there is a function called dblink.

I've never used it, but it is maintained and distributed with the rest of PostgreSQL. If you're using the version of PostgreSQL that came with your Linux distro, you might need to install a package called postgresql-contrib.

I have run into this before an came to the same conclusion about cross database queries as you. What I ended up doing was using schemas to divide the table space that way I could keep the tables grouped but still query them all.

dblink() -- executes a query in a remote database

dblink executes a query (usually a SELECT, but it can be any SQL statement that returns rows) in a remote database.

When two text arguments are given, the first one is first looked up as a persistent connection's name; if found, the command is executed on that connection. If not found, the first argument is treated as a connection info string as for dblink_connect, and the indicated connection is made just for the duration of this command.

one of the good example:

SELECT * 
FROM   table1 tb1 
LEFT   JOIN (
   SELECT *
   FROM   dblink('dbname=db2','SELECT id, code FROM table2')
   AS     tb2(id int, code text);
) AS tb2 ON tb2.column = tb1.column;

Note: I am giving this information for future reference. Reference