Mysql中Identity的一个特性

假如表中包含一列为auto_increment，

如果是Myisam类型的引擎，那么在删除了最新一笔数据，无论是否重启Mysql，下一次插入之后仍然会使用上次删除的最大ID+1.

mysql> create table test_myisam (id int not null auto_increment primary key, name char(5)) engine=myisam;
Query OK, 0 rows affected (0.04 sec)

mysql> insert into test_myisam (name) select 'a';
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> insert into test_myisam (name) select 'b';
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> insert into test_myisam (name) select 'c';
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> insert into test_myisam (name) select name from test_myisam;
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from test_myisam;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
|  4 | a    |
|  5 | b    |
|  6 | c    |
+----+------+
6 rows in set (0.00 sec)

mysql> delete from test_myisam where id=6;
Query OK, 1 row affected (0.00 sec)

mysql> insert into test_myisam(name) select 'd';
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from test_myisam;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
|  4 | a    |
|  5 | b    |
|  7 | d    |
+----+------+
6 rows in set (0.00 sec)

下面是对Innodb表的测试。

mysql> create table test_innodb(id int not null auto_increment primary key, name char(5)) engine=innodb;
Query OK, 0 rows affected (0.26 sec)

mysql> insert into test_innodb (name)select 'a';
Query OK, 1 row affected (0.06 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> insert into test_innodb (name)select 'b';
Query OK, 1 row affected (0.06 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> insert into test_innodb (name)select 'c';
Query OK, 1 row affected (0.07 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from test_innodb;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
+----+------+
3 rows in set (0.00 sec)

mysql> delete from test_innodb where id=3;
Query OK, 1 row affected (0.05 sec)

mysql> insert into test_innodb (name)select 'd';
Query OK, 1 row affected (0.20 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from test_innodb;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  4 | d    |
+----+------+
3 rows in set (0.00 sec)

mysql> exit
Bye
[2@a data]$ mysql -uroot -pwsdad
Welcome to the MySQL monitor.  Commands end with ; or g.
Your MySQL connection id is 5
Server version: 5.5.37-log Source distribution

Copyright (c) 2000, 2014, 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 wison
Database changed
mysql> delete from test_innodb where id=4;
Query OK, 1 row affected (0.07 sec)

mysql> exit
Bye
[2@a data]$ sudo service mysql restart
Shutting down MySQL... SUCCESS!
Starting MySQL.. SUCCESS!
[2@a data]$ mysql -uroot -pwison
Welcome to the MySQL monitor.  Commands end with ; or g.
Your MySQL connection id is 1
Server version: 5.5.37-log Source distribution

Copyright (c) 2000, 2014, 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 wison
Database changed
mysql> insert into test_innodb (name) select 'z';
Query OK, 1 row affected (0.07 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from test_innodb;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | z    |
+----+------+
3 rows in set (0.00 sec)

可以看到在mysql数据库没有重启时，innodb的表新插入数据会是之前被删除的数据再加1.

但是当Mysql服务被重启后，再向InnodB的自增表表里插入数据，那么会使用当前Innodb表里的最大的自增列再加1.

原因：

Myisam类型存储引擎的表将最大的ID值是记录到数据文件中，不管是否重启最大的ID值都不会丢失。但是InnoDB表的最大的ID值是存在内存中的，若不重启Mysql服务，新加入数据会使用内存中最大的数据+1.但是重启之后，会使用当前表中最大的值再+1