gabotechs commented on code in PR #20019:
URL: https://github.com/apache/datafusion/pull/20019#discussion_r2735934863
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datafusion/physical-plan/src/aggregates/mod.rs:
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@@ -144,6 +144,23 @@ pub enum AggregateMode {
/// This mode requires that the input has more than one partition, and is
/// partitioned by group key (like FinalPartitioned).
SinglePartitioned,
+ /// Combine multiple partial aggregations to produce a new partial
+ /// aggregation.
+ ///
+ /// Input is intermediate accumulator state (like Final), but output is
+ /// also intermediate accumulator state (like Partial). This enables
+ /// tree-reduce aggregation strategies where partial results from
+ /// multiple workers are combined in multiple stages before a final
+ /// evaluation.
+ ///
+ /// ```text
+ /// Final
+ /// / \
+ /// PartialReduce PartialReduce
+ /// / \ / \
+ /// Partial Partial Partial Partial
+ /// ```
+ PartialReduce,
Review Comment:
This is looking really good 👍 I don't think a partial reduce implementation
can get cleaner than this.
One question: do you think this could be beneficial for single-node
DataFusion plans at some point? I can imagine how this is useful for
distributed engine, but do you have any thoughts about single-node usage?
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