ahmed-mez commented on code in PR #18906:
URL: https://github.com/apache/datafusion/pull/18906#discussion_r2556052719
##########
datafusion/physical-plan/src/aggregates/group_values/row.rs:
##########
@@ -206,29 +233,43 @@ impl GroupValues for GroupValuesRows {
output
}
EmitTo::First(n) => {
- let groups_rows = group_values.iter().take(n);
- let output = self.row_converter.convert_rows(groups_rows)?;
- // Clear out first n group keys by copying them to a new Rows.
- // TODO file some ticket in arrow-rs to make this more
efficient?
- let mut new_group_values = self.row_converter.empty_rows(0, 0);
- for row in group_values.iter().skip(n) {
- new_group_values.push(row);
- }
- std::mem::swap(&mut new_group_values, &mut group_values);
-
- self.map.retain(|(_exists_hash, group_idx)| {
- // Decrement group index by n
- match group_idx.checked_sub(n) {
- // Group index was >= n, shift value down
- Some(sub) => {
- *group_idx = sub;
- true
- }
- // Group index was < n, so remove from table
- None => false,
+ if self.drain_mode {
+ let start = self.emission_offset;
+ let end = std::cmp::min(start + n,
group_values.num_rows());
+ let iter = group_values.iter().skip(start).take(end -
start);
+ let output = self.row_converter.convert_rows(iter)?;
+ self.emission_offset = end;
+ if self.emission_offset == group_values.num_rows() {
+ group_values.clear();
+ self.emission_offset = 0;
Review Comment:
The `emission_offset` optimization is just to avoid the expensive "copy
remaining rows" operation that the old `EmitTo::First` path did during input
processing. The real fix is replacing `emit(EmitTo::All)` (which blocks for
seconds on large group counts) with incremental drain
https://github.com/apache/datafusion/pull/18906/files#diff-69c8ecaca5e2c7005f2ed1facaa41f80b45bfd006f2357e53ff3072f535c287dR1196
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