InnoDB独有的密钥和索引

Question

我使用了以下create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只在user_id上放一个索引？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也有一个索引，它是唯一的。这对MySQL来说已经足够了。只有为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，因此它对外键没用。