mingmwang commented on issue #6278:
URL:
https://github.com/apache/arrow-datafusion/issues/6278#issuecomment-1540004152
@viirya @alamb
I think the major problem is that the `i256` math operation is not
efficient.
And the logic in the method `multiply_fixed_point()` in arrow-rs is not
consistent.
If required_scale == product_scale, it still use i128 for multiply(can still
overflow)
if product_scale > required_scale, use i256 for multiply and div, why ??
```rust
pub fn multiply_fixed_point(
left: &PrimitiveArray<Decimal128Type>,
right: &PrimitiveArray<Decimal128Type>,
required_scale: i8,
) -> Result<PrimitiveArray<Decimal128Type>, ArrowError> {
let product_scale = left.scale() + right.scale();
let precision = min(
left.precision() + right.precision() + 1,
DECIMAL128_MAX_PRECISION,
);
if required_scale == product_scale {
return multiply(left, right)?
.with_precision_and_scale(precision, required_scale);
}
if required_scale > product_scale {
return Err(ArrowError::ComputeError(format!(
"Required scale {} is greater than product scale {}",
required_scale, product_scale
)));
}
let divisor =
i256::from_i128(10).pow_wrapping((product_scale - required_scale) as
u32);
binary::<_, _, _, Decimal128Type>(left, right, |a, b| {
let a = i256::from_i128(a);
let b = i256::from_i128(b);
let mut mul = a.wrapping_mul(b);
mul = divide_and_round::<Decimal256Type>(mul, divisor);
mul.as_i128()
})
.and_then(|a| a.with_precision_and_scale(precision, required_scale))
}
```
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