Function union_extract_by_type 

pub fn union_extract_by_type(
    union_array: &UnionArray,
    target_type: &DataType,
    cast_options: &CastOptions<'_>,
) -> Result<ArrayRef, ArrowError>

Extracts the best-matching child array from a [UnionArray] for a given target type, and casts it to that type.

Rows where a different child array is active become NULL. If no child array matches, returns an error.

Example

let fields = UnionFields::try_new(
    [0, 1],
    [
        Field::new("int", DataType::Int32, true),
        Field::new("str", DataType::Utf8, true),
    ],
).unwrap();

let union = UnionArray::try_new(
    fields,
    vec![0, 1, 0].into(),
    None,
    vec![
        Arc::new(Int32Array::from(vec![Some(42), None, Some(99)])),
        Arc::new(StringArray::from(vec![None, Some("hello"), None])),
    ],
)
.unwrap();

// extract the Utf8 child array and cast to Utf8View
let result = union_extract_by_type(&union, &DataType::Utf8View, &CastOptions::default()).unwrap();
assert_eq!(result.data_type(), &DataType::Utf8View);
assert!(result.is_null(0));   // Int32 row -> NULL
assert!(!result.is_null(1));  // Utf8 row -> "hello"
assert!(result.is_null(2));   // Int32 row -> NULL