Function key_val_upd_counts 

fn key_val_upd_counts<'a, K, V, T, R>(
    chain: impl Iterator<Item = BorrowedOf<'a, ((K, V), T, R)>>,
) -> (usize, usize, usize)
where
    K: Columnar,
    V: Columnar,
    T: Columnar,
    R: Columnar,
    for<'b> Ref<'b, K>: Copy + Ord,
    for<'b> Ref<'b, V>: Copy + Ord,

Distinct key, distinct (key, val), and update counts over a chain of views, deduplicated within each view and summed.