Case：update中把in改写成join性能提高数倍

(1)优化前

如下一条SQL，把从1985-05-21入职前的员工薪资都增加500，执行约20.70 s，

从执行计划中可以看出对表salaries进行的是索引全扫描，扫描行数约260W行。

mysql> update salaries set salary=salary+500 where emp_no in (select emp_no from employees where hire_date<='1985-05-21');Query OK, 151583 rows affected (20.70 sec)Rows matched: 151583  Changed: 151583  Warnings: 0mysql> desc update salaries set salary=salary+500 where emp_no in (select emp_no from employees where hire_date<='1985-05-21');+----+--------------------+-----------+------------+-----------------+---------------+---------+---------+------+---------+----------+-------------+| id | select_type        | table     | partitions | type            | possible_keys | key     | key_len | ref  | rows    | filtered | Extra       |+----+--------------------+-----------+------------+-----------------+---------------+---------+---------+------+---------+----------+-------------+|  1 | UPDATE             | salaries  | NULL       | index           | NULL          | PRIMARY | 7       | NULL | 2674458 |   100.00 | Using where ||  2 | DEPENDENT SUBQUERY | employees | NULL       | unique_subquery | PRIMARY       | PRIMARY | 4       | func |       1 |    33.33 | Using where |+----+--------------------+-----------+------------+-----------------+---------------+---------+---------+------+---------+----------+-------------+2 rows in set, 1 warning (0.00 sec)

(2)优化后

把in改写成join后，虽然对employees是全表扫描，但是扫描行数近29W行，大大减少，所以SQL执行时间可以缩减到7.26s.

mysql> update salaries s join (select distinct e.emp_no from employees e where e.hire_date<='1985-05-21') e on s.emp_no=e.emp_no    -> set s.salary=salary+500;Query OK, 151583 rows affected (7.26 sec)Rows matched: 151583  Changed: 151583  Warnings: 0mysql> desc update salaries s join (select distinct e.emp_no from employees e where e.hire_date<='1985-05-21') e on s.emp_no=e.emp_no    -> set s.salary=salary+500;+----+-------------+------------+------------+------+----------------+---------+---------+----------+--------+----------+-------------+| id | select_type | table      | partitions | type | possible_keys  | key     | key_len | ref      | rows   | filtered | Extra       |+----+-------------+------------+------------+------+----------------+---------+---------+----------+--------+----------+-------------+|  1 | PRIMARY     |  | NULL       | ALL  | NULL           | NULL    | NULL    | NULL     |  99827 |   100.00 | NULL        ||  1 | UPDATE      | s          | NULL       | ref  | PRIMARY,emp_no | PRIMARY | 4       | e.emp_no |     10 |   100.00 | NULL        ||  2 | DERIVED     | e          | NULL       | ALL  | PRIMARY        | NULL    | NULL    | NULL     | 299512 |    33.33 | Using where |+----+-------------+------------+------------+------+----------------+---------+---------+----------+--------+----------+-------------+3 rows in set, 1 warning (0.00 sec)