Function first_value_no_list

src/expr/src/relation/func.rs:915–953

Like `first_value`, but doesn't perform the final wrapping in a list, returning an Iterator instead.

(
    datums: I,
    callers_temp_storage: &'b RowArena,
    order_by: &[ColumnOrder],
    window_frame: &WindowFrame,
)

/// Like `first_value`, but doesn't perform the final wrapping in a list, returning an Iterator
/// instead.
fn first_value_no_list<'a: 'b, 'b, I>(
    datums: I,
    callers_temp_storage: &'b RowArena,
    order_by: &[ColumnOrder],
    window_frame: &WindowFrame,
) -> impl Iterator<Item = Datum<'b>>
where
    I: IntoIterator<Item = Datum<'a>>,
{
    // Sort the datums according to the ORDER BY expressions and return the (OriginalRow, InputValue) record
    let datums = order_aggregate_datums(datums, order_by);

    // Decode the input (OriginalRow, InputValue) into separate datums
    let (orig_rows, args): (Vec<_>, Vec<_>) = datums
        .into_iter()
        .map(|d| {
            let mut iter = d.unwrap_list().iter();
            let original_row = iter.next().unwrap();
            let arg = iter.next().unwrap();

            (original_row, arg)
        })
        .unzip();

    let results = first_value_inner(args, window_frame);

    callers_temp_storage.reserve(results.len());
    results
        .into_iter()
        .zip_eq(orig_rows)
        .map(|(result_value, original_row)| {
            callers_temp_storage.make_datum(|packer| {
                packer.push_list_with(|packer| {
                    packer.push(result_value);
                    packer.push(original_row);
                });
            })
        })
}

fn first_value_inner<'a>(datums: Vec<Datum<'a>>, window_frame: &WindowFrame) -> Vec<Datum<'a>> {
    let length = datums.len();