The Ultimate Guide to High Availability with MariaDB

AUTO_INCREMENT

Contents

  1. Description
  2. Setting or Changing the Auto_Increment Value
  3. InnoDB
  4. Setting Explicit Values
  5. Missing Values
  6. Replication
  7. CHECK Constraints, DEFAULT Values and Virtual Columns
  8. Generating Auto_Increment Values When Adding the Attribute
  9. See Also

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 (or upon adding an AUTO_INCREMENT attribute with the ALTER TABLE statement), and the auto_increment field is NULL or DEFAULT (in the case of an INSERT), 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). In some storage engines (including the default InnoDB), if the key consists of multiple columns, the AUTO_INCREMENT column must be the first column. Storage engines that permit the column to be placed elsewhere are Aria, MyISAM, MERGE, Spider, TokuDB, BLACKHOLE, FederatedX and Federated.

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

AUTO_INCREMENT is persistent in InnoDB. Prior to MariaDB 10.2.3, InnoDB used an auto-increment counter that was stored in memory. When the server restarted, the counter was re-initialized to the highest value used in the table, which canceled the effects of any AUTO_INCREMENT = N option in the table statements).

See also AUTO_INCREMENT Handling in InnoDB.

Setting Explicit Values

It is possible to specify a value for an AUTO_INCREMENT column. If the key is primary or unique, the value must not already 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 behaviors:

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.

SET @@auto_increment_increment=3;

SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 3     |
| auto_increment_offset    | 1     |
+--------------------------+-------+

CREATE TABLE t (c INT NOT NULL AUTO_INCREMENT PRIMARY KEY);

INSERT INTO t VALUES (NULL), (NULL), (NULL);

SELECT * FROM t;
+---+
| c |
+---+
| 1 |
| 4 |
| 7 |
+---+

CREATE TABLE t2 (c INT NOT NULL AUTO_INCREMENT PRIMARY KEY);

SET @@auto_increment_offset=2;

SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 3     |
| auto_increment_offset    | 2     |
+--------------------------+-------+

INSERT INTO t2 VALUES (NULL), (NULL), (NULL);

SELECT * FROM t2;
+---+
| c |
+---+
| 2 |
| 5 |
| 8 |
+---+

If auto_increment_offset is larger than auto_increment_increment, the value of auto_increment_offset is ignored, and the offset reverts to the default of 1 instead:

SET @@auto_increment_offset=5;

SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 3     |
| auto_increment_offset    | 5     |
+--------------------------+-------+

CREATE TABLE t3 (c INT NOT NULL AUTO_INCREMENT PRIMARY KEY);

INSERT INTO t3 VALUES (NULL), (NULL), (NULL);

SELECT * FROM t3;
+---+
| c |
+---+
| 1 |
| 4 |
| 5 |
+---+

+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 3     |
| auto_increment_offset    | 3     |
+--------------------------+-------+

INSERT INTO t4 VALUES (NULL), (NULL), (NULL);

SELECT * FROM t4;
+---+
| c |
+---+
| 3 |
| 6 |
| 9 |
+---+

CHECK Constraints, DEFAULT Values and Virtual Columns

auto_increment columns are not permitted in CHECK constraints, DEFAULT value expressions and virtual columns. They were permitted until MariaDB 10.2.6, but did not work correctly. See MDEV-11117.

Generating Auto_Increment Values When Adding the Attribute

CREATE OR REPLACE TABLE t1 (a INT);
INSERT t1 VALUES (0),(0),(0);
ALTER TABLE t1 MODIFY a INT NOT NULL AUTO_INCREMENT PRIMARY KEY;
SELECT * FROM t1;
+---+
| a |
+---+
| 1 |
| 2 |
| 3 |
+---+

CREATE OR REPLACE TABLE t1 (a INT);
INSERT t1 VALUES (5),(0),(8),(0);
ALTER TABLE t1 MODIFY a INT NOT NULL AUTO_INCREMENT PRIMARY KEY;
SELECT * FROM t1;
+---+
| a |
+---+
| 5 |
| 6 |
| 8 |
| 9 |
+---+

If the NO_AUTO_VALUE_ON_ZERO SQL_MODE is set, zero values will not be automatically incremented: 

SET SQL_MODE='no_auto_value_on_zero';
CREATE OR REPLACE TABLE t1 (a INT);
INSERT t1 VALUES (3), (0);
ALTER TABLE t1 MODIFY a INT NOT NULL AUTO_INCREMENT PRIMARY KEY;
SELECT * FROM t1;
+---+
| a |
+---+
| 0 |
| 3 |
+---+

See Also

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