[sql] Possible to perform cross-database queries with PostgreSQL?

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...

In case someone needs a more involved example on how to do cross-database queries, here's an example that cleans up the databasechangeloglock table on every database that has it:

CREATE EXTENSION IF NOT EXISTS dblink;

DO 
$$
DECLARE database_name TEXT;
DECLARE conn_template TEXT;
DECLARE conn_string TEXT;
DECLARE table_exists Boolean;
BEGIN
    conn_template = 'user=myuser password=mypass dbname=';

    FOR database_name IN
        SELECT datname FROM pg_database
        WHERE datistemplate = false
    LOOP
        conn_string = conn_template || database_name;

        table_exists = (select table_exists_ from dblink(conn_string, '(select Count(*) > 0 from information_schema.tables where table_name = ''databasechangeloglock'')') as (table_exists_ Boolean));
        IF table_exists THEN
            perform dblink_exec(conn_string, 'delete from databasechangeloglock');
        END IF;     
    END LOOP;

END
$$

_{Just to add a bit more information.}

There is no way to query a database other than the current one. Because PostgreSQL loads database-specific system catalogs, it is uncertain how a cross-database query should even behave.

contrib/dblink allows cross-database queries using function calls. Of course, a client can also make simultaneous connections to different databases and merge the results on the client side.

PostgreSQL FAQ

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. Refrence

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.

Yes, you can by using DBlink (postgresql only) and DBI-Link (allows foreign cross database queriers) and TDS_LInk which allows queries to be run against MS SQL server.

I have used DB-Link and TDS-link before with great success.

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.

If performance is important and most queries are read-only, I would suggest to replicate data over to another database. While this seems like unneeded duplication of data, it might help if indexes are required.

This can be done with simple on insert triggers which in turn call dblink to update another copy. There are also full-blown replication options (like Slony) but that's off-topic.

I have checked and tried to create a foreign key relationships between 2 tables in 2 different databases using both dblink and postgres_fdw but with no result.

Having read the other peoples feedback on this, for example here and here and in some other sources it looks like there is no way to do that currently:

The dblink and postgres_fdw indeed enable one to connect to and query tables in other databases, which is not possible with the standard Postgres, but they do not allow to establish foreign key relationships between tables in different databases.