Map reduce cached data


I know that map reduce data incrementally updated and cached
But how it is possible for filtering and grouping result? it is unlimited filtering cases

How views works exactly? assume we emit([doc.location,],doc.sallary) and use _sum as reducer

Okay, we have a cached sum for whole data in view
But I choose group level 2 , is there cache for it?
What about key range filter?


Please reply …


@matthew.groves @siri can you help me?