I came across Cognos DMR recently as I reviewed some Framework Manager material. Cognos DMR (or Dimensionally Modelled Relational) is a hybrid of a relational model and an OLAP (or dimensional) model. It is covered as a theoretical concept in any Cognos Framework Manager course, but beyond a purely academic exercise I have never seen any call for it. Why is this? Because the way I see it, it delivers the worst of both worlds. It requires extra work creating a dimensional model on a relational database, and it delivers poor performance because it is not using an efficient pre-aggregated OLAP source. If you instead delivered a true OLAP solution with a Cognos Transformer model, you avoid both of these problems.
In my 12 years of Cognos consulting, I have never seen or recommended a DMR solution. That is not to say that there are no exceptional circumstances that could make DMR an ideal choice. But I would argue that these circumstances are rare, and may be the result of someone trying to do OLAP on the cheap. If you are considering a DMR solution, ask yourself why you are not using Cognos Transformer to deliver a true OLAP solution. If you have a valid reason (including cost), then DMR may be your ideal choice. But be aware of the cost of your choice (particularly in performance) and factor that into your decision too.
If anyone has any success stories with DMR I would be curious to hear about them!