Dandandan commented on code in PR #11718:
URL: https://github.com/apache/datafusion/pull/11718#discussion_r1696758426
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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.
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