I've worked in this space a lot and in my experience, the only true solution is some sort of layer below your reporting tool to aggregate the data. That being s

I've worked in this space a lot and in my experience, the only true solution is some sort of layer below your reporting tool to aggregate the data. That being said, it's not always a database that physically stores the data. It simply needs to be a semantic layer that models the way in which the data will be queried. Additionally, your OLTP systems are not optimized for reporting and you'll quickly choke yourself out trying to perform complex analysis on data structures attempting to solve a problem for which they weren't designed.