我一直在根据你告诉我的内容做一些实验,我想我会把它作为答案分享。
首先我创建一些测试表:
CREATE TABLE foo (
foo_id int(10) unsigned NOT NULL,
PRIMARY KEY (foo_id)
)
ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_spanish_ci;
CREATE TABLE bar (
bar_id int(10) unsigned NOT NULL,
PRIMARY KEY (bar_id)
)
ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_spanish_ci;
CREATE TABLE foo_bar (
foo_id int(10) unsigned NOT NULL,
bar_id int(10) unsigned NOT NULL
)
ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_spanish_ci;
到目前为止,不存在任何索引:
mysql> SHOW INDEXES FROM foo_bar;
Empty set (0.00 sec)
添加主键会生成索引:
mysql> ALTER TABLE foo_bar
-> ADD PRIMARY KEY (`foo_id`, `bar_id`);
Query OK, 0 rows affected (0.70 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> SHOW INDEXES FROM foo_bar;
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| foo_bar | 0 | PRIMARY | 1 | foo_id | A | 0 | NULL | NULL | | BTREE | |
| foo_bar | 0 | PRIMARY | 2 | bar_id | A | 0 | NULL | NULL | | BTREE | |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
2 rows in set (0.02 sec)
如果我添加外键foo_id
它重用主键索引,因为该列是索引中的第一个列:
mysql> ALTER TABLE foo_bar
-> ADD CONSTRAINT `foo_bar_fk1` FOREIGN KEY (`foo_id`) REFERENCES `foo` (`foo_id`) ON DELETE CASCADE ON UPDATE CASCADE;
Query OK, 0 rows affected (0.27 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> SHOW INDEXES FROM foo_bar;
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| foo_bar | 0 | PRIMARY | 1 | foo_id | A | 0 | NULL | NULL | | BTREE | |
| foo_bar | 0 | PRIMARY | 2 | bar_id | A | 0 | NULL | NULL | | BTREE | |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
2 rows in set (0.00 sec)
如果我添加外键bar_id
,它创建一个索引,因为没有现有索引可以重用:
mysql> ALTER TABLE foo_bar
-> ADD CONSTRAINT `foo_bar_fk2` FOREIGN KEY (`bar_id`) REFERENCES `bar` (`bar_id`) ON DELETE CASCADE ON UPDATE CASCADE;
Query OK, 0 rows affected (0.25 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> SHOW INDEXES FROM foo_bar;
+---------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| foo_bar | 0 | PRIMARY | 1 | foo_id | A | 0 | NULL | NULL | | BTREE | |
| foo_bar | 0 | PRIMARY | 2 | bar_id | A | 0 | NULL | NULL | | BTREE | |
| foo_bar | 1 | foo_bar_fk2 | 1 | bar_id | A | 0 | NULL | NULL | | BTREE | |
+---------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
3 rows in set (0.02 sec)
我们的外键之一是使用主键索引。这意味着我们无法删除这样的索引!
mysql> ALTER TABLE foo_bar
-> DROP PRIMARY KEY;
ERROR 1025 (HY000): Error on rename of '.\test\#sql-568_c7d' to '.\test\foo_bar' (errno: 150)
除非我们为外键创建索引或者删除键本身:
mysql> ALTER TABLE foo_bar
-> DROP FOREIGN KEY `foo_bar_fk1`;
Query OK, 0 rows affected (0.19 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE foo_bar
-> DROP PRIMARY KEY;
Query OK, 0 rows affected (0.23 sec)
Records: 0 Duplicates: 0 Warnings: 0
结论是,当功能需要时,MySQL 会自动创建索引(但前提是它们是绝对必要的)。