Joining Results from Two Separate Databases

Is it possible to JOIN rows from two separate postgres databases?

I am working with system with couple databases in one server and sometimes I really need such a feature.

According to http://wiki.postgresql.org/wiki/FAQ

> 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.

EDIT: 3 years later (march 2014), this FAQ entry has been revised and is more helpful:

> How do I perform queries using multiple databases? > > There is no way to directly 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. > > The SQL/MED support in PostgreSQL allows a "foreign data wrapper" to > be created, linking tables in a remote database to the local database. > The remote database might be another database on the same PostgreSQL > instance, or a database half way around the world, it doesn't matter. > postgres_fdw is built-in to PostgreSQL 9.3 and includes read/write > support; a read-only version for 9.2 can be compiled and installed as > a contrib module. > > contrib/dblink allows cross-database queries using function calls and > is available for much older PostgreSQL versions. Unlike postgres_fdw > it can't "push down" conditions to the remote server, so it'll often > land up fetching a lot more data than you need. > > Of course, a client can also make simultaneous connections to > different databases and merge the results on the client side.

Forget about dblink!

Say hello to Postgres_FDW:

> To prepare for remote access using postgres_fdw: > > 1. Install the postgres_fdw extension using CREATE EXTENSION. > > 2. Create a foreign server object, using CREATE SERVER, to represent each remote database you want to connect to. Specify connection > information, except user, and password, as options of the server > object. > > 3. Create a user mapping, using CREATE USER MAPPING, for each database user you want to allow to access each foreign server. Specify > the remote user name and password to use as user and password options > of the user mapping. > > 4. Create a foreign table, using CREATE FOREIGN TABLE or IMPORT FOREIGN SCHEMA, for each remote table you want to access. The columns > of the foreign table must match the referenced remote table. You can, > however, use table and/or column names different from the remote > table's, if you specify the correct remote names as options of the > foreign table object. > > Now you need only SELECT from a foreign table to access the data > stored in its underlying remote table.

It's really useful even on large data.

Yes, it is possible to do this using dblink albeit with significant performance considerations.

The following example will require the current SQL user to have permissions on both databases. If db2 is not located on the same cluster, then you will need to replace dbname=db2 with the full connection string defined in the dblink documentation.

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;

If table2 is very large, you could have performance issues because the sub-query loads up the entire table2 before performing the join.

No you can't. You could use dblink to connect from one database to another database, but that won't help if you're looking for JOIN's.

You can't use different SCHEMA's within a single database to store all you data?

Just a few steps and You can reach the goal: follow this reference step by step

WE HAVE BEEN CONNECTED TO DB2 WITH TABLE TBL2 AND COLUMN COL2
ALSO THERE IS DB1 WITH TBL1 AND COLUMN COL1

 *** connecting to second db ie db2
    Now just **copy paste the 1-7 processes** (make sure u use correct username and password and ofcourse db name)

    1.**CREATE EXTENSION dblink;**
    
    2.**SELECT pg_namespace.nspname, pg_proc.proname 
    FROM pg_proc, pg_namespace 
    WHERE pg_proc.pronamespace=pg_namespace.oid 
       AND pg_proc.proname LIKE '%dblink%';**
    
    3.**SELECT dblink_connect('host=localhost user=postgres password=postgres dbname=db1');**
    
    4.**CREATE FOREIGN DATA WRAPPER postgres VALIDATOR postgresql_fdw_validator;**
    
    5.**CREATE SERVER postgres2 FOREIGN DATA WRAPPER postgres OPTIONS (hostaddr '127.0.0.1', dbname 'db1');**
    
    6.**CREATE USER MAPPING FOR postgres SERVER postgres2 OPTIONS (user 'postgres', password 'postgres');**
    
    7.**SELECT dblink_connect('postgres2');**
    
    ---Now, you can SELECT the data of Database_One from Database_Two and even join both db results:
    
    **SELECT * FROM public.dblink
    ('postgres2','SELECT col1,um_name FROM public.tbl1 ') 
    AS DATA(um_userid INTEGER),tbl2 where DATA.col1=tbl2.col2;**


You can also Check this :[How to join two tables of different databases together in postgresql [\[working finely in version 9.4\]][1]

You need to use dblink...as araqnid mentioned above, something like this works fine:

> select ST.Table_Name, ST.Column_Name, DV.Table_Name, DV.Column_Name, * from information_schema.Columns ST full outer join dblink('dbname=otherdatabase','select Table_Name, Column_Name from information_schema.Columns') DV(Table_Name text, Column_Name text) on ST.Table_Name = DV.Table_name and ST.Column_Name = DV.Column_Name where ST.Column_Name is null or DV.Column_Name is NULL

You have use dblink extension of postgresql.

Reference take from this Article:

DbLink extension of PostgreSQL which is used to connect one database to another database.

Install DbLink extension.

CREATE EXTENSION dblink;

Verify DbLink:

SELECT pg_namespace.nspname, pg_proc.proname 
FROM pg_proc, pg_namespace 
WHERE pg_proc.pronamespace=pg_namespace.oid 
   AND pg_proc.proname LIKE '%dblink%';

I have already prepared full demonstration on this. Please visit my post to learn step by step for executing cross database query in Postgresql.