mysql 的 fiter push down 优化

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原SQL执行了 4秒:

mysql> select * from employees e inner join (select emp_no,count(*) from salaries group by emp_no) s on s.emp_no=e.emp
o where e.emp_no BETWEEN 10001 and 10010;

|  10008 | 1958-02-19 | Saniya     | Kalloufi  | M      | 1994-09-15 |  10008 |        3 |
|  10009 | 1952-04-19 | Sumant     | Peac      | F      | 1985-02-18 |  10009 |       18 |
|  10010 | 1963-06-01 | Duangkaew  | Piveteau  | F      | 1989-08-24 |  10010 |        6 |
+--------+------------+------------+-----------+--------+------------+--------+----------+
10 rows in set (4.11 sec)

　　

执行计划, 中间有个auto_key 是mysql 临时在内存创建索引 , salaries表id 是 2  优先执行 , 如果这个表统计数据量很多,那么先等待它执行完之后才能再跟employee表关联增加执行时间 , 子查询有group by 就不能合并到视图里面去 , 这就是  “filter push-down” , “filter push-down”的目的是推动视图内的限制或不能合并的内联视图。 , 并没有重复使用mysql 优化器优势 :

mysql> explain select * from employees e inner join (select emp_no,count(*) from salaries group by emp_no) s on s.emp_no=e.emp_no where e.emp_no BETWEEN 10001 and 10010;
+----+-------------+------------+------------+-------+----------------------+-------------+---------+--------------------+---------+----------+-------------+
| id | select_type | table      | partitions | type  | possible_keys        | key         | key_len | ref                | rows    | filtered | Extra       |
+----+-------------+------------+------------+-------+----------------------+-------------+---------+--------------------+---------+----------+-------------+
|  1 | PRIMARY     | e          | NULL       | range | PRIMARY              | PRIMARY     | 4       | NULL               |      10 |      100 | Using where |
|  1 | PRIMARY     | <derived2> | NULL       | ref   | <auto_key0>          | <auto_key0> | 4       | employees.e.emp_no |      10 |      100 | NULL        |
|  2 | DERIVED     | salaries   | NULL       | index | PRIMARY,emp_no,idx_s | emp_no      | 4       | NULL               | 2694129 |      100 | Using index |
+----+-------------+------------+------------+-------+----------------------+-------------+---------+--------------------+---------+----------+-------------+

　　

那么现在就是要想把emp 表 和 sal 表 同时统计同进进行关联提高效率 ,  做法就是把两个表放到一个子查询里面 , 并且摆脱了mysql 临时在内存中创建索引 , 虽然还有using filesort , 但并不是重点  , 效率提升到 0.01 秒 :

select * from employees e inner join
(
select s.emp_no,count(*) from employees e inner join salaries s on s.emp_no = e.emp_no where e.emp_no BETWEEN 10001 and 10010 group by s.emp_no
) s where e.emp_no BETWEEN 10001 and 10010;

| 10008 | 1958-02-19 | Saniya | Kalloufi | M | 1994-09-15 | 10010 | 6 |
| 10009 | 1952-04-19 | Sumant | Peac | F | 1985-02-18 | 10010 | 6 |
| 10010 | 1963-06-01 | Duangkaew | Piveteau | F | 1989-08-24 | 10010 | 6 |
+--------+------------+------------+-----------+--------+------------+--------+----------+
100 rows in set (0.01 sec)

mysql> explain select * from employees e inner join
(
select s.emp_no,count(*) from employees e inner join salaries s on s.emp_no = e.emp_no where e.emp_no BETWEEN 10001 and 10010 group by s.emp_no
) s where e.emp_no BETWEEN 10001 and 10010;
+----+-------------+------------+------------+-------+----------------------+---------+---------+--------------------+------+----------+-----------------------------------------------------------+
| id | select_type | table      | partitions | type  | possible_keys        | key     | key_len | ref                | rows | filtered | Extra                                                     |
+----+-------------+------------+------------+-------+----------------------+---------+---------+--------------------+------+----------+-----------------------------------------------------------+
|  1 | PRIMARY     | e          | NULL       | range | PRIMARY              | PRIMARY | 4       | NULL               |   10 |      100 | Using where                                               |
|  1 | PRIMARY     | <derived2> | NULL       | ALL   | NULL                 | NULL    | NULL    | NULL               |   93 |      100 | Using join buffer (Block Nested Loop)                     |
|  2 | DERIVED     | e          | NULL       | range | PRIMARY              | PRIMARY | 4       | NULL               |   10 |      100 | Using where; Using index; Using temporary; Using filesort |
|  2 | DERIVED     | s          | NULL       | ref   | PRIMARY,emp_no,idx_s | emp_no  | 4       | employees.e.emp_no |    9 |      100 | Using index                                               |
+----+-------------+------------+------------+-------+----------------------+---------+---------+--------------------+------+----------+-----------------------------------------------------------+