Conceptual question about "dimension usage"

I have a 2 dimensions:

UNDERLYING 0-8 INSTRUMENT [one underlying has zero or many instruments]

then I have several fact tables, say FACT1, FACT2, FACT3, which reference instrument_id as a foreign key.

Is it the case that in order to aggregate the fact tables by underlying, I need to define a "Referenced' relationship using the dimension usage tab for each fact table separately?

Either I'm missing something (probable!) but this seems unnecessary to me. Why can't I just define the UNDERLYING dimension as the parent of the INSTRUMENT dimension, and then every time the instrument dimension gets joined to the fact table, it simply follows that the corresponding fact can be rolled up by <correction>underlying [was: instrument]</correction>.

Any clarifications gratefully received.

tx,

JG

Are the Underlying and Instrument dimensions always paired like this? If so, one option is to build a single dimension based on the two "tables" in your DSV.

SSAS gives you a lot of flexibility in how you structure your cube. The trade-off is you must be explicit about that structure. So, one rule is that for a dimension to be associated with a fact, there must be an explicit relationship ("path") defined betewen the measure group and the dimension. Think about the alternative. Given any dimension in a conformed data warehouse, you could probably find a path to any measure group using intermediary measure groups and their dimension relationships. You'd end up with a real mess (and inappropriate results).

One thing you can do to ease the burden of setting these up is to insure you have relationships defined in your DSV. The Cube Designer will detect these and take a first stab at setting up relationships in the cube. (Still, I don't believe it will detect referenced relationships per the reason above.)

Good luck,
Bryan