Return reference from UnionArray::child (#2035)

arrow/src/array/array_union.rs

@@ -231,10 +231,10 @@ impl UnionArray {
     ///
     /// Panics if the `type_id` provided is less than zero or greater than the number of types
     /// in the `Union`.
-    pub fn child(&self, type_id: i8) -> ArrayRef {
+    pub fn child(&self, type_id: i8) -> &ArrayRef {
         assert!(0 <= type_id);
         assert!((type_id as usize) < self.boxed_fields.len());
-        self.boxed_fields[type_id as usize].clone()
+        &self.boxed_fields[type_id as usize]
     }
 
     /// Returns the `type_id` for the array slot at `index`.

arrow/src/ipc/writer.rs

@@ -226,7 +226,7 @@ impl IpcDataGenerator {
             }
             DataType::Union(fields, _, _) => {
                 let union = as_union_array(column);
-                for (field, ref column) in fields
+                for (field, column) in fields
                     .iter()
                     .enumerate()
                     .map(|(n, f)| (f, union.child(n as i8)))

arrow/src/util/display.rs

@@ -434,7 +434,7 @@ fn union_to_string(
     let name = fields.get(field_idx).unwrap().name();
 
     let value = array_value_to_string(
-        &list.child(type_id),
+        list.child(type_id),
         match mode {
             UnionMode::Dense => list.value_offset(row) as usize,
             UnionMode::Sparse => row,