Mingyu Zhong created ARROW-9913:
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Summary: Outputs of Decimal128::FromString depend on presence of
one another
Key: ARROW-9913
URL: https://issues.apache.org/jira/browse/ARROW-9913
Project: Apache Arrow
Issue Type: Bug
Components: C++
Reporter: Mingyu Zhong
Assignee: Mingyu Zhong
[This
block|[https://github.com/apache/arrow/blame/bfac60dd73bffa5f7bcefc890486268036182278/cpp/src/arrow/util/decimal.cc#L365-L373]]
in Decimal128::FromString makes *out depend on whether scale is null, and
makes *scale depend on whether out is null. For example, given an input "1e2",
# if out is not null and scale is not null, then *out is 1
# if out is null and scale is not null, then *scale is -2
# if neither out nor scale is null, then *out is 100 and *scale is 0
It is very counter-intuitive that when an additional output-only pointer is
given for receiving extra info, it alters the value of another output.
Similarly, *precision is also affected by presence of out and scale.
The block of adjustment was added in
[https://github.com/apache/arrow/commit/bfac60dd73bffa5f7bcefc890486268036182278]
for preventing negative scale output
(https://issues.apache.org/jira/browse/ARROW-2177). While the motivation looks
reasonable, the change is not sufficient (see case #2 above).
I think we should make the outputs independent of one-another. For the input
"1e2", *out should be 100 if out is not null, and *scale should be 0 if scale
is not null.
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