mongodb explain总是将millis返回为0

Question

我正在使用node.js mongodb本机驱动程序，我有一个包含大量文档的集合，如：

{
    name: "cat",
}

我有一个查询，我试图测试速度：

collection.find({name: { $gt: 'ca', $lt: 'cb' } }).explain(function(err, docs){
    console.log( docs );
});

但是毫秒总是0

 { cursor: 'BasicCursor',
  isMultiKey: false,
  n: 2,
  nscannedObjects: 8,
  nscanned: 8,
  nscannedObjectsAllPlans: 8,
  nscannedAllPlans: 8,
  scanAndOrder: false,
  indexOnly: false,
  nYields: 0,
  nChunkSkips: 0,
  millis: 0,
  allPlans: 
   [ { cursor: 'BasicCursor',
       isMultiKey: false,
       n: 2,
       nscannedObjects: 8,
       nscanned: 8,
       scanAndOrder: false,
       indexOnly: false,
       nChunkSkips: 0 } ],
  server: 'h003723.mongolab.com:33453',
  filterSet: false,
  stats: 
   { type: 'COLLSCAN',
     works: 10,
     yields: 0,
     unyields: 0,
     invalidates: 0,
     advanced: 2,
     needTime: 7,
     needFetch: 0,
     isEOF: 1,
     docsTested: 8,
     children: [] } }

我似乎无法弄清楚这一点，显然我不能像这样测试这个查询，我错过了什么吗？