I need to create a system using heterogeneous database. I must use at least 4 different database. My choice would be MySQL, MS SQL Server, Oracle and PostgreSQL since these are among the most popular and matured database or if you have any database that is more suitable please tell me? I am trying to connect to different database on the database level, not on the application level. Did you have any ways how to connect all the database become distributed database? Please help me to solve this problem..