viirya opened a new pull request, #19571:
URL: https://github.com/apache/datafusion/pull/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.
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