alamb commented on code in PR #21154:
URL: https://github.com/apache/datafusion/pull/21154#discussion_r2991228968
##########
datafusion/functions-aggregate/src/string_agg.rs:
##########
@@ -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:
> Then assemble the actual results in evaluate() (this is similar to #20504
for array_agg). This would be quite a bit more complicated than this PR,
We also need to ensure we aren't keeping around too much "garbage" (memory)
if we go this approach as well
##########
datafusion/functions-aggregate/src/string_agg.rs:
##########
@@ -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).
Review Comment:
I think it would help to add a link the ticket in the code so others can
more easily find the relevant code/improvement
```suggestion
/// Accumulated string per group. `None` means no values have been seen
/// (the group's output will be NULL).
/// A potential improvement is to avoid this String allocation
/// See <https://github.com/apache/datafusion/issues/21156>
```
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