MySql: AUTO_INCREMENT is missing from tables - mysql

Question

MySql: AUTO_INCREMENT is missing from some tables after running for about one month.
Initially: (show create table Foo)
CREATE TABLE `Foo` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`name` varchar(10) NOT NULL,
`type` tinyint(2) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8
After one month:
CREATE TABLE `Foo` (
`id` bigint(20) NOT NULL,
`name` varchar(10) NOT NULL,
`type` tinyint(2) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
AUTO_INCREMENT is missing. What is the cause?
Mysql Server version: 5.6.25, Linux

Answer 1

Someone must have changed it. This change does not happen spontaneously.
I can reproduce this change myself:
CREATE TABLE Foo ( id BIGINT AUTO_INCREMENT, ...
ALTER TABLE Foo MODIFY COLUMN id BIGINT;
SHOW CREATE TABLE Foo\G
*************************** 1. row ***************************
Table: foo
Create Table: CREATE TABLE `foo` (
`id` bigint(20) NOT NULL,
`name` varchar(10) NOT NULL,
`type` tinyint(2) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
Now the column shows it is BIGINT but not AUTO_INCREMENT.
Every time you MODIFY COLUMN or CHANGE COLUMN, you must repeat all the column options like NOT NULL and AUTO_INCREMENT and DEFAULT, or else it will revert to defaults (i.e. not auto-increment).
So I would interpret this shows that someone did an ALTER TABLE and didn't remember to include the AUTO_INCREMENT column option.

Answer 2

Just a thought.
If you have binary logs, you may see the alter query on the logs and when it was run. :)
Check if the binary log is enabled by
show variable like 'log_bin';
If binary log is enabled, find the likely period that the query could have been executed and then use mysqlbinlog to help you find it.
If binary log is not enabled, bad luck - as the previous post by Bill Karwin has suggested mysql does not change it on its own - someone must have changed it.