(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: 0
mysql> 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: 0
mysql> 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 | <derived2> | 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)
