[英]MySQL foreign key not created when used within composite primary key
CREATE TABLE test
(
user_id int unsigned not null,
post_id int unsigned not null,
primary key (user_id, post_id),
foreign key test_user_id (user_id) references user (id),
foreign key test_post_id (post_id) references post (id)
);
show index from test;
+-------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| test | 0 | PRIMARY | 1 | user_id | A | 0 | NULL | NULL | | BTREE | | |
| test | 0 | PRIMARY | 2 | post_id | A | 0 | NULL | NULL | | BTREE | | |
| test | 1 | test_post_id | 1 | post_id | A | 0 | NULL | NULL | | BTREE | | |
+-------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
添加id
列作为主键将产生:
show index from test;
+-------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| test | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| test | 1 | test_user_id | 1 | user_id | A | 0 | NULL | NULL | | BTREE | | |
| test | 1 | test_post_id | 1 | post_id | A | 0 | NULL | NULL | | BTREE | | |
+-------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
当使用由外键约束内使用的列组成的复合主键时,似乎未创建索引test_user_id
。 应该是这样吗? 有办法克服吗? (MySQL 5.7)
创建FOREIGN KEY约束时,必须有一个索引。 如有必要,创建FOREIGN KEY会自动创建一个新索引(至少由于某些旧版本的MySQL(例如4.0或类似版本),但是一次,即使是较旧的版本也不会自动创建IIRC FK索引)。
但是,如果已经存在合适的索引,则MySQL无需创建新索引。 InnoDB至少是那么聪明。
包括FK列作为索引的最左侧子集的任何索引都将满足要求。 PRIMARY KEY索引(即表的聚集索引)可以正常工作。
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