Rachelint commented on code in PR #11718:
URL: https://github.com/apache/datafusion/pull/11718#discussion_r1696770853
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datafusion/physical-plan/src/aggregates/group_values/row.rs:
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@@ -120,12 +120,13 @@ impl GroupValues for GroupValuesRows {
batch_hashes.resize(n_rows, 0);
create_hashes(cols, &self.random_state, batch_hashes)?;
- for (row, &hash) in batch_hashes.iter().enumerate() {
- let entry = self.map.get_mut(hash, |(_hash, group_idx)| {
+ for (row, &target_hash) in batch_hashes.iter().enumerate() {
+ let entry = self.map.get_mut(target_hash, |(exist_hash,
group_idx)| {
// verify that a group that we are inserting with hash is
// actually the same key value as the group in
// existing_idx (aka group_values @ row)
- group_rows.row(row) == group_values.row(*group_idx)
+ target_hash == *exist_hash
Review Comment:
> @Rachelint My understanding is the similar, collision will happen quite
often in `get_mut`, equality of `u64` hashes will be faster than retrieving /
comparing rows. Hash collisions are usually very low, even for high
cardinality, but `RawTable::get_mut` doesn't check for equality itself, just
finds a first without guaranteeing hash values are the same (equality check
should be in the provided equality function). In other implementations we also
check for hash values to be equal first.
Yes, so we can just see improvement in the case like clickbench q32.
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