[GitHub] [arrow] lidavidm commented on a diff in pull request #34267: GH-34266: [C++] Add a pivot_longer node

["lidavidm (via GitHub)" <gi...@apache.org>]

lidavidm commented on code in PR #34267:
URL: https://github.com/apache/arrow/pull/34267#discussion_r1118931541


##########
cpp/src/arrow/compute/exec/options.h:
##########
@@ -582,6 +582,94 @@ class ARROW_EXPORT TableSinkNodeOptions : public ExecNodeOptions {
   bool sequence_output = false;
 };
 
+struct ARROW_EXPORT PivotLongerRowTemplate {
+  PivotLongerRowTemplate(std::vector<std::string> feature_values,
+                         std::vector<std::optional<FieldRef>> measurement_values)
+      : feature_values(std::move(feature_values)),
+        measurement_values(std::move(measurement_values)) {}
+  /// A (typically unique) set of feature values for the template, usually derived from a
+  /// column name
+  ///
+  /// These will be used to populate the feature columns
+  std::vector<std::string> feature_values;
+  /// The fields containing the measurements to use for this row
+  ///
+  /// These will be used to populate the measurement columns.  If nullopt then nulls
+  /// will be inserted for the given value.
+  std::vector<std::optional<FieldRef>> measurement_values;
+};
+
+/// \brief Reshape a table by turning some columns into additional rows
+///
+/// This operation is sometimes also referred to as UNPIVOT
+///
+/// This is typically done when there are multiple observations in each row in order to
+/// transform to a table containing a single observation per row.
+///
+/// For example:
+///
+/// | time | left_temp | right_temp |
+/// | ---- | --------- | ---------- |
+/// | 1    | 10        | 20         |
+/// | 2    | 15        | 18         |
+///
+/// The above table contains two observations per row.  There is an implicit feature
+/// "location" (left vs right) and a measurement "temp".  What we really want is:
+///
+/// | time | location | temp |
+/// | 1    | left     | 10   |

Review Comment:
   ```suggestion
   /// | time | location | temp |
   /// | ---- | -------- | --- |
   /// | 1    | left     | 10   |
   ```



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