3cbe9da3_pReduce___MongoDB_Manual_3_0_5__Description
[Field] sort [Type] document [Description] 
Optional. Sorts the input documents. This option is useful for optimization. For example, specify the sort key to be the same as the emit key so that there are fewer reduce operations. The sort key must be in an existing index for this collection.
mapReduce — MongoDB Manual 3.0.5
Description