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https://issues.apache.org/jira/browse/ARROW-17601?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17600547#comment-17600547
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Yibo Cai commented on ARROW-17601:
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Hmm, there are some difficulties:
- If the total digits of the two input decimal128 types to be multiplied is
larger than 38, the output type cannot be represented by decimal128 without
losing precision. It has to fail.
- Decimal overlow, like floating points, should lead to {{Inf}}, whether
checked or non-checked. Currently we don't support decimal {{Inf}}.
> [C++] Error when creating Expression on Decimal128 types: precision out of
> range
> --------------------------------------------------------------------------------
>
> Key: ARROW-17601
> URL: https://issues.apache.org/jira/browse/ARROW-17601
> Project: Apache Arrow
> Issue Type: Bug
> Components: C++
> Reporter: Neal Richardson
> Assignee: Yibo Cai
> Priority: Major
>
> Reproducer in R:
> {code}
> library(arrow)
> library(dplyr)
> tab <- Table$create(col1 = 1:4, col2 = 5:8)
> tab <- tab$cast(schema(col1 = decimal128(33, 4), col2 = decimal128(15, 2)))
> tab %>% mutate(col1 * col2)
> # Error: Invalid: Decimal precision out of range [1, 38]: 49
> # /Users/me/arrow/cpp/src/arrow/compute/kernels/scalar_arithmetic.cc:1078
> DecimalType::Make(left_type.id(), precision, scale)
> # /Users/me/arrow/cpp/src/arrow/compute/exec/expression.cc:413
> call.kernel->signature->out_type().Resolve(&kernel_context, types)
> {code}
> We don't have this problem integers and floats (see comment below). For
> consistency with the other arithmetic functions, what I would expect would be
> that we would expand the precision as much as we could within Decimal128–in
> this case, Decimal128(38, 6)–and the compute function would either error _if_
> there is an overflow (in the _checked version) or just overflow in the
> non-checked version. But it wouldn't error on determining the output type.
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