Dynamic Groups in SSRS Reports with MDX

Providing a dynamic group in SSRS with SQL is not all that hard. It is also a common business requirement when combining reports. In example, I see very often an old system providing multiple similar reports to its users - that is reports with exactly the same layout but with different items on rows or columns. I also see quite often SSRS requirements asking me to mock the old functionality and build multiple almost identical reports. My personal preference is to combine them in one if possible, thus reducing the maintenance and even development effort and cost. In SQL we have the convenience to choose how to name the output columns. As long as we have a stable dataset - that is a constant number of columns with the same data types called the same way, SSRS does not differentiate between them and treats them the same way. This allows us to flexibly change their actual contents. The standard approach in MDX would be to create all possible "groups" in a large dataset and then use one or another in the report as required through expressions. This, however, leads to slower execution times, larger chunks of data going around the network and the need to aggregate within SSRS at almost every data cell.