auto_increment 问题：

原文： https://yq.aliyun.com/articles/59263?spm=5176.8091938.0.0.bjzoFN

官方文档：https://dev.mysql.com/doc/refman/5.6/en/innodb-auto-increment-handling.html

导致auto_increment变小的几种情况：

1、 alter table xx auto_increment = yy;

2、 truncate table

3、 restart mysql

第三种的复现方法：

一张刚创建的innoDB表，目前自增是1.

插入3条记录后，auto_increment=4.

然后再删除掉这三条记录,auto_increment=4 没变

重启MySQL，会发现auto_increment值被清空了。我们插入的话，自动从1开始编号了

官方对于自增序号的初始化的几种情况的说明： 

InnoDB AUTO_INCREMENT Counter Initialization  

This section describes how InnoDB initializes AUTO_INCREMENT counters.

If you specify an AUTO_INCREMENT column for an InnoDB table, the table handle in the InnoDB data dictionary contains a special counter called the auto-increment counter that is used in assigning new values for the column. This counter is stored only in main memory, not on disk.

To initialize an auto-increment counter after a server restart, InnoDB executes the equivalent of the following statement on the first insert into a table containing an AUTO_INCREMENT column.

SELECT MAX(ai_col) FROM table_name FOR UPDATE;

InnoDB increments the value retrieved by the statement and assigns it to the column and to the auto-increment counter for the table. By default, the value is incremented by 1. This default can be overridden by the auto_increment_increment configuration setting.

If the table is empty, InnoDB uses the value 1. This default can be overridden by the auto_increment_offset configuration setting.

If a SHOW TABLE STATUS statement examines the table before the auto-increment counter is initialized, InnoDB initializes but does not increment the value. The value is stored for use by later inserts. This initialization uses a normal exclusive-locking read on the table and the lock lasts to the end of the transaction. InnoDB follows the same procedure for initializing the auto-increment counter for a newly created table.

After the auto-increment counter has been initialized, if you do not explicitly specify a value for an AUTO_INCREMENT column, InnoDB increments the counter and assigns the new value to the column. If you insert a row that explicitly specifies the column value, and the value is greater than the current counter value, the counter is set to the specified column value.

InnoDB uses the in-memory auto-increment counter as long as the server runs. When the server is stopped and restarted, InnoDB reinitializes the counter for each table for the first INSERT to the table, as described earlier.

A server restart also cancels the effect of the AUTO_INCREMENT = N table option in CREATE TABLE and ALTER TABLE statements, which you can use with InnoDB tables to set the initial counter value or alter the current counter value.

auto_increment 自增键的一些说明