Question

I’m going to guess that the answer is ‘no’ based on the below error message (and this Google result), but is there anyway to perform a cross-database query using PostgreSQL?

databaseA=# select * from databaseB.public.someTableName; ERROR:  cross-database references are not implemented:  'databaseB.public.someTableName' 

I’m working with some data that is partitioned across two databases although data is really shared between the two (userid columns in one database come from the users table in the other database). I have no idea why these are two separate databases instead of schema, but c’est la vie…

Answer 1

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.