I see `add_index ~ unique: true` statement in `schema.rb`and think uniqueness is constraint for **table**, not for **index**.Using index is one of way realizing uniqueness, Programmer should not designate to the RDBMS "how" and index quicken searching but take costs inserting. In fact, is there another way to keep uniqueness constraint? If not, why Rails provide us only `add_index` statement for uniqueness constraint?