MongoDB mapreduce

🏷️ MongoDB

实现功能

生成一份报表，对每个国家中，包含同样数字的电话号码进行计数。

实现方法

• 首先，创建一个辅助函数。它抽取所有不同的数字，构成一个数组。

  点击查看代码

  > distinctDigits = function(phone) {
  ...         var 
  ...                 number = phone.components.number + '',
  ...                 seen = ,
  ...                 result = ,
  ...                 i = number.length;
  ...         while(i--) {
  ...                 seen[+number] = 1;
  ...         }
  ...         for(i=0; i<10; i++) {
  ...                 if (seen[i]) {
  ...                         result[result.length] = i;
  ...                 }
  ...         }
  ...         return result;
  ... }
  function (phone) {
          var 
                  number = phone.components.number + '',
                  seen = ,
                  result = ,
                  i = number.length;
          while(i--) {
                  seen[+number[i]] = 1;
          }
          for(i=0; i<10; i++) {
                  if (seen[i]) {
                          result[result.length] = i;
                  }
          }
          return result;
  }
  > db.system.js.save({_id : 'distinctDigits', value: distinctDigits})
  WriteResult({
          "nMatched" : 0,
          "nUpserted" : 1,
          "nModified" : 0,
          "_id" : "distinctDigits"
  })
  > db.eval("distinctDigits(db.phones.findOne({ 'components.number': 5551213 }))")
  WARNING: db.eval is deprecated
  [ 1, 2, 3, 5 ]

• 其次，准备 map 和 reduce 函数

  点击查看代码

  > map = function() {
  ...         var digits = distinctDigits(this);
  ...         emit({ digits : digits, country : this.components.country }, { count : 1 });
  ... }
  function () {
          var digits = distinctDigits(this);
          emit({ digits : digits, country : this.components.country }, { count : 1 });
  }
  > reduce = 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 };
  }

• 最后，执行 mapreduce 操作

  点击查看代码

  > results = db.runCommand({
  ...         mapReduce: 'phones',
  ...         map: map,
  ...         reduce: reduce,
  ...         out: 'phones.report'
  ... });
  {
          "result" : "phones.report",
          "timeMillis" : 9690,
          "counts" : {
                  "input" : 316229,
                  "emit" : 316229,
                  "reduce" : 76620,
                  "output" : 3491
          },
          "ok" : 1
  }

• 因为通过 out 参数设置了集合的名称（ out: 'phones.report' ），所以可以像其它集合那样查询结果。

  点击查看查询结果

  > db.phones.report.find({'_id.country' : 8})
  { "_id" : { "digits" : [ 0, 1, 2, 3, 4, 5, 6 ], "country" : 8 }, "value" : { "count" : 46 } }
  { "_id" : { "digits" : [ 0, 1, 2, 3, 5 ], "country" : 8 }, "value" : { "count" : 5 } }
  { "_id" : { "digits" : [ 0, 1, 2, 3, 5, 6 ], "country" : 8 }, "value" : { "count" : 170 } }
  { "_id" : { "digits" : [ 0, 1, 2, 3, 5, 6, 7 ], "country" : 8 }, "value" : { "count" : 36 } }
  { "_id" : { "digits" : [ 0, 1, 2, 3, 5, 6, 8 ], "country" : 8 }, "value" : { "count" : 33 } }
  { "_id" : { "digits" : [ 0, 1, 2, 3, 5, 6, 9 ], "country" : 8 }, "value" : { "count" : 33 } }
  { "_id" : { "digits" : [ 0, 1, 2, 3, 5, 7 ], "country" : 8 }, "value" : { "count" : 8 } }
  { "_id" : { "digits" : [ 0, 1, 2, 3, 5, 8 ], "country" : 8 }, "value" : { "count" : 5 } }
  { "_id" : { "digits" : [ 0, 1, 2, 3, 5, 9 ], "country" : 8 }, "value" : { "count" : 13 } }
  { "_id" : { "digits" : [ 0, 1, 2, 4, 5 ], "country" : 8 }, "value" : { "count" : 12 } }
  { "_id" : { "digits" : [ 0, 1, 2, 4, 5, 6 ], "country" : 8 }, "value" : { "count" : 162 } }
  { "_id" : { "digits" : [ 0, 1, 2, 4, 5, 6, 7 ], "country" : 8 }, "value" : { "count" : 31 } }
  { "_id" : { "digits" : [ 0, 1, 2, 4, 5, 6, 8 ], "country" : 8 }, "value" : { "count" : 28 } }
  { "_id" : { "digits" : [ 0, 1, 2, 4, 5, 6, 9 ], "country" : 8 }, "value" : { "count" : 38 } }
  { "_id" : { "digits" : [ 0, 1, 2, 4, 5, 7 ], "country" : 8 }, "value" : { "count" : 5 } }
  { "_id" : { "digits" : [ 0, 1, 2, 4, 5, 8 ], "country" : 8 }, "value" : { "count" : 8 } }
  { "_id" : { "digits" : [ 0, 1, 2, 4, 5, 9 ], "country" : 8 }, "value" : { "count" : 10 } }
  { "_id" : { "digits" : [ 0, 1, 2, 5 ], "country" : 8 }, "value" : { "count" : 24 } }
  { "_id" : { "digits" : [ 0, 1, 2, 5, 6 ], "country" : 8 }, "value" : { "count" : 264 } }
  { "_id" : { "digits" : [ 0, 1, 2, 5, 6, 7 ], "country" : 8 }, "value" : { "count" : 139 } }
  Type "it" for more

  输入 it 可以继续遍历结果集。