Dandandan commented on code in PR #21154:
URL: https://github.com/apache/datafusion/pull/21154#discussion_r2990006154
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datafusion/functions-aggregate/src/string_agg.rs:
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@@ -315,10 +323,134 @@ fn filter_index<T: Clone>(values: &[T], index: usize) ->
Vec<T> {
.collect::<Vec<_>>()
}
-/// StringAgg accumulator for the simple case (no order or distinct specified)
-/// This accumulator is more efficient than `StringAggAccumulator`
-/// because it accumulates the string directly,
-/// whereas `StringAggAccumulator` uses `ArrayAggAccumulator`.
+/// GroupsAccumulator for `string_agg` without DISTINCT or ORDER BY.
+#[derive(Debug)]
+struct StringAggGroupsAccumulator {
+ /// The delimiter placed between concatenated values.
+ delimiter: String,
+ /// Accumulated string per group. `None` means no values have been seen
+ /// (the group's output will be NULL).
+ values: Vec<Option<String>>,
Review Comment:
Perhaps he values can be collected first to a single buffer first and
collected afterwards (from offsets / lengths).
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