MongoDB MapReduce原理实战之坑注意点

MongoDB MapReduce分组后的每一组数据如果大于100，那么他每次会取100条，归并完变成1条又返回给emit里面作为输入，继续判断余数是否大于100，大于100，取100条，周而复始，直到最后一次取数不大于100，运算完成后就结束。

需要特别说明的是，如果最后一次虽然不大于100，但是如果是1的话，那么不好意思，他直接就给到finalize了，跳过reduce函数。

原理剖析：

  

 

 

 

下面给出两个相同业务的运算逻辑。

 

第一种：注意costTime += (val.endTime - val.beginTime)+val.costTime;

var m =  function(){
        emit(this.url,{count:1,beginTime:this.rt,endTime:this.rpt,costTime:0})
    }
    var r =function(key,values){
        var cnt=0;
        var costTime = 0;
        var obj;
       values.forEach(function(val){
            if(val.endTime  != null && val.beginTime != null){
                cnt += val.count;
                costTime += (val.endTime - val.beginTime)+val.costTime;
            }
        })
        return {"count":cnt,beginTime:0,endTime:0,"costTime":costTime};
    }
    
    var f =function (key,reduceResult){
         //reduceResult.price =(reduceResult.beginTime).toFixed(2);
         if(reduceResult.count != 0 ){
             
              reduceResult.avgCost =(reduceResult.costTime/reduceResult.count).toFixed(2);
         }
        
         return reduceResult;
    }
    //"url":{"$in":["/agent-fx-web/area/findManageAreas.do","/agent-fx-web/api/distributor/expectCommission.do"]},
    var q = {"rt":{"$gt":ISODate("2017-10-10 07:00:25"),"$lt":ISODate("2017-10-10 07:30:25")}}
    
    db.runCommand({
        mapreduce:"traceIndex",
        map:m,
        reduce:r,
        query:q,
        finalize:f,
        out:{merge:"test_map_reduce_4"}
        }
    )

 第二种：注意totalCount += val.totalCount;

var m =  function(){
        emit(this.url,{"costTime":(this.rpt&&this.rt)?(this.rpt - this.rt):0,"totalCount":1})
    }
    var r =function(key,values){
        var totalCostTime = 0;
        var totalCount = 0;

        values.forEach(function(val){
            //if(val.costTime > 0){
                totalCount += val.totalCount;
                totalCostTime += val.costTime;
            //}
            //console.log(val);
        })
        var result = {"costTime":totalCostTime,"totalCount":totalCount};
        print(result);
        return result;
    }
    
    var f =function (key,reduceResult){
         //reduceResult.price =(reduceResult.beginTime).toFixed(2);
         if(reduceResult.count != 0 ){
              reduceResult.avgCost =(reduceResult.costTime/reduceResult.totalCount).toFixed(2);
         }  
        // db.getCollection("test_map_reduce_5").update({ca:key},result,{upsert:true});
         return reduceResult;
    }
    //"url":{"$in":["/agent-fx-web/area/findManageAreas.do","/agent-fx-web/api/distributor/expectCommission.do"]},
    var q = {"rt":{"$gt":ISODate("2017-10-10 07:00:25"),"$lt":ISODate("2017-10-10 07:30:25")}}
    
    db.runCommand({
        mapreduce:"traceIndex",
        map:m,
        reduce:r,
        query:q,
        finalize:f,
        out:{merge:"test_map_reduce_4"}
        }
    )