Ingres 9.0 provides support for applications built against previous versions of Ingres, back to and including Ingres 6.4.

You can run applications built with any version of OpenIngres 1.x or Ingres II 2.x, accessing an Ingres 9.0 server, without rebuilding the application.

Applications built against Ingres 6.4 expect to access an older version of the Ingres message files. You can run applications built with Ingres 6.4 against an Ingres 9.0 server, as long as either the application is running across Ingres Net, or Ingres 9.0 was installed with the 6.4 message files. However, in all cases, we recommend that you rebuild all applications with Ingres 9.0.