相关疑难解决方法(0)

例如,我有这些文件:

{
  "addr": "address1",
  "book": "book1"
},
{
  "addr": "address2",
  "book": "book1"
},
{
  "addr": "address1",
  "book": "book5"
},
{
  "addr": "address3",
  "book": "book9"
},
{
  "addr": "address2",
  "book": "book5"
},
{
  "addr": "address2",
  "book": "book1"
},
{
  "addr": "address1",
  "book": "book1"
},
{
  "addr": "address15",
  "book": "book1"
},
{
  "addr": "address9",
  "book": "book99"
},
{
  "addr": "address90",
  "book": "book33"
},
{
  "addr": "address4",
  "book": "book3"
},
{
  "addr": "address5",
  "book": "book1"
},
{
  "addr": "address77",
  "book": "book11"
},
{ …

地图缩小示例我看到使用像count这样的聚合函数,但是使用map reduce来说出每个类别中前三项的最佳方法是什么.

我假设我也可以使用组功能,但很奇怪,因为他们声明分片环境不能使用group().但是,我实际上也有兴趣看到一个group()示例.