InnoDB的唯一键和索引

Question

我用下面的创建语句：

CREATE TABLE `subscr` (
`id` int(11) NOT NULL AUTO_INCREMENT, 
`media_id` int(11) NOT NULL, 
`user_id` int(11) NOT NULL, 
`date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP, 
PRIMARY KEY (`id`), 
UNIQUE KEY (`media_id`, `user_id`), 
CONSTRAINT FOREIGN KEY (`media_id`) REFERENCES `media` (`id`) ON DELETE CASCADE, 
CONSTRAINT FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE 
) ENGINE=InnoDB 

为什么InnoDB的只是把一个指数USER_ID？我认为它需要所有外键引用的索引。

CREATE TABLE `subscr` (
`id` int(11) NOT NULL AUTO_INCREMENT, 
`media_id` int(11) NOT NULL, 
`user_id` int(11) NOT NULL, 
`date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP, 
PRIMARY KEY (`id`), 
UNIQUE KEY `media_id` (`media_id`,`user_id`), 
KEY `user_id` (`user_id`), 
CONSTRAINT `subscriptions_media_ibfk_1` FOREIGN KEY (`media_id`) REFERENCES `media` (`id`) ON DELETE CASCADE, 
CONSTRAINT `subscriptions_media_ibfk_2` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE 
) ENGINE=InnoDB 

Answer 1

media_id也具有索引，并且它是唯一的。这对于MySQL来说已经足够了。只为user_id创建了一个单独的索引，因为唯一索引中的user_id是第二个索引，因此对于外键是没用的。