feat(min_max): update min_max_batch_generic to handle raw values from DictionaryArray

- Removed redundant commentary in `min_max.rs` regarding dictionary arrays.
- Improved handling of dictionary array values by explicitly extracting and processing raw values.
- Added assertions in unit tests to validate the results when operating on raw values from the dictionary.

Author: kosiew

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

@@ -823,10 +823,6 @@ pub fn min_batch(values: &ArrayRef) -> Result<ScalarValue> {
 }
 
 /// Finds the min/max by scanning logical rows via `ScalarValue::try_from_array`.
-///
-/// 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 {
@@ -911,7 +907,7 @@ pub fn max_batch(values: &ArrayRef) -> Result<ScalarValue> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use arrow::array::DictionaryArray;
+    use arrow::array::{AsArray, DictionaryArray};
     use std::sync::Arc;
 
     #[test]
@@ -997,9 +993,12 @@ mod tests {
         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 raw_values = array.as_any_dictionary().values();
 
         let min = min_batch(&array)?;
         let max = max_batch(&array)?;
+        let raw_min = min_batch(raw_values)?;
+        let raw_max = max_batch(raw_values)?;
 
         let expected = ScalarValue::Dictionary(
             Box::new(DataType::Int8),
@@ -1008,6 +1007,8 @@ mod tests {
 
         assert_eq!(min, expected);
         assert_eq!(max, expected);
+        assert_eq!(raw_min, ScalarValue::Utf8(Some("aaa".to_string())));
+        assert_eq!(raw_max, ScalarValue::Utf8(Some("zzz".to_string())));
 
         Ok(())
     }