Support casting a FixedSizedList<T>[1] to T

arrow-array/src/array/fixed_size_list_array.rs

@@ -183,7 +183,8 @@ impl FixedSizeListArray {
                 || nulls
                     .as_ref()
                     .map(|n| n.expand(size as _).contains(&a))
-                    .unwrap_or_default();
+                    .unwrap_or_default()
+                || (nulls.is_none() && a.null_count() == 0);
 
             if !nulls_valid {
                 return Err(ArrowError::InvalidArgumentError(format!(

arrow-cast/src/cast/list.rs

@@ -41,6 +41,15 @@ pub(crate) fn cast_values_to_fixed_size_list(
     Ok(Arc::new(list))
 }
 
+pub(crate) fn cast_single_element_fixed_size_list_to_values(
+    array: &dyn Array,
+    to: &DataType,
+    cast_options: &CastOptions,
+) -> Result<ArrayRef, ArrowError> {
+    let values = array.as_fixed_size_list().values();
+    cast_with_options(values, to, cast_options)
+}
+
 pub(crate) fn cast_fixed_size_list_to_list<OffsetSize>(
     array: &dyn Array,
 ) -> Result<ArrayRef, ArrowError>

arrow-cast/src/cast/mod.rs

@@ -153,6 +153,8 @@ pub fn can_cast_types(from_type: &DataType, to_type: &DataType) -> bool {
         (_, LargeList(list_to)) => can_cast_types(from_type, list_to.data_type()),
         (_, FixedSizeList(list_to,size)) if *size == 1 => {
             can_cast_types(from_type, list_to.data_type())},
+        (FixedSizeList(list_from,size), _) if *size == 1 => {
+            can_cast_types(list_from.data_type(), to_type)},
         // cast one decimal type to another decimal type
         (Decimal128(_, _), Decimal128(_, _)) => true,
         (Decimal256(_, _), Decimal256(_, _)) => true,
@@ -703,6 +705,9 @@ pub fn cast_with_options(
         (_, FixedSizeList(ref to, size)) if *size == 1 => {
             cast_values_to_fixed_size_list(array, to, *size, cast_options)
         }
+        (FixedSizeList(_, size), _) if *size == 1 => {
+            cast_single_element_fixed_size_list_to_values(array, to_type, cast_options)
+        }
         (Decimal128(_, s1), Decimal128(p2, s2)) => {
             cast_decimal_to_decimal_same_type::<Decimal128Type>(
                 array.as_primitive(),
@@ -6660,6 +6665,85 @@ mod tests {
         assert_eq!(&expect.value(0), &actual.value(0));
     }
 
+    #[test]
+    fn test_cast_single_element_fixed_size_list() {
+        // FixedSizeList<T>[1] => T
+        let from_array = Arc::new(FixedSizeListArray::from_iter_primitive::<Int16Type, _, _>(
+            [(Some([Some(5)]))],
+            1,
+        )) as ArrayRef;
+        let casted_array = cast(&from_array, &DataType::Int32).unwrap();
+        let actual: &Int32Array = casted_array.as_primitive();
+        let expected = Int32Array::from(vec![Some(5)]);
+        assert_eq!(&expected, actual);
+
+        // FixedSizeList<T>[1] => FixedSizeList<U>[1]
+        let from_array = Arc::new(FixedSizeListArray::from_iter_primitive::<Int16Type, _, _>(
+            [(Some([Some(5)]))],
+            1,
+        )) as ArrayRef;
+        let to_field = Arc::new(Field::new("dummy", DataType::Float32, false));
+        let actual = cast(&from_array, &DataType::FixedSizeList(to_field.clone(), 1)).unwrap();
+        let expected = Arc::new(FixedSizeListArray::new(
+            to_field.clone(),
+            1,
+            Arc::new(Float32Array::from(vec![Some(5.0)])) as ArrayRef,
+            None,
+        )) as ArrayRef;
+        assert_eq!(*expected, *actual);
+
+        // FixedSizeList<T>[1] => FixedSizeList<FixdSizedList<U>[1]>[1]
+        let from_array = Arc::new(FixedSizeListArray::from_iter_primitive::<Int16Type, _, _>(
+            [(Some([Some(5)]))],
+            1,
+        )) as ArrayRef;
+        let to_field_inner = Arc::new(Field::new("item", DataType::Float32, false));
+        let to_field = Arc::new(Field::new(
+            "dummy",
+            DataType::FixedSizeList(to_field_inner.clone(), 1),
+            false,
+        ));
+        let actual = cast(&from_array, &DataType::FixedSizeList(to_field.clone(), 1)).unwrap();
+        let expected = Arc::new(FixedSizeListArray::new(
+            to_field.clone(),
+            1,
+            Arc::new(FixedSizeListArray::new(
+                to_field_inner.clone(),
+                1,
+                Arc::new(Float32Array::from(vec![Some(5.0)])) as ArrayRef,
+                None,
+            )) as ArrayRef,
+            None,
+        )) as ArrayRef;
+        assert_eq!(*expected, *actual);
+
+        // T => FixedSizeList<T>[1] (non-nullable)
+        let field = Arc::new(Field::new("dummy", DataType::Float32, false));
+        let from_array = Arc::new(Int8Array::from(vec![Some(5)])) as ArrayRef;
+        let casted_array = cast(&from_array, &DataType::FixedSizeList(field.clone(), 1)).unwrap();
+        let actual = casted_array.as_fixed_size_list();
+        let expected = Arc::new(FixedSizeListArray::new(
+            field.clone(),
+            1,
+            Arc::new(Float32Array::from(vec![Some(5.0)])) as ArrayRef,
+            None,
+        )) as ArrayRef;
+        assert_eq!(expected.as_ref(), actual);
+
+        // T => FixedSizeList<T>[1] (nullable)
+        let field = Arc::new(Field::new("nullable", DataType::Float32, true));
+        let from_array = Arc::new(Int8Array::from(vec![None])) as ArrayRef;
+        let casted_array = cast(&from_array, &DataType::FixedSizeList(field.clone(), 1)).unwrap();
+        let actual = casted_array.as_fixed_size_list();
+        let expected = Arc::new(FixedSizeListArray::new(
+            field.clone(),
+            1,
+            Arc::new(Float32Array::from(vec![None])) as ArrayRef,
+            None,
+        )) as ArrayRef;
+        assert_eq!(expected.as_ref(), actual);
+    }
+
     #[test]
     fn test_cast_list_containers() {
         // large-list to list