xavlee commented on code in PR #24697:
URL: https://github.com/apache/datafusion/pull/24697#discussion_r3936344921
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datafusion/physical-plan/src/aggregates/order/mod.rs:
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@@ -28,6 +28,50 @@ use crate::InputOrderMode;
pub use full::GroupOrderingFull;
pub use partial::GroupOrderingPartial;
+/// Describes how an aggregate can determine that groups are complete.
+///
+/// This is distinct from [`InputOrderMode`], which describes the ordering of
+/// the input relative to the grouping expressions. Input ordering is one way
+/// to establish a group-completion mode, but the execution machinery only
+/// needs to know when it can safely emit completed groups.
+///
+/// For example, when grouping by `key`, both inputs have fully contiguous
+/// groups within the input partition:
+///
+/// ```text
+/// sorted: A A B B C C
+/// not sorted: C C A A B B
+/// ```
+///
+/// In both cases, once the key changes, the previous key will not appear
again,
+/// so its group is complete and can be emitted.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub(crate) enum GroupCompletionMode {
+ /// No group can be known complete before the input ends.
+ None,
+ /// Rows with the same values at these grouping-expression indices form one
+ /// contiguous range. When those values change, every group in the previous
+ /// range is complete and can be emitted.
+ ///
+ /// For example, with `GROUP BY (a, b)`, `Partial(vec![0])` means all rows
+ /// for each value of `a` are contiguous, while an `(a, b)` tuple may recur
+ /// within that range.
Review Comment:
Exactly
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