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Built-in _stats Function

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    The built-in _stats reduce function produces statistical calculations for the input data.

    As with the _sum function, the corresponding value in the emit call should be a number. The generated statistics include the sum, count, minimum ( min ), maximum ( max ) and sum squared ( sumsqr ) of the input rows.

    For example, using the sales data, a slightly truncated output at group level one shows the same fields in the output value for each of the reduced output rows.

    {
       "rows" : [
          {
             "value" : {
                "count" : 3,
                "min" : 7000,
                "sumsqr" : 699000000,
                "max" : 19000,
                "sum" : 43000
             },
             "key" : [
                "Adam"
             ]
          },
          {
             "value" : {
                "count" : 3,
                "min" : 5000,
                "sumsqr" : 594000000,
                "max" : 20000,
                "sum" : 38000
             },
             "key" : [
                "James"
             ]
          },
          {
             "value" : {
                "count" : 3,
                "min" : 3000,
                "sumsqr" : 542000000,
                "max" : 22000,
                "sum" : 32000
             },
             "key" : [
                "John"
             ]
          }
       ]
    }