Mondrian is generally very smart in how it chooses to implement queries. Over the last month or so, I have learned some lessons about how hard can be to make Mondrian smarter.

As a ROLAP engine (I prefer to call it 'ROLAP with caching'), Mondrian's evaluation strategy has always been a blend of in-memory processing, caching, and native SQL execution. Naturally there is always SQL involved, because Mondrian doesn't store any of its own data, but the question is how much of the processing Mondrian pushes down to the DBMS and how much it does itself, based on data in its cache.

The trends are towards native SQL execution. Data volumes are growing across the board, Mondrian is being deployed to larger enterprises with large data sets (in some cases displacing more established, and expensive, engines). …