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1详细说明

https://docs.mongodb.org/manual/core/map-reduce-sharded-collections/

2.out 设置sharded:true

db.coll.mapReduce(map, reduce, {
   out: {
   reduce: "events", "sharded": true}})

https://docs.mongodb.org/manual/reference/command/mapReduce/#dbcmd.mapReduce

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