korowa commented on code in PR #13681:
URL: https://github.com/apache/datafusion/pull/13681#discussion_r1933264875
##########
datafusion/functions-aggregate/src/median.rs:
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@@ -230,6 +276,212 @@ impl<T: ArrowNumericType> Accumulator for
MedianAccumulator<T> {
}
}
+/// The median groups accumulator accumulates the raw input values
+///
+/// For calculating the accurate medians of groups, we need to store all values
+/// of groups before final evaluation.
+/// So values in each group will be stored in a `Vec<T>`, and the total group
values
+/// will be actually organized as a `Vec<Vec<T>>`.
+///
+#[derive(Debug)]
+struct MedianGroupsAccumulator<T: ArrowNumericType + Send> {
+ data_type: DataType,
+ group_values: Vec<Vec<T::Native>>,
Review Comment:
There was some improvements, but overall results for clickbench q9 (I was
mostly looking at this query) were like x2.63 for GroupsAccumulator, and x2.30
for the regular Accumulator -- so it would be like 13-15% overall difference,
which is not as massive as this PR results.
However, maybe things has changed in GroupsAccumulator implementation, and
now even plain `Vec<HashSet<>>` will be way faster.
UPD: and, yes, maybe producing state, as pointed out by @alamb above, was
(at least partially) the cause of non-significant improvement -- in count
distinct it was implemented via `ListArray::from_iter_primitive`
([commit](https://github.com/apache/datafusion/pull/8721/commits/aa7199e1aab401c816e8089d4a4dab79e6e04855)),
instead of building it from single flattened array and its offsets.
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