mysql外键索引
2015-08-07 16:42
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mysql在创建外键的时候会自动添加索引,oracle就不会,这样避免了死锁的产生,提高查询效率
mysql> create table f(id int primary key);
Query OK, 0 rows affected (0.05 sec)
mysql> create table c(id int , foreign key(id) references f(id));
Query OK, 0 rows affected (0.02 sec)
mysql> show index from c;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+--------
-------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_c
omment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+--------
-------+
| c | 1 | id | 1 | id | A | 0 | NULL | NULL | YES | BTREE | |
|
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+--------
-------+
1 row in set (0.00 sec)
mysql> drop index id on c;
ERROR 1553 (HY000): Cannot drop index 'id': needed in a foreign key constraint
mysql>
mysql> create table f(id int primary key);
Query OK, 0 rows affected (0.05 sec)
mysql> create table c(id int , foreign key(id) references f(id));
Query OK, 0 rows affected (0.02 sec)
mysql> show index from c;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+--------
-------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_c
omment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+--------
-------+
| c | 1 | id | 1 | id | A | 0 | NULL | NULL | YES | BTREE | |
|
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+--------
-------+
1 row in set (0.00 sec)
mysql> drop index id on c;
ERROR 1553 (HY000): Cannot drop index 'id': needed in a foreign key constraint
mysql>
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