Function array_array_concat

src/expr/src/scalar/func.rs:2846–2947

(
    a: Option<Array<'a>>,
    b: Option<Array<'a>>,
    temp_storage: &'a RowArena,
)

    introduces_nulls = false
)]
fn array_array_concat<'a>(
    a: Option<Array<'a>>,
    b: Option<Array<'a>>,
    temp_storage: &'a RowArena,
) -> Result<Option<Array<'a>>, EvalError> {
    let Some(a_array) = a else {
        return Ok(b);
    };
    let Some(b_array) = b else {
        return Ok(a);
    };

    let a_dims: Vec<ArrayDimension> = a_array.dims().into_iter().collect();
    let b_dims: Vec<ArrayDimension> = b_array.dims().into_iter().collect();

    let a_ndims = a_dims.len();
    let b_ndims = b_dims.len();

    // Per PostgreSQL, if either of the input arrays is zero dimensional,
    // the output is the other array, no matter their dimensions.
    if a_ndims == 0 {
        return Ok(b);
    } else if b_ndims == 0 {
        return Ok(a);
    }

    // Postgres supports concatenating arrays of different dimensions,
    // as long as one of the arrays has the same type as an element of
    // the other array, i.e. `int[2][4] || int[4]` (or `int[4] || int[2][4]`)
    // works, because each element of `int[2][4]` is an `int[4]`.
    // This check is separate from the one below because Postgres gives a
    // specific error message if the number of dimensions differs by more
    // than one.
    // This cast is safe since MAX_ARRAY_DIMENSIONS is 6
    // Can be replaced by .abs_diff once it is stabilized
    // TODO(benesch): remove potentially dangerous usage of `as`.
    #[allow(clippy::as_conversions)]
    if (a_ndims as isize - b_ndims as isize).abs() > 1 {
        return Err(EvalError::IncompatibleArrayDimensions {
            dims: Some((a_ndims, b_ndims)),
        });
    }

    let mut dims;

    // After the checks above, we are certain that:
    // - neither array is zero dimensional nor empty
    // - both arrays have the same number of dimensions, or differ
    //   at most by one.
    match a_ndims.cmp(&b_ndims) {
        // If both arrays have the same number of dimensions, validate
        // that their inner dimensions are the same and concatenate the
        // arrays.
        Ordering::Equal => {
            if &a_dims[1..] != &b_dims[1..] {
                return Err(EvalError::IncompatibleArrayDimensions { dims: None });
            }
            dims = vec![ArrayDimension {