support as_decimal_array api (#1356)

Author: liukun4515

arrow/src/array/cast.rs

@@ -96,13 +96,26 @@ array_downcast_fn!(as_null_array, NullArray);
 array_downcast_fn!(as_struct_array, StructArray);
 array_downcast_fn!(as_union_array, UnionArray);
 array_downcast_fn!(as_map_array, MapArray);
+array_downcast_fn!(as_decimal_array, DecimalArray);
 
 #[cfg(test)]
 mod tests {
     use std::sync::Arc;
 
     use super::*;
 
+    #[test]
+    fn test_as_decimal_array_ref() {
+        let array: DecimalArray = vec![Some(123), None, Some(1111)]
+            .into_iter()
+            .collect::<DecimalArray>()
+            .with_precision_and_scale(10, 2)
+            .unwrap();
+        assert!(!as_decimal_array(&array).is_empty());
+        let result_decimal = as_decimal_array(&array);
+        assert_eq!(result_decimal, &array);
+    }
+
     #[test]
     fn test_as_primitive_array_ref() {
         let array: Int32Array = vec![1, 2, 3].into_iter().map(Some).collect();

arrow/src/array/mod.rs

@@ -513,7 +513,7 @@ pub use self::ord::{build_compare, DynComparator};
 // --------------------- Array downcast helper functions ---------------------
 
 pub use self::cast::{
-    as_boolean_array, as_dictionary_array, as_generic_binary_array,
+    as_boolean_array, as_decimal_array, as_dictionary_array, as_generic_binary_array,
     as_generic_list_array, as_large_list_array, as_largestring_array, as_list_array,
     as_map_array, as_null_array, as_primitive_array, as_string_array, as_struct_array,
     as_union_array,