The Ultimate Guide to High Availability with MariaDB

AUTO_INCREMENT

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Description

The AUTO_INCREMENT attribute can be used to generate a unique identity for new rows. When you insert a new record to the table, and the auto_increment field is NULL or DEFAULT, the value will automatically be incremented. This also applies to 0, unless the NO_AUTO_VALUE_ON_ZERO SQL_MODE is enabled.

AUTO_INCREMENT columns start from 1 by default. The automatically generated value can never be lower than 0.

Each table can have only one AUTO_INCREMENT column. It must defined as a key (not necessarily the PRIMARY KEY or UNIQUE key). If the key consists of multiple columns, the AUTO_INCREMENT column must be the first one, unless the storage engine is Aria or MyISAM.

CREATE TABLE animals (
     id MEDIUMINT NOT NULL AUTO_INCREMENT,
     name CHAR(30) NOT NULL,
     PRIMARY KEY (id)
 );

INSERT INTO animals (name) VALUES
    ('dog'),('cat'),('penguin'),
    ('fox'),('whale'),('ostrich');

SELECT * FROM animals;
+----+---------+
| id | name    |
+----+---------+
|  1 | dog     |
|  2 | cat     |
|  3 | penguin |
|  4 | fox     |
|  5 | whale   |
|  6 | ostrich |
+----+---------+

SERIAL is an alias for BIGINT UNSIGNED NOT NULL AUTO_INCREMENT UNIQUE.

CREATE TABLE t (id SERIAL, c CHAR(1)) ENGINE=InnoDB;

SHOW CREATE TABLE t \G
*************************** 1. row ***************************
       Table: t
Create Table: CREATE TABLE `t` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `c` char(1) DEFAULT NULL,
  UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

Setting or changing the auto_increment value

You can use an ALTER TABLE statement to assign a new value to the auto_increment table option, or set the insert_id server system variable to change the next AUTO_INCREMENT value inserted by the current session.

LAST_INSERT_ID() can be used to see the last AUTO_INCREMENT value inserted by the current session.

ALTER TABLE animals AUTO_INCREMENT=8;

INSERT INTO animals (name) VALUES ('aardvark');

SELECT * FROM animals;
+----+-----------+
| id | name      |
+----+-----------+
|  1 | dog       |
|  2 | cat       |
|  3 | penguin   |
|  4 | fox       |
|  5 | whale     |
|  6 | ostrich   |
|  8 | aardvark  |
+----+-----------+

SET insert_id=12;

INSERT INTO animals (name) VALUES ('gorilla');

SELECT * FROM animals;
+----+-----------+
| id | name      |
+----+-----------+
|  1 | dog       |
|  2 | cat       |
|  3 | penguin   |
|  4 | fox       |
|  5 | whale     |
|  6 | ostrich   |
|  8 | aardvark  |
| 12 | gorilla   |
+----+-----------+

InnoDB/XtraDB

InnoDB and XtraDB use an auto-increment counter that is stored in memory. When the server restarts, the counter is re-initialized to the highest value used in the table, which cancels the effects of any AUTO_INCREMENT = N option in the table statements.

Setting explicit values

It is possible to specify a value for an AUTO_INCREMENT column. The value must not exist in the key.

If the new value is higher than the current maximum value, the AUTO_INCREMENT value is updated, so the next value will be higher. If the new value is lower than the current maximum value, the AUTO_INCREMENT value remains unchanged.

The following example demonstrates these behaviours:

CREATE TABLE t (id INTEGER UNSIGNED AUTO_INCREMENT PRIMARY KEY) ENGINE = InnoDB;

INSERT INTO t VALUES (NULL);
SELECT id FROM t;
+----+
| id |
+----+
|  1 |
+----+

INSERT INTO t VALUES (10); -- higher value
SELECT id FROM t;
+----+
| id |
+----+
|  1 |
| 10 |
+----+

INSERT INTO t VALUES (2); -- lower value
INSERT INTO t VALUES (NULL); -- auto value
SELECT id FROM t;
+----+
| id |
+----+
|  1 |
|  2 |
| 10 |
| 11 |
+----+

The ARCHIVE storage engine does not allow to insert a value that is lower than the current maximum.

Missing values

An AUTO_INCREMENT column normally has missing values. This happens because if a row is deleted, or an AUTO_INCREMENT value is explicitly updated, old values are never re-used. The REPLACE statement also deletes a row, and its value is wasted. With InnoDB, values can be reserved by a transaction; but if the transaction fails (for example, because of a ROLLBACK) the reserved value will be lost.

Thus AUTO_INCREMENT values can be used to sort results in a chronological order, but not to create a numeric sequence.

Replication

To make master-master or Galera safe to use AUTO_INCREMENT one should use the system variables auto_increment_increment and auto_increment_offset to generate unique values for each server.

See Also

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