Perf fix merged into arrow-rs: the code labeled "fast path" was the slow one
Abhishekcur · x · 2026-08-19
A developer got a performance fix merged into apache/arrow-rs. Dictionary encoding is meant for low-cardinality data — a few distinct values repeated across many rows — and that exact workload was hitting the slower code path.
The funny part: the comment in arrow-cast/src/cast/dictionary.rs called it the "fast path" while claiming the other path incurs unnecessary data copies — the comment was the opposite of reality, and the perf fix targets that mismatched path.
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