mzabaluev commented on code in PR #23648:
URL: https://github.com/apache/datafusion/pull/23648#discussion_r3602302229
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datafusion/physical-plan/src/aggregates/group_values/multi_group_by/list.rs:
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@@ -218,20 +214,11 @@ impl<O: OffsetSizeTrait> GroupColumn for
ListGroupValueBuilder<O> {
let cut_offset = self.offsets[n];
let cut = cut_offset.as_usize();
- // First-n offsets: 0, off[1], ..., off[n].
- let first_n_offsets: Vec<O> = self.offsets[..=n].to_vec();
-
- // Remaining offsets shifted so that what was offsets[n] becomes 0.
- // Overwrite the array in place.
- // SAFETY: the write range is at most as large as offsets.len().
- // Values in the possible overlap are read before being overwritten.
- unsafe {
- let dst = self.offsets.as_mut_ptr();
- for (i, &off) in self.offsets[n..].iter().enumerate() {
- *dst.add(i) = off - cut_offset;
- }
- }
- self.offsets.truncate(self.offsets.len() - n);
Review Comment:
@KonaeAkira if we're going into micro-benchmarking territory, the previous
code might be better in smaller take case when the remaining vector is drained
in place, because it's subtract-while-copying vs. copy, then subtract in place.
But this highly depends on vectorization, and the other case benefits from a
smaller alloc-and-copy. Maybe there needs to be another split helper with a map
closure to address both cases.
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