Aggregation operator $sqrt output for NumberDecimal is the same as $exp

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    • Fully Compatible
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    • v4.0, v3.6, v3.4
    • Query 2019-02-25
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      Using the following documents: 

      db.test.insert({value:NumberDecimal("25")});
db.test.insert({value:NumberDecimal(25)});

      Running aggregation operator $sqrt as below: 

      db.test.aggregate([{
    "$addFields":{
        "sqrt": {"$sqrt": "$value"}, 
        "exp": {"$exp": "$value"}, 
    }
}])

      Produces: 

            {
        "_id": ObjectId("..."),
        "value": NumberDecimal("25"),
        "sqrt": NumberDecimal("72004899337.38587252416135146612616"),
        "exp": NumberDecimal("72004899337.38587252416135146612616")
      },
      {
        "_id": ObjectId("..."),
        "value": NumberDecimal("25.0000000000000"),
        "sqrt": NumberDecimal("72004899337.38587252416135146612616"),
        "exp": NumberDecimal("72004899337.38587252416135146612616")
      }

      Note that the output of the $sqrt is the same as the output of $exp.
      Looking at the code, the method Decimal128::squareRoot is exactly the same as Decimal128::exponential :

      Decimal128::squareRoot:
      https://github.com/mongodb/mongo/blob/r4.1.6/src/mongo/platform/decimal128.cpp#L720

      Decimal128::exponential:
      https://github.com/mongodb/mongo/blob/r4.1.6/src/mongo/platform/decimal128.cpp#L627

      A workaround is to convert the decimal to double first i.e. $toDouble, to bypass the squareRoot method call in expression: https://github.com/mongodb/mongo/blob/r4.1.6/src/mongo/db/pipeline/expression.cpp#L4091

      For example: 

      db.test.aggregate([{
    "$addFields":{
        "sqrt": {"$sqrt": {"$toDouble":"$value"}}, 
        "exp": {"$exp": "$value"}, 
    }
}])

            Assignee:
            James Wahlin
            Reporter:
            Wan Bachtiar
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              Created:
              Updated:
              Resolved: