with recursive lookup(root, base_msn) as (
      select mailbox_select_msn_tree_id, 0::numeric from mailbox_select_msn_tree where parent is null and mailbox_id = :mailboxId
      union all
    (with children as (
        select m.* from mailbox_select_msn_tree m, lookup
            where m.parent = root
    ),
    rolling as (
        select mailbox_select_msn_tree_id, sum(size) over (order by mailbox_select_msn_tree_id) as sum
            from children order by mailbox_select_msn_tree_id
    )
    select mailbox_select_msn_tree_id, sum
        from rolling
        where sum >= :msn
        limit 1)
)
select last_value(root) over (), base_msn from lookup limit 1;