Amogh Margoor created IMPALA-10654:
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Summary: Improve the precision of DecimalValue<T>::ToDouble
Key: IMPALA-10654
URL: https://issues.apache.org/jira/browse/IMPALA-10654
Project: IMPALA
Issue Type: Sub-task
Reporter: Amogh Margoor
Assignee: Amogh Margoor
>From discussion of IMPALA-10350, it was noted that
>[DecimalValue<T>::ToDouble|https://github.com/apache/impala/blob/b28da054f3595bb92873433211438306fc22fbc7/be/src/runtime/decimal-value.inline.h#L725]
> is not accurate.
Current approach is:
{code:java}
static_cast<double>(value_) / pow(10.0, scale).
{code}
Inaccuracy is due to fact that only integers from −2^53 to 2^53 can be
represented accurately by double precision without any loss. Hence, above
approach would not work for numbers like -0.43149576573887316. For DecimalValue
representing -0.43149576573887316, value_ would be -43149576573887316 and scale
would be 17. As value_ < -2^53, result would not be accurate.
Hence through discussion in IMPALA-10350, we propose to use thirdparty library
https://github.com/lemire/fast_double_parser, which handles above scenario in a
performant manner. Library's internal representation of Decimal is similar to
the Impala's DecimalValue and function
[compute_float_64|https://github.com/lemire/fast_double_parser/blob/e4f6319bfa9cbc829f7f99ae88c1d2fb205c15e8/include/fast_double_parser.h#L232]
can be used for the conversion.
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