实现MongoDB map-reduce
Map-reduce是一种数据处理范例，用于将大量数据压缩为有用的聚合结果。
让我们创建一个包含文档的集合-
> db.demo280.insertOne({"CustomerName":"Chris","isMarried":true});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e49116edd099650a5401a62")
}
> db.demo280.insertOne({"CustomerName":"Mike","isMarried":false});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e491170dd099650a5401a63")
}
> db.demo280.insertOne({"CustomerName":"David","isMarried":false});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e491170dd099650a5401a64")
}
> db.demo280.insertOne({"CustomerName":"Bob","isMarried":true});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e491171dd099650a5401a65")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo280.find();
这将产生以下输出-
{ "_id" : ObjectId("5e49116edd099650a5401a62"), "CustomerName" : "Chris", "isMarried" : true }
{ "_id" : ObjectId("5e491170dd099650a5401a63"), "CustomerName" : "Mike", "isMarried" : false }
{ "_id" : ObjectId("5e491170dd099650a5401a64"), "CustomerName" : "David", "isMarried" : false }
{ "_id" : ObjectId("5e491171dd099650a5401a65"), "CustomerName" : "Bob", "isMarried" : true }以下是实现MongoDBmapreduce的查询-
> db.demo280.mapReduce(
... function() { emit(this.isMarried,true); },
...
... function(key, values) {return Array.sum(values)}, {
... query:{isMarried:true},
... out:"Output"
... }
...)这将产生以下输出，显示总共2个与查询匹配的文档（输入：2个），并发出2个结果（发出：2个）-
{
"result" : "Output",
"timeMillis" : 1241,
"counts" : {
"input" : 2,
"emit" : 2,
"reduce" : 1,
"output" : 1
},
"ok" : 1
}