MariaDB AUTO_INCREMENT Jump to Max (unsigned) int - mysql

Question

EDIT: it was a bug in or application
What could cause the auto_increment value to jump up billions of values to strike max int?
Our core table uses an int unsigned auto_increment primary key column. Today it skipped every value all the way up to 4294967295.
I was able to resolve this by deleting the offensive row and running
ALTER TABLE [table] AUTO_INCREMENT = [new value];
but I cannot explain how or why it happened.
Our environment is MariaDB 10.2.27 under PHP/Apache.

Answer 1

Any INSERT can specify a high value, and this will implicitly advance the next auto-increment value. In other words, an auto-increment for a table will never be less than the largest value currently inserted in the table, and any client can override the auto-increment mechanism simply by specifying a value in an INSERT.
Demo:
mysql> create table mytable (id int unsigned auto_increment primary key);
mysql> insert into mytable (id) values (DEFAULT);
Query OK, 1 row affected (0.01 sec)
That should have inserted id value 1, so the next AI will be 2:
mysql> show create table mytable\G
*************************** 1. row ***************************
Table: mytable
Create Table: CREATE TABLE `mytable` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4
Now I specify a large value, and the AI is updated accordingly:
mysql> insert into mytable (id) values(1000000000);
Query OK, 1 row affected (0.01 sec)
mysql> show create table mytable\G
*************************** 1. row ***************************
Table: mytable
Create Table: CREATE TABLE `mytable` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1000000001 DEFAULT CHARSET=utf8mb4
Note a value greater than the max unsigned int value 232-1 will be truncated to that value.
mysql> insert into mytable (id) values(1000000000000);
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> show create table mytable\G
*************************** 1. row ***************************
Table: mytable
Create Table: CREATE TABLE `mytable` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4294967295 DEFAULT CHARSET=utf8mb4
So someone must have inserted a large value. Any value larger than 4294967295 will be truncated to 4294967295, and then subsequent inserts that try to use the AI sequence will fail.