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Type:
Bug
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Resolution: Fixed
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Priority:
Critical - P2
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Affects Version/s: 4.0.5, 4.1.6
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Component/s: Aggregation Framework
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None
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Fully Compatible
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ALL
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v4.0, v3.6, v3.4
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Query 2019-02-25
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"},
}
}])