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AUTO_RANDOM New in v3.1.0

User scenario

When you write data intensively into TiDB and TiDB has the table with a primary key of the auto-increment integer type, hotspot issue might occur. To solve the hotspot issue, you can use the AUTO_RANDOM attribute. Refer to Highly Concurrent Write Best Practices for details.

Take the following created table as an example:

CREATE TABLE t (a bigint PRIMARY KEY AUTO_INCREMENT, b varchar(255))

On this t table, you execute a large number of INSERT statements that do not specify the values of the primary key as below:

INSERT INTO t(b) VALUES ('a'), ('b'), ('c')

In the above statement, values of the primary key (column a) are not specified, so TiDB uses the continuous auto-increment row values as the row IDs, which might cause write hotspot in a single TiKV node and affect the performance. To avoid such write hotspot, you can specify the AUTO_RANDOM attribute rather than the AUTO_INCREMENT attribute for the column a when you create the table. See the follow examples:

CREATE TABLE t (a bigint PRIMARY KEY AUTO_RANDOM, b varchar(255))

or

CREATE TABLE t (a bigint AUTO_RANDOM, b varchar(255), PRIMARY KEY (a))

Then execute the INSERT statement such as INSERT INTO t(b) VALUES.... Now the results will be as follows:

  • Implicitly allocating values: If the INSERT statement does not specify the values of the integer primary key column (column a) or specify the value as NULL, TiDB automatically allocates values to this column. These values are not necessarily auto-increment or continuous but are unique, which avoids the hotspot problem caused by continuous row IDs.
  • Explicitly inserting values: If the INSERT statement explicitly specifies the values of the integer primary key column, TiDB saves these values, which works similarly to the AUTO_INCREMENT attribute. Note that if you do not set NO_AUTO_VALUE_ON_ZERO in the @@sql_mode system variable, TiDB will automatically allocate values to this column even if you explicitly specify the value of the integer primary key column as 0.

TiDB automatically allocates values in the following way:

The highest five digits (ignoring the sign bit) of the row value in binary (namely, shard bits) are determined by the starting time of the current transaction. The remaining digits are allocated values in an auto-increment order.

To use different number of shard bits, append a pair of parentheses to AUTO_RANDOM and specify the desired number of shard bits in the parentheses. See the following example:

CREATE TABLE t (a bigint PRIMARY KEY AUTO_RANDOM(3), b varchar(255))

In the above CREATE TABLE statement, 3 shard bits are specified. The range of the number of shard bits is [1, field_max_bits). field_max_bits is the length of bits occupied by the primary key column.

After creating the table, use the SHOW WARNINGS statement to see the maximum number of implicit allocations supported by the current table:

SHOW WARNINGS

+-------+------+----------------------------------------------------------+
| Level | Code | Message                                                  |
+-------+------+----------------------------------------------------------+
| Note  | 1105 | Available implicit allocation times: 1152921504606846976 |
+-------+------+----------------------------------------------------------+

In addition, to view the shard bit number of the table with the AUTO_RANDOM attribute, you can 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.

Values allocated to the AUTO_RANDOM column affect last_insert_id(). You can use SELECT last_insert_id () to get the ID that TiDB last implicitly allocates. For example:

INSERT INTO t (b) VALUES ("b")
SELECT * FROM t;
SELECT last_insert_id()

You might see the following result:

+------------+---+
| a          | b |
+------------+---+
| 1073741825 | b |
+------------+---+
+------------------+
| last_insert_id() |
+------------------+
| 1073741825       |
+------------------+

Compatibility

TiDB supports parsing the version comment syntax. See the following example:

CREATE TABLE t (a bigint PRIMARY KEY /*T![auto_rand] auto_random */)

CREATE TABLE t (a bigint PRIMARY KEY AUTO_RANDOM)

The above two statements have the same meaning.

In the result of SHOW CREATE TABLE, the AUTO_RANDOM attribute is commented out. This comment includes an attribute identifier (for example, /*T![auto_rand] auto_random */). Here auto_rand represents the AUTO_RANDOM attribute. Only the version of TiDB that implements the feature corresponding to this identifier can parse the SQL statement fragment properly.

This attribute supports forward compatibility, namely, downgrade compatibility. TiDB of earlier versions that do not implement this feature ignore the AUTO_RANDOM attribute of a table (with the above comment) and can also use the table with the attribute.

Restrictions

Pay attention to the following restrictions when you use AUTO_RANDOM:

  • Specify this attribute for the primary key column ONLY of integer type. Otherwise, an error might occur. 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 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.
  • 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.
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