pitrou commented on code in PR #45562:
URL: https://github.com/apache/arrow/pull/45562#discussion_r1976370007
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cpp/src/arrow/compute/api_aggregate.h:
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@@ -175,6 +175,88 @@ class ARROW_EXPORT TDigestOptions : public FunctionOptions
{
uint32_t min_count;
};
+/// \brief Control Pivot kernel behavior
+///
+/// These options apply to the "pivot_wider" and "hash_pivot_wider" functions.
+///
+/// Constraints:
+/// - The corresponding `Aggregate::target` must have two FieldRef elements;
+/// the first one points to the pivot key column, the second points to the
+/// pivoted data column.
+/// - The pivot key column must be string-like; its values will be matched
+/// against `key_names` in order to dispatch the pivoted data into the
+/// output.
+///
+/// "pivot_wider" example
+/// ---------------------
+///
+/// Assuming the following two input columns with types utf8 and int16
(respectively):
+/// ```
+/// width | 11
+/// height | 13
+/// ```
+/// and the options `PivotWiderOptions(.key_names = {"height", "width"})`
+///
+/// then the output will be a scalar with the type
+/// `struct{"height": int16, "width": int16}`
+/// and the value `{"height": 13, "width": 11}`.
+///
+/// "hash_pivot_wider" example
+/// --------------------------
+///
+/// Assuming the following input with schema
+/// `{"group": int32, "key": utf8, "value": int16}`:
+/// ```
+/// group | key | value
+/// -----------------------------
+/// 1 | height | 11
+/// 1 | width | 12
+/// 2 | width | 13
+/// 3 | height | 14
+/// 3 | depth | 15
+/// ```
+/// and the following settings:
+/// - a hash grouping key "group"
+/// - Aggregate(
+/// .function = "hash_pivot_wider",
+/// .options = PivotWiderOptions(.key_names = {"height", "width"}),
+/// .target = {"key", "value"},
+/// .name = {"properties"})
+///
+/// then the output will have the schema
+/// `{"group": int32, "properties": struct{"height": int16, "width": int16}}`
+/// and the following value:
+/// ```
+/// group | properties
+/// | height | width
+/// -----------------------------
+/// 1 | 11 | 12
+/// 2 | null | 13
+/// 3 | 14 | null
+/// ```
+class ARROW_EXPORT PivotWiderOptions : public FunctionOptions {
Review Comment:
Well, because there is already a node named "pivot_longer".
https://github.com/apache/arrow/blob/0fbf9823542233c5f32c26534c34cc97ce3f0be2/cpp/src/arrow/acero/pivot_longer_node.cc#L4
Besides, I find "pivot_wider" / "pivot_longer" more explanatory than "pivot"
/ "melt" or other alternatives.
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