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perf: optimize left function by eliminating double chars() iteration #19571

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Merged
viirya merged 3 commits into from
Jan 10, 2026
+12 −3
Merged

perf: optimize left function by eliminating double chars() iteration #19571

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perf: optimize left function by eliminating double chars() iteration
viirya Dec 30, 2025
adee2ed
fix: Avoid abs() panic on i64::MIN in left() function
viirya Jan 4, 2026
745120e
Merge branch 'main' into left_optimize
alamb Jan 9, 2026
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15 changes: 12 additions & 3 deletions datafusion/functions/src/unicode/left.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -139,14 +139,23 @@ fn left_impl<'a, T: OffsetSizeTrait, V: ArrayAccessor<Item = &'a str>>(
n_array: &Int64Array,
) -> Result<ArrayRef> {
let iter = ArrayIter::new(string_array);
let mut chars_buf = Vec::new();
let result = iter
.zip(n_array.iter())
.map(|(string, n)| match (string, n) {
(Some(string), Some(n)) => match n.cmp(&0) {
Ordering::Less => {
let len = string.chars().count() as i64;
Some(if n.abs() < len {
string.chars().take((len + n) as usize).collect::<String>()
// Collect chars once and reuse for both count and take
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@alamb alamb Jan 9, 2026

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You can probably use https://doc.rust-lang.org/std/primitive.str.html#method.char_indices here to avoid copying the strings into chars_buf entirely -- you could just find the correct offset and then copy the relevant bytes

If you wanted to get super fancy, you could add specializations for:

  1. Scalar n (no need to expand it out to an array)
  2. StringViewArray (no need to copy strings at all, you could just adjust the views)

However, this code seems better than what is on main! So we can merge it too and keep iterating

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@viirya viirya Jan 10, 2026

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Yea, let me merge it first and probably improve it later.

chars_buf.clear();
chars_buf.extend(string.chars());
let len = chars_buf.len() as i64;

// For negative n, take (len + n) chars if n > -len (avoiding abs() which panics on i64::MIN)
Some(if n > -len {
chars_buf
.iter()
.take((len + n) as usize)
.collect::<String>()
} else {
"".to_string()
})
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