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AUTO_RANDOM New in v3.1.0

User scenario

Since the value of AUTO_RANDOM is random and unique, AUTO_RANDOM is often used in place of AUTO_INCREMENT to avoid write hotspot in a single storage node caused by TiDB assigning consecutive IDs. If the current AUTO_INCREMENT column is a primary key and the type is BIGINT, you can execute the ALTER TABLE t MODIFY COLUMN id BIGINT AUTO_RANDOM(5); statement to switch from AUTO_INCREMENT to AUTO_RANDOM.

For more information about how to handle highly concurrent write-heavy workloads in TiDB, see Highly concurrent write best practices.

Basic concepts

AUTO_RANDOM is a column attribute that is used to automatically assign values to a BIGINT column. Values assigned automatically are random and unique.

To create a table with an AUTO_RANDOM column, you can use the following statements. The AUTO_RANDOM column must be included in a primary key, and the AUTO_RANDOM column is the first column in the primary key.

CREATE TABLE t (a BIGINT AUTO_RANDOM, b VARCHAR(255), PRIMARY KEY (a));
CREATE TABLE t (a BIGINT PRIMARY KEY AUTO_RANDOM, b VARCHAR(255));
CREATE TABLE t (a BIGINT AUTO_RANDOM(6), b VARCHAR(255), PRIMARY KEY (a));
CREATE TABLE t (a BIGINT AUTO_RANDOM(5, 54), b VARCHAR(255), PRIMARY KEY (a));
CREATE TABLE t (a BIGINT AUTO_RANDOM(5, 54), b VARCHAR(255), PRIMARY KEY (a, b));

You can wrap the keyword AUTO_RANDOM in an executable comment. For more details, refer to TiDB specific comment syntax.

CREATE TABLE t (a bigint /*T![auto_rand] AUTO_RANDOM */, b VARCHAR(255), PRIMARY KEY (a));
CREATE TABLE t (a bigint PRIMARY KEY /*T![auto_rand] AUTO_RANDOM */, b VARCHAR(255));
CREATE TABLE t (a BIGINT /*T![auto_rand] AUTO_RANDOM(6) */, b VARCHAR(255), PRIMARY KEY (a));
CREATE TABLE t (a BIGINT  /*T![auto_rand] AUTO_RANDOM(5, 54) */, b VARCHAR(255), PRIMARY KEY (a));

When you execute an INSERT statement:

If you explicitly specify the value of the AUTO_RANDOM column, it is inserted into the table as is.
If you do not explicitly specify the value of the AUTO_RANDOM column, TiDB generates a random value and inserts it into the table.

tidb> CREATE TABLE t (a BIGINT PRIMARY KEY AUTO_RANDOM, b VARCHAR(255));
Query OK, 0 rows affected, 1 warning (0.01 sec)

tidb> INSERT INTO t(a, b) VALUES (1, 'string');
Query OK, 1 row affected (0.00 sec)

tidb> SELECT * FROM t;
+---+--------+
| a | b      |
+---+--------+
| 1 | string |
+---+--------+
1 row in set (0.01 sec)

tidb> INSERT INTO t(b) VALUES ('string2');
Query OK, 1 row affected (0.00 sec)

tidb> INSERT INTO t(b) VALUES ('string3');
Query OK, 1 row affected (0.00 sec)

tidb> SELECT * FROM t;
+---------------------+---------+
| a                   | b       |
+---------------------+---------+
|                   1 | string  |
| 1152921504606846978 | string2 |
| 4899916394579099651 | string3 |
+---------------------+---------+
3 rows in set (0.00 sec)

The AUTO_RANDOM(S, R) column value automatically assigned by TiDB has a total of 64 bits:

S is the number of shard bits. The value ranges from 1 to 15. The default value is 5.
R is the total length of the automatic allocation range. The value ranges from 32 to 64. The default value is 64.

