KonaeAkira commented on code in PR #23648:
URL: https://github.com/apache/datafusion/pull/23648#discussion_r3602428224
##########
datafusion/physical-plan/src/aggregates/group_values/multi_group_by/list.rs:
##########
@@ -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:
One more thing that just occurred to me: `split_vec_min_alloc(&mut
self.offsets, n)` returns a vector with exact capacity in case `n * 2 <=
self.offsets.len()`:
https://github.com/apache/datafusion/blob/fb8fe7ae61efc12c6c09a00d025e35c750e2ee86/datafusion/common/src/utils/mod.rs#L405-L412
in which case `first_n_offsets.push(cut_offset);` will always reallocate,
which is bad.
This affects `bytes.rs` as well.
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