MapRedue 的输出

Use reduce, it will calculate your existing results and newly calculated results in the result set using the reduce function (reducer) you provided, and save the final result. The most important thing here is to make the intermediate results output by the mapper have the same format as the final results. Such a reducer is used in two places, once in reduce and once in output. If they have the same format, especially the hierarchy, it will be much easier to write. The reducer can be understood as being only responsible for merging multiple results. If the mapper only outputs an intermediate result, it should be directly used as the final result without going through the reducer. In fact, in MongoDB, when a key corresponds to only one mapper output, it is indeed output directly without going through the reducer.

http://docs.mongodb.org/manual/reference/method/db.collection.mapReduce/#output-to-a-collection-with-an-action

Merge the new result with the existing result if the output collection already exists. If an existing document has the same key as the new result, apply the reduce function to both the new and the existing documents and overwrite the existing document with the result.