Function order_aggregate_datums

src/expr/src/relation/func.rs:331–342

Assuming datums is a List, sort them by the 2nd through Nth elements corresponding to order_by, then return the 1st element. Near the usages of this function, we sometimes want to produce Datums with a shorter lifetime than 'a. We have to actually perform the shortening of the lifetime here, inside this function, because if we were to simply return `impl Iterator >`, that wouldn't

(
    datums: I,
    order_by: &[ColumnOrder],
)

/// we perform the shortening _inside_ this function: the input of the `map` is known to
/// specifically be `std::vec::IntoIter`, which is known to be covariant.)
pub fn order_aggregate_datums<'a: 'b, 'b, I>(
    datums: I,
    order_by: &[ColumnOrder],
) -> impl Iterator<Item = Datum<'b>>
where
    I: IntoIterator<Item = Datum<'a>>,
{
    order_aggregate_datums_with_rank_inner(datums, order_by)
        .into_iter()
        // (`payload` is coerced here to `Datum<'b>` in the argument of the closure)
        .map(|(payload, _order_datums)| payload)
}

/// Assuming datums is a List, sort them by the 2nd through Nth elements
/// corresponding to order_by, then return the 1st element and computed order by expression.