CREATE INDEX
CREATE INDEX 语句用于在已有表中添加新索引，功能等同于 ALTER TABLE .. ADD INDEX。包含该语句提供了 MySQL 兼容性。
语法图
CreateIndexStmt:
CreateIndexStmtUnique:
Identifier:
IndexTypeOpt:
TableName:
IndexColNameList:
IndexOptionList:
IndexOption:
示例
CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, c1 INT NOT NULL);
Query OK, 0 rows affected (0.10 sec)
INSERT INTO t1 (c1) VALUES (1),(2),(3),(4),(5);
Query OK, 5 rows affected (0.02 sec)
Records: 5 Duplicates: 0 Warnings: 0
EXPLAIN SELECT * FROM t1 WHERE c1 = 3;
+---------------------+----------+------+-------------------------------------------------------------+
| id | count | task | operator info |
+---------------------+----------+------+-------------------------------------------------------------+
| TableReader_7 | 10.00 | root | data:Selection_6 |
| └─Selection_6 | 10.00 | cop | eq(test.t1.c1, 3) |
| └─TableScan_5 | 10000.00 | cop | table:t1, range:[-inf,+inf], keep order:false, stats:pseudo |
+---------------------+----------+------+-------------------------------------------------------------+
3 rows in set (0.00 sec)
CREATE INDEX c1 ON t1 (c1);
Query OK, 0 rows affected (0.30 sec)
EXPLAIN SELECT * FROM t1 WHERE c1 = 3;
+-------------------+-------+------+-----------------------------------------------------------------+
| id | count | task | operator info |
+-------------------+-------+------+-----------------------------------------------------------------+
| IndexReader_6 | 10.00 | root | index:IndexScan_5 |
| └─IndexScan_5 | 10.00 | cop | table:t1, index:c1, range:[3,3], keep order:false, stats:pseudo |
+-------------------+-------+------+-----------------------------------------------------------------+
2 rows in set (0.00 sec)
ALTER TABLE t1 DROP INDEX c1;
Query OK, 0 rows affected (0.30 sec)
CREATE UNIQUE INDEX c1 ON t1 (c1);
Query OK, 0 rows affected (0.31 sec)
相关系统变量
和 CREATE INDEX 语句相关的系统变量有 tidb_ddl_reorg_worker_cnt 、tidb_ddl_reorg_batch_size 和 tidb_ddl_reorg_priority，具体可以参考TiDB 特定系统变量。
MySQL 兼容性
- 不支持
FULLTEXT，HASH和SPATIAL索引。 - 不支持降序索引 （类似于 MySQL 5.7）。
- 默认无法向表中添加
PRIMARY KEY，在开启alter-primary-key配置项后可支持此功能，详情参考：alter-primary-key。