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Increase output scale for decimal division #4640

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tustvold wants to merge 2 commits into from
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Increase output scale for decimal division #4640

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f8931b9
Increase output scale for decimal division
tustvold Aug 3, 2023
919acd8
Add doc links
tustvold Aug 3, 2023
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18 changes: 13 additions & 5 deletions arrow-arith/src/numeric.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -738,6 +738,8 @@ fn decimal_op<T: DecimalType>(

// Follow the Hive decimal arithmetic rules
// https://cwiki.apache.org/confluence/download/attachments/27362075/Hive_Decimal_Precision_Scale_Support.pdf
// And the Calcite rules
// https://github.com/apache/arrow/blob/36ddbb531cac9b9e512dfa3776d1d64db588209f/java/gandiva/src/main/java/org/apache/arrow/gandiva/evaluator/DecimalTypeUtil.java#L46
let array: PrimitiveArray<T> = match op {
Op::Add | Op::AddWrapping | Op::Sub | Op::SubWrapping => {
// max(s1, s2)
Expand Down Expand Up @@ -794,9 +796,8 @@ fn decimal_op<T: DecimalType>(
}

Op::Div => {
// Follow postgres and MySQL adding a fixed scale increment of 4
// s1 + 4
let result_scale = s1.saturating_add(4).min(T::MAX_SCALE);
// max(6, s1 + p2 + 1)
let result_scale = 6.max(s1 + *p2 as i8 + 1).min(T::MAX_SCALE);
let mul_pow = result_scale - s1 + s2;

// p1 - s1 + s2 + result_scale
Expand Down Expand Up @@ -1126,10 +1127,17 @@ mod tests {
);

let result = div(&a, &b).unwrap();
assert_eq!(result.data_type(), &DataType::Decimal128(17, 7));
assert_eq!(result.data_type(), &DataType::Decimal128(26, 16));
assert_eq!(
result.as_primitive::<Decimal128Type>().values(),
&[27777, 0, 162078, 11133333, -1300000, 402]
&[
27777777777777,
0,
162078651685393,
11133333333333333,
-1300000000000000,
402684563758
]
);

let result = rem(&a, &b).unwrap();
Expand Down