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 : <collection>, map : <mapfunction>, reduce : <reducefunction> [, query : <query filter object>] [, sort : <sort the query.useful for optimization>] [, limit : <number of objects to return from collection>] [, out : <output-collection name>] [, keeptemp: <true|false>] [, finalize : <finalizefunction>] [, scope : <object where fields go into javascript global 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<values.length;i++) ... total += values[i].count; ... return {count:total}; ... }; function (key, values) { var total = 0; for (var i = 0; i < values.length; i++) { total += values[i].count; } return {count:total}; } > 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[res.result].find() { "_id" : "cat", "value" : { "count" : 3 } } { "_id" : "dog", "value" : { "count" : 2 } } { "_id" : "mouse", "value" : { "count" : 1 } } > db[res.result].drop() true > db[res.result].find() > 以下有几个MapReduce的参考例子：http://www.mongodb.org/display/DOCS/MapReducehttp://github.com/mongodb/mongo/ ... sts/mr_bigobject.jshttp://github.com/mongodb/mongo/blob/master/jstests/mr5.jshttp://github.com/mongodb/mongo/blob/master/jstests/mr4.jshttp://github.com/mongodb/mongo/blob/master/jstests/mr3.jshttp://github.com/mongodb/mongo/blob/master/jstests/mr2.jshttp://github.com/mongodb/mongo/blob/master/jstests/mr1.js 本文转自 不得闲 博客园博客，原文链接： http://www.cnblogs.com/DxSoft/archive/2010/10/21/1857367.html，如需转载请自行联系原作者
