aweltsch commented on code in PR #7021:
URL: https://github.com/apache/arrow-rs/pull/7021#discussion_r1952068132
##########
arrow-cast/src/cast/decimal.rs:
##########
@@ -99,10 +100,24 @@ where
I::Native: DecimalCast + ArrowNativeTypeOp,
O::Native: DecimalCast + ArrowNativeTypeOp,
{
+ validate_decimal_precision_and_scale::<O>(output_precision, output_scale)?;
Review Comment:
The validation inside the `is_infallible_cast` branch is conceptually a
little different to what is going on in the other branches. We can check the
validity _before running the kernel_. It would look something like this
```rust
if is_infallible_cast {
let precision_check =
validate_decimal_precision_and_scale::<O>(output_precision,
output_scale);
if precision_check.is_err() {
if cast_options.safe {
return Ok(array.unary_opt(|_| None));
}
precision_check?
}
let g = |x: I::Native| f(x).unwrap(); // unwrapping is safe since
the result is guaranteed
// to fit into the target type
array.unary(g)
```
so the performance on the benchmarks is not affected. It might be affected
if the output_precision / output_scale are invalid, but even in that case I
think it will be faster (though I didn't run a benchmark yet).
The main problem is really that it is potentially possible to call the
functions `convert_to_smaller_scale_decimal` or
`convert_to_bigger_or_equal_scale_decimal` with an invalid `output_precision`
(e.g. precision 39 for `Decimal128Type`).
This would be an example:
```rust
convert_to_smaller_scale_decimal::<Decimal256Type, Decimal128Type>(
&array,
50, // input scale: valid
38, // input_precision: valid
39, // output_precision: invalid, DECIMAL128_MAX_PRECISION =
38
37, // output_scale: valid
&CastOptions {
safe: true,
..Default::default()
},
);
```
In the current version on the main branch this would return an array of all
nulls, since the `output_precision` is invalid for every entry in the array.
From my POV there's already a precondition that has failed, when you try to
convert to a decimal type with a precision outside of the allowed range, but to
stay consistent with the current behaviour on the main branch, I think we need
some special handling in the `is_infallible_cast` branch.
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