Function unnest_list_array

datafusion/physical-plan/src/unnest.rs:926–957

Unnest a list array according the target length array. Consider a list array like this: ```ignore [1], [2, 3, 4], null, [5], [], ``` and the length array is: ```ignore [2, 3, 2, 1, 2] ``` If the length of a certain list is less than the target length, pad with NULLs. So the unnested array will look like this: ```ignore [1, null, 2, 3, 4, null, null, 5, null, null] ```

(
    list_array: &dyn ListArrayType,
    length_array: &PrimitiveArray<Int64Type>,
    capacity: usize,
)

/// [1, null, 2, 3, 4, null, null, 5, null, null]
/// ```
fn unnest_list_array(
    list_array: &dyn ListArrayType,
    length_array: &PrimitiveArray<Int64Type>,
    capacity: usize,
) -> Result<ArrayRef> {
    let values = list_array.values();
    let mut take_indices_builder = PrimitiveArray::<Int64Type>::builder(capacity);
    for row in 0..list_array.len() {
        let mut value_length = 0;
        if !list_array.is_null(row) {
            let (start, end) = list_array.value_offsets(row);
            value_length = end - start;
            for i in start..end {
                take_indices_builder.append_value(i)
            }
        }
        let target_length = length_array.value(row);
        debug_assert!(
            value_length <= target_length,
            "value length is beyond the longest length"
        );
        // Pad with NULL values
        for _ in value_length..target_length {
            take_indices_builder.append_null();
        }
    }
    Ok(kernels::take::take(
        &values,
        &take_indices_builder.finish(),
        None,
    )?)
}

/// Creates take indices that will be used to expand all columns except for the list type
/// [`columns`](UnnestExec::list_column_indices) that is being unnested.