zeroshade commented on code in PR #43957:
URL: https://github.com/apache/arrow/pull/43957#discussion_r1761361053
##########
cpp/src/arrow/type.cc:
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@@ -480,7 +494,7 @@ Result<std::shared_ptr<DataType>> MaybeMergeNumericTypes(
ARROW_ASSIGN_OR_RAISE(const int32_t precision,
MaxDecimalDigitsForInteger(other_type->id()));
ARROW_ASSIGN_OR_RAISE(const auto promoted_decimal,
- DecimalType::Make(promoted_type->id(), precision,
0));
+ DecimalType::Make(promoted_type->id(), precision -
1, 0));
Review Comment:
The values we use for `MaxDecimalDigitsForInteger` is actually 1 +
numeric_limits<T>::digits10 (i.e. for int32/uint32 we return 10 instead of 9,
for int64 we return 19 instead of 18). As such we were passing a precision that
was higher than the maximum precision allowed for Decimal32 and Decimal64 here
(9 and 18 respectively).
it felt safer to me to put the -1 here than to change the value returned in
`MaxDecimalDigitsForInteger` (which is used elsewhere and caused other
complications in tests)
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