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fix: Avoid unnecessary type casts in concat_ws #20436

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Merged
alamb merged 3 commits into from
Mar 11, 2026
+380 −68
Merged

fix: Avoid unnecessary type casts in concat_ws #20436

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b976e16
Initial work
neilconway Feb 19, 2026
51a7e87
Fix bug with NULL separators, add tests
neilconway Feb 19, 2026
9c0b4f4
cargo fmt
neilconway Feb 19, 2026
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42 changes: 19 additions & 23 deletions datafusion/functions/src/string/concat.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -88,37 +88,33 @@ impl ScalarUDFImpl for ConcatFunc {
&self.signature
}

/// Match the return type to the input types to avoid unnecessary casts. On
/// mixed inputs, prefer Utf8View; prefer LargeUtf8 over Utf8 to avoid
/// potential overflow on LargeUtf8 input.
fn return_type(&self, arg_types: &[DataType]) -> Result<DataType> {
use DataType::*;
let mut dt = &Utf8;
arg_types.iter().for_each(|data_type| {
if data_type == &Utf8View {
dt = data_type;
}
if data_type == &LargeUtf8 && dt != &Utf8View {
dt = data_type;
}
});

Ok(dt.to_owned())
if arg_types.contains(&Utf8View) {
Ok(Utf8View)
} else if arg_types.contains(&LargeUtf8) {
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@Omega359 Omega359 Feb 25, 2026

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I had a thought about this. I think LargeUtf8 should take precedence over Utf8View because you cannot necessarily fit data from a LargeUtf8 column into Utf8View (i64 vs i32) https://docs.rs/arrow/latest/arrow/datatypes/enum.DataType.html#variant.LargeUtf8

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@neilconway neilconway Feb 25, 2026

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Interesting point. I believe the typical precedence today is Utf8View > LargeUtf8 > Utf8, partly on the grounds that "StringArray to StringViewArray is cheap but not vice versa". I can see arguments for both sides; if we want to reconsider this, seems like a distinct issue?

datafusion/datafusion/expr-common/src/type_coercion/binary.rs

Lines 1663 to 1667 in e894a03

/// Coercion rules for string view types (Utf8/LargeUtf8/Utf8View):
/// If at least one argument is a string view, we coerce to string view
/// based on the observation that StringArray to StringViewArray is cheap but not vice versa.
///
/// Between Utf8 and LargeUtf8, we coerce to LargeUtf8.

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@Omega359 Omega359 Feb 26, 2026

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Likely it should be another issue as it likely occurs in a few places. I am fairly certain I am correct on the proper type ordering here but in the wild I doubt it would be encountered much - just how many columns would have > 2 billion bytes?

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@Omega359 Omega359 Feb 26, 2026

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Also, this is really only pertinent in areas that pick a return type based on multiple columns. For the typical case where the udf is operating on a single column the existing logic should be fine - such as in btrim

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@alamb alamb Feb 26, 2026

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I think LargeUtf8 should take precedence over Utf8View because you cannot necessarily fit data from a LargeUtf8 column into Utf8View (i64 vs i32)

I think the only type of data that can't be stored in a Utf8View that a LargeUtf8 an handle is individual strings that are longer than 2GB

Otherwise, data from a LargeUtf8 will work just fine in Utf8View (the view will have multiple buffers rather than one large one)

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@Omega359 Omega359 Feb 26, 2026

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I think LargeUtf8 should take precedence over Utf8View because you cannot necessarily fit data from a LargeUtf8 column into Utf8View (i64 vs i32)

I think the only type of data that can't be stored in a Utf8View that a LargeUtf8 an handle is individual strings that are longer than 2GB

Otherwise, data from a LargeUtf8 will work just fine in Utf8View (the view will have multiple buffers rather than one large one)

Indeed, that was the point I was trying to get across. It's a rare ... but possible. Though honestly I expect DF would fail somewhere else pretty quickly if a column with data that big was ever encountered.

Ok(LargeUtf8)
} else {
Ok(Utf8)
}
}

/// Concatenates the text representations of all the arguments. NULL arguments are ignored.
/// concat('abcde', 2, NULL, 22) = 'abcde222'
fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
let ScalarFunctionArgs { args, .. } = args;

let mut return_datatype = DataType::Utf8;
args.iter().for_each(|col| {
if col.data_type() == DataType::Utf8View {
return_datatype = col.data_type();
}
if col.data_type() == DataType::LargeUtf8
&& return_datatype != DataType::Utf8View
{
return_datatype = col.data_type();
}
});
let return_datatype = if args.iter().any(|c| c.data_type() == DataType::Utf8View)
{
DataType::Utf8View
} else if args.iter().any(|c| c.data_type() == DataType::LargeUtf8) {
DataType::LargeUtf8
} else {
DataType::Utf8
};

let array_len = args.iter().find_map(|x| match x {
ColumnarValue::Array(array) => Some(array.len()),
Expand Down Expand Up @@ -247,7 +243,7 @@ impl ScalarUDFImpl for ConcatFunc {
builder.append_offset();
}

let string_array = builder.finish();
let string_array = builder.finish(None);
Ok(ColumnarValue::Array(Arc::new(string_array)))
}
DataType::LargeUtf8 => {
Expand Down
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