Hibernate: @Formula maps a computed column

@Entity
class Order {
    @Id Long id;

    @Formula("(select count(*) from order_items i where i.order_id = id)")
    int itemCount;
}

select o.id, (select count(*) from order_items i where i.order_id = o.id) as itemCount
from orders o

It runs as a subquery on every load - fine for one row, costly for a list.