在复合主键中使用时未创建 MySQL 外键

Question

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 列作为主键会产生:

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      |         |               |
+-------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

使用由外键约束中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？有办法克服这个问题吗？ (MySQL 5.7)

Answer 1

当您创建 FOREIGN KEY 约束时，必须有一个索引。如果有必要，创建 FOREIGN KEY 将自动创建一个新索引(至少从某些旧版本的 MySQL 如 4.0 或其他版本开始，但有一次，即使旧版本也不会自动创建 FK 索引 IIRC )。

但是如果合适的索引已经存在，MySQL不需要创建新索引。 InnoDB 至少是那么智能。

任何包含 FK 列作为索引最左侧子集的索引都将满足要求。 PRIMARY KEY 索引(即表的聚集索引)工作正常。