MongoDB数据库的MapReduce简单操作（转）
MongoDB也简单的实现了MapReduce的功能来提供分布式的数据查询服务,MapReduce的分布是功能主要用在Shard上db.runCommand(
{ mapreduce : ,
map : ,
reduce :
[, query : ]
[, sort : ]
[, limit : ]
[, out : ]
[, keeptemp: ]
[, finalize : ]
[, scope : ]
[, verbose : true]
}
); 下面是对MapReduce的简单测试
此例子来源于：http://www.mongodb.org/display/DOCS/MapReduce
> db.things.insert({_id:1,tags:['dog','cat']});
> db.things.insert({_id:2,tags:['cat']});
> db.things.insert({_id:3,tags:['mouse','cat','dog']});
> db.things.insert({_id:4,tags:[]});
> m = function(){
... this.tags.forEach(
... function(z){
... emit(z,{count:1});
... }
...);
};
function () {
this.tags.forEach(function (z) {emit(z, {count:1});});
}
> r=function(key,values){
... var total = 0;
... for(var i=0;i res=db.things.mapReduce(m,r);
{
"result" : "tmp.mr.mapreduce_1268577545_1",
"timeMillis" : 25,
"counts" : {
"input" : 4,
"emit" : 6,
"output" : 3
},
"ok" : 1,
"ok" : 1,
}
> res
{
"result" : "tmp.mr.mapreduce_1268577545_1",
"timeMillis" : 25,
"counts" : {
"input" : 4,
"emit" : 6,
"output" : 3
},
"ok" : 1,
"ok" : 1,
}
> db.find()
{ "_id" : "cat", "value" : { "count" : 3 } }
{ "_id" : "dog", "value" : { "count" : 2 } }
{ "_id" : "mouse", "value" : { "count" : 1 } }
> db.drop()
true
> db.find()
> 以下有几个MapReduce的参考例子：
http://www.mongodb.org/display/DOCS/MapReduce
http://github.com/mongodb/mongo/ ... sts/mr_bigobject.js
http://github.com/mongodb/mongo/blob/master/jstests/mr5.js
http://github.com/mongodb/mongo/blob/master/jstests/mr4.js
http://github.com/mongodb/mongo/blob/master/jstests/mr3.js
http://github.com/mongodb/mongo/blob/master/jstests/mr2.js
http://github.com/mongodb/mongo/blob/master/jstests/mr1.js
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