docs: update helper documentation in min_max.rs for dictionary routing responsibility

- Clarified caller responsibility for dictionary routing in the helper docs.
- Maintained caller-side correctness routing in min_max.rs at lines 820 and 906.
- Added regression coverage with a new test in min_max.rs at line 996 to ensure min/max are computed from logical rows instead of raw dictionary values, including unreferenced entries and null-key positions.

Author: kosiew

datafusion/functions-aggregate-common/src/min_max.rs

@@ -824,9 +824,9 @@ pub fn min_batch(values: &ArrayRef) -> Result<ScalarValue> {
 
 /// Finds the min/max by scanning logical rows via `ScalarValue::try_from_array`.
 ///
-/// This path is required for dictionary arrays because comparing
-/// `dictionary.values()` is not semantically correct: it can include
-/// unreferenced values and ignore null key positions.
+/// Callers are responsible for routing dictionary arrays to this helper.
+/// Passing `dictionary.values()` is semantically incorrect because it can
+/// include unreferenced dictionary entries and ignore null key positions.
 fn min_max_batch_generic(values: &ArrayRef, ordering: Ordering) -> Result<ScalarValue> {
     let mut index = 0;
     let mut extreme = loop {
@@ -911,6 +911,8 @@ pub fn max_batch(values: &ArrayRef) -> Result<ScalarValue> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use arrow::array::DictionaryArray;
+    use std::sync::Arc;
 
     #[test]
     fn min_max_dictionary_and_scalar_compare_by_inner_value() -> Result<()> {
@@ -989,4 +991,24 @@ mod tests {
         );
         Ok(())
     }
+
+    #[test]
+    fn min_max_batch_dictionary_uses_logical_rows() -> Result<()> {
+        let keys = Int8Array::from(vec![Some(1), None, Some(1), Some(1)]);
+        let values = Arc::new(StringArray::from(vec!["zzz", "bbb", "aaa"]));
+        let array = Arc::new(DictionaryArray::new(keys, values)) as ArrayRef;
+
+        let min = min_batch(&array)?;
+        let max = max_batch(&array)?;
+
+        let expected = ScalarValue::Dictionary(
+            Box::new(DataType::Int8),
+            Box::new(ScalarValue::Utf8(Some("bbb".to_string()))),
+        );
+
+        assert_eq!(min, expected);
+        assert_eq!(max, expected);
+
+        Ok(())
+    }
 }