[jira] [Resolved] (IMPALA-10654) Improve the precision of DecimalValue::ToDouble

["Amogh Margoor (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/IMPALA-10654?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Amogh Margoor resolved IMPALA-10654.
------------------------------------
    Resolution: Fixed

Resolving this JIRA as the patch submitted for this fixes this issue.

> 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
>
> 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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