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new 458b49109a perf: optimize left function by eliminating double chars()
iteration (#19571)
458b49109a is described below
commit 458b49109af58e678520edebbad9fb3edfd26992
Author: Liang-Chi Hsieh <[email protected]>
AuthorDate: Sat Jan 10 16:34:41 2026 +0800
perf: optimize left function by eliminating double chars() iteration
(#19571)
For negative n values, the function was calling string.chars() twice:
1. Once to count total characters
2. Again to take the prefix
This optimization collects chars into a reusable buffer once per row for
the negative n case, eliminating the redundant iteration.
Benchmark results (negative n, which triggers the optimization):
- size=1024: 71.323 µs → 52.760 µs (26.0% faster)
- size=4096: 289.62 µs → 212.23 µs (26.7% faster)
Benchmark results (positive n, minimal overhead):
- size=1024: 24.465 µs → 24.691 µs (0.9% slower)
- size=4096: 96.129 µs → 97.078 µs (1.0% slower)
The dramatic improvement for negative n cases far outweighs the
negligible overhead for positive n cases.
## Which issue does this PR close?
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- Closes #.
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---------
Co-authored-by: Claude Sonnet 4.5 <[email protected]>
Co-authored-by: Andrew Lamb <[email protected]>
---
datafusion/functions/src/unicode/left.rs | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/datafusion/functions/src/unicode/left.rs
b/datafusion/functions/src/unicode/left.rs
index ecff8f8699..db27d900b6 100644
--- a/datafusion/functions/src/unicode/left.rs
+++ b/datafusion/functions/src/unicode/left.rs
@@ -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
+ 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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