The structure of an AUTO_RANDOM value is as follows:

Total number of bits	Sign bit	Reserved bits	Shard bits	Auto-increment bits
64 bits	0/1 bit	(64-R) bits	S bits	(R-1-S) bits

The length of the sign bit is determined by the existence of an UNSIGNED attribute. If there is an UNSIGNED attribute, the length is 0. Otherwise, the length is 1.
The length of the reserved bits is 64-R. The reserved bits are always 0.
The content of the shard bits is obtained by calculating the hash value of the starting time of the current transaction. To use a different length of shard bits (such as 10), you can specify AUTO_RANDOM(10) when creating the table.
The value of the auto-increment bits is stored in the storage engine and allocated sequentially. Each time a new value is allocated, the value is incremented by 1. The auto-increment bits ensure that the values of AUTO_RANDOM are unique globally. When the auto-increment bits are exhausted, an error Failed to read auto-increment value from storage engine is reported when the value is allocated again.

Note

Selection of shard bits (S):

Since there is a total of 64 available bits, the shard bits length affects the auto-increment bits length. That is, as the shard bits length increases, the length of auto-increment bits decreases, and vice versa. Therefore, you need to balance the randomness of allocated values and available space.
The best practice is to set the shard bits as log(2, x), in which x is the current number of storage engines. For example, if there are 16 TiKV nodes in a TiDB cluster, you can set the shard bits as log(2, 16), that is 4. After all regions are evenly scheduled to each TiKV node, the load of bulk writes can be uniformly distributed to different TiKV nodes to maximize resource utilization.

Selection of range (R):

Typically, the R parameter needs to be set when the numeric type of the application cannot represent a full 64-bit integer.
For example, the range of JSON number is [-2^53+1, 2^53-1]. TiDB can easily assign an integer outside this range to a column of AUTO_RANDOM(5), causing unexpected behaviors when the application reads the column. In this case, you can replace AUTO_RANDOM(5) with AUTO_RANDOM(5, 54) and TiDB does not assign an integer greater than 9007199254740991 (2^53-1) to the column.

Values allocated implicitly to the AUTO_RANDOM column affect last_insert_id(). To get the ID that TiDB last implicitly allocates, you can use the SELECT last_insert_id () statement.

To view the shard bits number of the table with an AUTO_RANDOM column, you can execute the SHOW CREATE TABLE statement. You can also see the value of the PK_AUTO_RANDOM_BITS=x mode in the TIDB_ROW_ID_SHARDING_INFO column in the information_schema.tables system table. x is the number of shard bits.

Restrictions

Pay attention to the following restrictions when you use AUTO_RANDOM:

To insert values explicitly, you need to set the value of the @@allow_auto_random_explicit_insert system variable to 1 (0 by default). It is not recommended that you explicitly specify a value for the column with the AUTO_RANDOM attribute when you insert data. Otherwise, the numeral values that can be automatically allocated for this table might be used up in advance.
Specify this attribute for the primary key column ONLY as the BIGINT type. Otherwise, an error occurs. In addition, when the attribute of the primary key is NONCLUSTERED, AUTO_RANDOM is not supported even on the integer primary key. For more details about the primary key of the CLUSTERED type, refer to clustered index.
You cannot use ALTER TABLE to modify the AUTO_RANDOM attribute, including adding or removing this attribute.
You cannot use ALTER TABLE to change from AUTO_INCREMENT to AUTO_RANDOM if the maximum value is close to the maximum value of the column type.
You cannot change the column type of the primary key column that is specified with AUTO_RANDOM attribute.
You cannot specify AUTO_RANDOM and AUTO_INCREMENT for the same column at the same time.
You cannot specify AUTO_RANDOM and DEFAULT (the default value of a column) for the same column at the same time.
WhenAUTO_RANDOM is used on a column, it is difficult to change the column attribute back to AUTO_INCREMENT because the auto-generated values might be very large.