Partial Results Mode

I am struggling to get partial results mode working. I have two oracle databases in a multi-source setup. If one of these databases is down, the VDB should still provide results from the remaining one. I have set partialResultsMode=True on the conenction URL of the VDB, but this does not make any difference.
An clear example of how the enable this would be most welcome.
Ideally the partial results mode should work with an internal materialized view.