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This statement adds a new index to an existing table. It is an alternative syntax to ALTER TABLE .. ADD INDEX, and included for MySQL compatibility.




mysql> CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, c1 INT NOT NULL); Query OK, 0 rows affected (0.10 sec) mysql> INSERT INTO t1 (c1) VALUES (1),(2),(3),(4),(5); Query OK, 5 rows affected (0.02 sec) Records: 5 Duplicates: 0 Warnings: 0 mysql> EXPLAIN SELECT * FROM t1 WHERE c1 = 3; +-------------------------+----------+-----------+---------------+--------------------------------+ | id | estRows | task | access object | operator info | +-------------------------+----------+-----------+---------------+--------------------------------+ | TableReader_7 | 10.00 | root | | data:Selection_6 | | └─Selection_6 | 10.00 | cop[tikv] | | eq(test.t1.c1, 3) | | └─TableFullScan_5 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo | +-------------------------+----------+-----------+---------------+--------------------------------+ 3 rows in set (0.00 sec) mysql> CREATE INDEX c1 ON t1 (c1); Query OK, 0 rows affected (0.30 sec) mysql> EXPLAIN SELECT * FROM t1 WHERE c1 = 3; +------------------------+---------+-----------+------------------------+---------------------------------------------+ | id | estRows | task | access object | operator info | +------------------------+---------+-----------+------------------------+---------------------------------------------+ | IndexReader_6 | 10.00 | root | | index:IndexRangeScan_5 | | └─IndexRangeScan_5 | 10.00 | cop[tikv] | table:t1, index:c1(c1) | range:[3,3], keep order:false, stats:pseudo | +------------------------+---------+-----------+------------------------+---------------------------------------------+ 2 rows in set (0.00 sec) mysql> ALTER TABLE t1 DROP INDEX c1; Query OK, 0 rows affected (0.30 sec) mysql> CREATE UNIQUE INDEX c1 ON t1 (c1); Query OK, 0 rows affected (0.31 sec)

Expression index

In some scenarios, the filtering condition of a query is based on a certain expression. In these scenarios, the query performance is relatively poor because ordinary indexes cannot take effect, the query can only be executed by scanning the entire table. The expression index is a type of special index that can be created on an expression. Once an expression index is created, TiDB can use the index for the expression-based query, which significantly improves the query performance.

For example, if you want to create an index based on lower(col1), execute the following SQL statement:

CREATE INDEX idx1 ON t1 ((lower(col1)));

Or you can execute the following equivalent statement:

ALTER TABLE t1 ADD INDEX idx1((lower(col1)));

You can also specify the expression index when you create the table:

CREATE TABLE t1(col1 char(10), col2 char(10), index((lower(col1))));

You can drop an expression index in the same way as dropping an ordinary index:

DROP INDEX idx1 ON t1;

Expression index involves various kinds of expressions. To ensure correctness, only some fully tested functions are allowed for creating an expression index. This means that only these functions are allowed in expressions in a production environment. You can get these functions by querying tidb_allow_function_for_expression_index variable. In future versions, more functions might be added to the list.

mysql> select @@tidb_allow_function_for_expression_index; +--------------------------------------------+ | @@tidb_allow_function_for_expression_index | +--------------------------------------------+ | lower, md5, reverse, upper, vitess_hash | +--------------------------------------------+ 1 row in set (0.00 sec)

For the functions that are not included in the returned result above, those functions are not fully tested and not recommended for a production environment, which can be seen as experimental. Other expressions such as operators, cast, and case when are also seen as experimental and not recommended for production.

If you still want to use those experimental expressions, you can make the following configuration in the TiDB configuration file:

allow-expression-index = true

When the expression in a query statement matches the expression in an expression index, the optimizer can choose the expression index for the query. In some cases, the optimizer might not choose an expression index depending on statistics. In this situation, you can force the optimizer to select an expression index by using optimizer hints.

In the following examples, suppose that you create the expression index idx on the expression lower(col1):

If the results of the query statement are the same expressions, the expression index applies. Take the following statement as an example:

SELECT lower(col1) FROM t;

If the same expression is included in the filtering conditions, the expression index applies. Take the following statements as an example:

SELECT * FROM t WHERE lower(col1) = "a"; SELECT * FROM t WHERE lower(col1) > "a"; SELECT * FROM t WHERE lower(col1) BETWEEN "a" AND "b"; SELECT * FROM t WHERE lower(col1) in ("a", "b"); SELECT * FROM t WHERE lower(col1) > "a" AND lower(col1) < "b"; SELECT * FROM t WHERE lower(col1) > "b" OR lower(col1) < "a";

When the queries are sorted by the same expression, the expression index applies. Take the following statement as an example:

SELECT * FROM t ORDER BY lower(col1);

If the same expression is included in the aggregate (GROUP BY) functions, the expression index applies. Take the following statements as an example:

SELECT max(lower(col1)) FROM t; SELECT min(col1) FROM t GROUP BY lower(col1);

To see the expression corresponding to the expression index, execute show index, or check the system tables information_schema.tidb_indexes and the table information_schema.STATISTICS. The Expression column in the output indicates the corresponded expression. For the non-expression indexes, the column shows NULL.

The cost of maintaining an expression index is higher than that of maintaining other indexes, because the value of the expression needs to be calculated whenever a row is inserted or updated. The value of the expression is already stored in the index, so this value does not require recalculation when the optimizer selects the expression index.

Therefore, when the query performance outweighs the insert and update performance, you can consider indexing the expressions.

Expression indexes have the same syntax and limitations as in MySQL. They are implemented by creating indexes on generated virtual columns that are invisible, so the supported expressions inherit all limitations of virtual generated columns.

Invisible index

Invisible indexes are indexes that are ignored by the query optimizer:


For details, see ALTER INDEX.

Associated system variables

The system variables associated with the CREATE INDEX statement are tidb_ddl_reorg_worker_cnt, tidb_ddl_reorg_batch_size, tidb_enable_auto_increment_in_generated, and tidb_ddl_reorg_priority. Refer to system variables for details.

MySQL compatibility

  • FULLTEXT, HASH and SPATIAL indexes are not supported.
  • Descending indexes are not supported (similar to MySQL 5.7).
  • Adding the primary key of the CLUSTERED type to a table is not supported. For more details about the primary key of the CLUSTERED type, refer to clustered index.
  • Expression indexes are incompatible with views. When a query is executed using a view, the expression index cannot be used at the same time.
  • Expression indexes have compatibility issues with bindings. When the expression of an expression index has a constant, the binding created for the corresponding query expands its scope. For example, suppose that the expression in the expression index is a+1, and the corresponding query condition is a+1 > 2. In this case, the created binding is a+? > ?, which means that the query with the condition such as a+2 > 2 is also forced to use the expression index and results in a poor execution plan. In addition, this also affects the baseline capturing and baseline evolution in SQL Plan Management (SPM).

See also

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