Major Description
The DiffIndex relies on implementation details of the query engine, specially that "query" is called with the exact same parameters as "getCost". This is currently always the case, but it is not a specified contract.
It's OK to do that right now, but the index should verify that the uuid of "query" does not match that of "getCost". This is to avoid returning the wrong data in case the query engine does no longer works like this.
The problem is that the current behavior can lead to hard to detect problems if the query engine is changed (for example if caching is introduced).