Detailed introduction to Identity in Mysql

If the table contains a column auto_increment,

If it is a Myisam type engine, then after deleting the latest data, regardless of whether Mysql is restarted, the last deleted maximum ID + 1 will still be used after the next insertion.

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)

The following is a test of Innodb table.

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)

You can see that when the mysql database is not restarted, the newly inserted data in the innodb table will be the previously deleted data plus 1.

But when the Mysql service is restarted, the auto-increment table in InnodB will be added. Insert data into, then the largest auto-increment column in the current Innodb table will be used plus 1.

Reason:

Myisam type storage engine table will record the largest ID value into the data file, regardless of whether to restart the largest None of the ID values ​​will be lost. However, the maximum ID value of the InnoDB table is stored in the memory. If the Mysql service is not restarted, the newly added data will use the maximum data in the memory + 1. But after restarting, the maximum value in the current table will be used and +1

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