Previously we introduced the following index (not sure if that would be a good name, maybe aggregation or projection?):
Let us consider how it works from the point of view of the system. The easy case is when we have just one event to work on. We are going to run it through the map and then through the reduce, and that would be about it.
What about the next step? Well, on the second event, all we actually need to do is run it through the map, then run it and the previous result through the reduce. The only thing we need to remember is the final result. No need to remember all the pesky details in the middle, because we don’t have the notion of updates / deletes to require them.
This make the entire process so simple it is ridiculous. I am actually looking forward to doing this, if only because I have to dig through a lot of complexity to get RavenDB’s map/reduce’s indexes to where they are now.