MySQL中的auto_increment的问题
发现一个看似很简单的MySQL问题,却引起了大家的广泛关注:
这是一道很早的面试题:
一张表,里面有ID自增主键,当insert了17条记录之后,删除了第15,16,17条记录,再把Mysql重启,再insert一条记录,这条记录的ID是18还是15 。
答案:
如果表的类型是MyISAM,那么是18。
因为MyISAM表会把自增主键的最大ID记录到数据文件里,重启MySQL自增主键的最大ID也不会丢失。
如果表的类型是InnoDB,那么是15。
InnoDB表只是把自增主键的最大ID记录到内存中,所以重启数据库或者是对表进行OPTIMIZE操作,都会导致最大ID丢失。
自己也做了个实验,结果证实了上面的说法。真是惭愧啊,看似简单的问题自己也打错了。
- mysql> select * from test1;
- +----+-----------+
- | id | name |
- +----+-----------+
- | 1 | 陈兵辉 |
- | 2 | chen |
- | 3 | chen |
- | 4 | chen |
- | 5 | chen |
- | 6 | chen |
- | 7 | chen |
- | 8 | chen |
- | 9 | chen |
- | 10 | chen |
- | 11 | chen |
- +----+-----------+
- 11 rows in set (0.00 sec)
-
- mysql> delete from test1 where id in (10,11,9);
Query OK, 3 rows affected (0.03 sec)
mysql> show create table test1;
CREATE TABLE `test1` (
`id` int(11) NOT NULL auto_increment,
`name` varchar(10) default NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=<span style="color:#ff0000;">12</span> DEFAULT CHARSET=utf8 |
mysql> exit;
Bye
[root@fsailing1 ~]# service mysqld restart
停止 MySQL: [确定]
启动 MySQL: [确定]
[root@fsailing1 ~]# mysql -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.0.95 Source distribution
Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> show create table test1;
| CREATE TABLE `test1` (
`id` int(11) NOT NULL auto_increment,
`name` varchar(10) default NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=<span style="color:#ff0000;">9</span> DEFAULT CHARSET=utf8 |
2,另外还有一个就是获取当前数据库表的自增字段数。
- mysql> select last_insert_id();
- +------------------+
- | last_insert_id() |
- +------------------+
- | 0 |
- +------------------+
- 1 row in set (0.00 sec)
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