flatten should support Datatype::FixedSizeList

[universalmind303]

Describe the bug

flatten only supports List not FixedSizeList

To Reproduce

select flatten(some_fixed_size_list);

Error: Internal error: Not reachable, data_type should be List.
This was likely caused by a bug in DataFusion's code and we would welcome that you file an bug report in our issue tracker

Expected behavior

flatten behaves the same on both List and FixedSizeList

Additional context

 BuiltinScalarFunction::Flatten => {
                fn get_base_type(data_type: &DataType) -> Result<DataType> {
                    match data_type {
                        DataType::List(field) => match field.data_type() {
                            DataType::List(_) => get_base_type(field.data_type()),
                            _ => Ok(data_type.to_owned()),
                        },
                        _ => internal_err!("Not reachable, data_type should be List"),
                    }
                }

                let data_type = get_base_type(&input_expr_types[0])?;
                Ok(data_type)
            }

probably just need to add a branch here for FixedSizeList

[Weijun-H]

This issue can be fixed by #9108

[alamb]

This issue can be fixed by #9108

Thanks @Weijun-H -- I will put #9108 on my review queue

[jonahgao]

Close it because it is already supported.

DataFusion CLI v38.0.0

> select flatten(arrow_cast(make_array([[1], [2]], [[3], [4]]), 'FixedSizeList(2, List(List(Int64)))')) as a;
+--------------+
| a            |
+--------------+
| [1, 2, 3, 4] |
+--------------+
1 row(s) fetched.
Elapsed 0.011 seconds.