2010YOUY01 commented on code in PR #14946:
URL: https://github.com/apache/datafusion/pull/14946#discussion_r1976265670
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datafusion/physical-plan/src/aggregates/mod.rs:
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@@ -57,41 +57,53 @@ mod row_hash;
mod topk;
mod topk_stream;
-/// Hash aggregate modes
+/// Aggregation modes
///
/// See [`Accumulator::state`] for background information on multi-phase
/// aggregation and how these modes are used.
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum AggregateMode {
+ /// One of multiple layers of aggregation, any input partitioning
+ ///
/// Partial aggregate that can be applied in parallel across input
/// partitions.
///
/// This is the first phase of a multi-phase aggregation.
Partial,
+ /// *Final* of multiple layers of aggregation, in exactly one partition
+ ///
/// Final aggregate that produces a single partition of output by combining
/// the output of multiple partial aggregates.
///
/// This is the second phase of a multi-phase aggregation.
+ ///
+ /// This mode requires that the input is a single partition
Review Comment:
```suggestion
/// This mode requires that the input is a single partition
///
/// Note: Adjacent `Partial` and `Final` mode aggregation is equivalent
to a `Single`
/// mode aggregation node. The `Final` mode is required since this is
used in an
/// intermediate step. The [`CombinePartialFinalAggregate`] physical
optimizer rule
/// will replace this combination with `Single` mode for more efficient
execution.
```
At first this `Final` mode looks redundant to me, then I found one optimizer
rule to explain
https://github.com/apache/datafusion/blob/main/datafusion/physical-optimizer/src/combine_partial_final_agg.rs
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