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Posted to issues-all@impala.apache.org by "Amogh Margoor (Jira)" <ji...@apache.org> on 2021/04/13 08:51:00 UTC
[jira] [Created] (IMPALA-10654) Improve the precision of
DecimalValue::ToDouble
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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