引背用策优化复制SHOW INDEXFROM employees.titles;  +--------+------------+----------+--------------+-------------+-----------+-------------+------+------------+ | Table  | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Null | Index_type |  +--------+------------+----------+--------------+-------------+-----------+-------------+------+------------+ | titles |          0 | PRIMARY  |            1 | emp_no      | A         |        NULL |      | BTREE      |  | titles |          0 | PRIMARY  |            2 | title       | A         |        NULL |      | BTREE      |  | titles |          0 | PRIMARY  |            3 | from_date   | A         |      443308 |      | BTREE      |  | titles |          1 | emp_no   |            1 | emp_no      | A         |      443308 |      | BTREE      |  +--------+------------+----------+--------------+-------------+-----------+-------------+------+------------+ 1.2.3.4.5.6.7.8.9.