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perf: Optimize array_to_string to avoid a copy #20639

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Merged
comphead merged 2 commits into from
Mar 2, 2026
+39 −41
Merged

perf: Optimize array_to_string to avoid a copy #20639

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2bc60c8
perf: Optimize array_to_string
neilconway Mar 2, 2026
4a7984b
Rename `buf` to `w` for consistency with new type
neilconway Mar 2, 2026
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80 changes: 39 additions & 41 deletions datafusion/functions-nested/src/string.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -29,7 +29,7 @@ use datafusion_common::utils::ListCoercion;
use datafusion_common::{DataFusionError, Result, not_impl_err};

use std::any::Any;
use std::fmt::Write;
use std::fmt::{self, Write};

use crate::utils::make_scalar_function;
use arrow::array::{
Expand Down Expand Up @@ -324,7 +324,6 @@ fn generate_string_array<O: OffsetSizeTrait>(
null_strings: &[Option<&str>],
) -> Result<StringArray> {
let mut builder = StringBuilder::with_capacity(list_arr.len(), 0);
let mut buf = String::new();

for ((arr, &delimiter), &null_string) in list_arr
.iter()
Expand All @@ -336,17 +335,16 @@ fn generate_string_array<O: OffsetSizeTrait>(
continue;
};

buf.clear();
let mut first = true;
compute_array_to_string(&mut buf, &arr, delimiter, null_string, &mut first)?;
builder.append_value(&buf);
compute_array_to_string(&mut builder, &arr, delimiter, null_string, &mut first)?;
builder.append_value("");
}

Ok(builder.finish())
}

fn compute_array_to_string(
buf: &mut String,
w: &mut impl Write,
arr: &ArrayRef,
delimiter: &str,
null_string: Option<&str>,
Expand All @@ -358,7 +356,7 @@ fn compute_array_to_string(
for i in 0..$list_array.len() {
if !$list_array.is_null(i) {
compute_array_to_string(
buf,
w,
&$list_array.value(i),
delimiter,
null_string,
Expand All @@ -368,9 +366,9 @@ fn compute_array_to_string(
if *first {
*first = false;
} else {
buf.push_str(delimiter);
w.write_str(delimiter)?;
}
buf.push_str(ns);
w.write_str(ns)?;
}
}
};
Expand Down Expand Up @@ -399,71 +397,69 @@ fn compute_array_to_string(
DataFusionError::from(e)
.context("Casting dictionary to values in compute_array_to_string")
})?;
compute_array_to_string(buf, &values, delimiter, null_string, first)
compute_array_to_string(w, &values, delimiter, null_string, first)
}
Null => Ok(()),
data_type => {
macro_rules! str_leaf {
($ARRAY_TYPE:ident) => {
write_leaf_to_string(
buf,
w,
downcast_arg!(arr, $ARRAY_TYPE),
delimiter,
null_string,
first,
|buf, x: &str| buf.push_str(x),
)
|w, x: &str| w.write_str(x),
)?
};
}
macro_rules! bool_leaf {
($ARRAY_TYPE:ident) => {
write_leaf_to_string(
buf,
w,
downcast_arg!(arr, $ARRAY_TYPE),
delimiter,
null_string,
first,
|buf, x: bool| {
|w, x: bool| {
if x {
buf.push_str("true");
w.write_str("true")
} else {
buf.push_str("false");
w.write_str("false")
}
},
)
)?
};
}
macro_rules! int_leaf {
($ARRAY_TYPE:ident) => {
write_leaf_to_string(
buf,
w,
downcast_arg!(arr, $ARRAY_TYPE),
delimiter,
null_string,
first,
|buf, x| {
|w, x| {
let mut itoa_buf = itoa::Buffer::new();
buf.push_str(itoa_buf.format(x));
w.write_str(itoa_buf.format(x))
},
)
)?
};
}
macro_rules! float_leaf {
($ARRAY_TYPE:ident) => {
write_leaf_to_string(
buf,
w,
downcast_arg!(arr, $ARRAY_TYPE),
delimiter,
null_string,
first,
|buf, x| {
// TODO: Consider switching to a more efficient
// floating point display library (e.g., ryu). This
// might result in some differences in the output
// format, however.
write!(buf, "{}", x).unwrap();
},
)
// TODO: Consider switching to a more efficient
// floating point display library (e.g., ryu). This
// might result in some differences in the output
// format, however.
|w, x| write!(w, "{}", x),
)?
};
}
match data_type {
Expand All @@ -487,7 +483,7 @@ fn compute_array_to_string(
.context("Casting to string in array_to_string")
})?;
return compute_array_to_string(
buf,
w,
&str_arr,
delimiter,
null_string,
Expand All @@ -506,17 +502,18 @@ fn compute_array_to_string(
}

/// Appends the string representation of each element in a leaf (non-list)
/// array to `buf`, separated by `delimiter`. Null elements are rendered
/// array to `w`, separated by `delimiter`. Null elements are rendered
/// using `null_string` if provided, or skipped otherwise. The `append`
/// closure controls how each non-null element is written to the buffer.
fn write_leaf_to_string<'a, A, T>(
buf: &mut String,
/// closure controls how each non-null element is written.
fn write_leaf_to_string<'a, W: Write, A, T>(
w: &mut W,
arr: &'a A,
delimiter: &str,
null_string: Option<&str>,
first: &mut bool,
append: impl Fn(&mut String, T),
) where
append: impl Fn(&mut W, T) -> fmt::Result,
) -> Result<()>
where
&'a A: IntoIterator<Item = Option<T>>,
{
for x in arr {
Expand All @@ -528,14 +525,15 @@ fn write_leaf_to_string<'a, A, T>(
if *first {
*first = false;
} else {
buf.push_str(delimiter);
w.write_str(delimiter)?;
}

match x {
Some(x) => append(buf, x),
None => buf.push_str(null_string.unwrap()),
Some(x) => append(w, x)?,
None => w.write_str(null_string.unwrap())?,
}
}
Ok(())
}

/// String_to_array SQL function
Expand Down