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https://issues.apache.org/jira/browse/IMPALA-10654?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Quanlong Huang updated IMPALA-10654:
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Fix Version/s: Impala 4.0.0
> 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
> Components: Backend
> Reporter: Amogh Margoor
> Assignee: Amogh Margoor
> Priority: Major
> Labels: ramp-up
> Fix For: Impala 4.0.0
>
>
> 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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