The current map/reduce query builder is getting unmaintainable. Begin work to break it down into separate classes.
First pass at implementing the object that will represent a composed map/reduce operation
Write a compositor class for sticking composable map/reduce queries together
Add the 'Only' query
Move over the current set of implementations into the new format (except CountUniqueValues which needs chaining)
Add CountUniqueValues
Build up the chain as we go