QinXi-ai commented on code in PR #25988:
URL: https://github.com/apache/datafusion/pull/25988#discussion_r4505364483
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datafusion/optimizer/src/single_distinct_to_groupby.rs:
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@@ -154,12 +158,47 @@ fn is_single_distinct_agg(
if aggregate_count != aggr_expr.len() || fields_set.len() != 1 {
return Ok(false);
}
+ // With one target partition, a native GroupsAccumulator avoids building an
+ // extra aggregate with a row per (group, distinct value) pair. With
multiple
+ // partitions, the rewrite also distributes deduplication by both keys;
+ // keeping DISTINCT instead repartitions lists of values by group and
merges
+ // them in the final accumulator. That can concentrate work on a few
groups.
+ // Even one target partition can have multiple input partitions (e.g. UNION
+ // ALL or an existing partitioned table), requiring Partial/Final state
merging.
+ // This is opt-in: the rule cannot see memory limits or cost the input, and
+ // native DISTINCT must fit each group's distinct values in memory.
+ if use_native_grouped_distinct
+ && distinct_aggs.len() == aggregate_count
+ // An unknown answer must preserve the existing rewrite.
+ && distinct_groups_support(&distinct_aggs, input_schema)?
+ .iter()
+ .all(|support| *support == Some(true))
Review Comment:
Added both EXPLAINs in 764869ea, before the global aggregate case. They pin
the rewrite for mixed aggregates and unknown DISTINCT support with the flag on.
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