Map-Reduce是一种计算模型，简单的说就是将大批量的工作（数据）分解（MAP）执行，然后再将结果合并成最终结果（REDUCE）。
MongoDB提供的Map-Reduce非常灵活，以下是常用样例：

1、mongodb mapreduce 查询集合中所有字段类型

db.runCommand({
    "mapreduce" : "collectionName",
    "map" : function () {
        for (var key in this) { 
            emit(key, this); 
        }
    },
    "reduce" :function (key, values) {
        var e = values[1][key];
        var r = typeof e;
       // js判断数据类型 方法二
       // var a = Object.prototype.toString;
       // var r = a.call(e);
        return r;
    }, 
    "out": { inline : 1}
})

2、java 执行mongodb mapreduce 的例子

import java.net.UnknownHostException;
 
import com.mongodb.DB;
import com.mongodb.DBCollection;
import com.mongodb.DBObject;
import com.mongodb.MapReduceCommand;
import com.mongodb.MapReduceOutput;
import com.mongodb.MongoClient;
 
/**
 * Java + MongoDB MapReduce  count()
 * 
 */
public class MapReduce {
 
  public static void main(String[] args) {
 
    // Connect to mongodb
    MongoClient mongo = null;
    try {
        mongo = new MongoClient("localhost", 27017);
    } catch (UnknownHostException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
 
    // get database 
    // if database doesn't exists, mongodb will create it for you
    DB db = mongo.getDB("test");
 
    // get collection
    // if collection doesn't exists, mongodb will create it for you
    DBCollection collection = db.getCollection("person");
 
    String map ="function () {"+
            "emit('size', {count:1});"+
    "}";
 
    String reduce = "function (key, values) { "+
        " total = 0; "+
        " for (var i in values) { "+
            " total += values[i].count; "+
        " } "+
        " return {count:total} }";
 
    MapReduceCommand cmd = new MapReduceCommand(collection, map, reduce,
                     null, MapReduceCommand.OutputType.INLINE, null);
 
    MapReduceOutput out = collection.mapReduce(cmd);
 
    for (DBObject o : out.results()) {
        System.out.println(o.toString());
    }
    System.out.println("Done");
 
  }
}