When it comes to expression evaluation, Mondrian keeps things simple. It doesn't tend to cache the results of expressions, but calculates them each time they are evaluated. Eventually the calculation tunnels through all calculated members and ends up at an atomic cell. Atomic cells are retrieved from the database, and stored in the cell value cache, so they are only calculated once.

By the way, an atomic cell is not necessarily at the lowest level of the hierarchy; Mondrian would prefer to load cells at a coarse granularity, and leave the hard work of aggregating values to the database, or even better, an aggregate table. And Mondrian does its best to retrieve atomic cells in batches. It gathers together requests for lots of cells of the same granularity and generates a single SQL statement to retrieve them all at once.

Mondrian's 'keep it simple' scheme comes unstuck when a particular calculation is repeated many times …