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  • 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

     

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  • 原文地址:https://www.cnblogs.com/Wison-Ho/p/4182728.html
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