环境：mysql 5.7/8.0

导入测试数据：

git clone https://github.com/datacharmer/test_db
cd test_db
mysql -u root -p < employees.sql

employees : 300024 条记录

salaries : 2844047 条记录

1、执行一个两表关联统计SQL，执行速度非常快，整个过程扫描了122行。

mysql> select VARIABLE_VALUE into @a from performance_schema.session_status where variable_name = 'Innodb_rows_read';
Query OK, 1 row affected (0.00 sec)

mysql> select e.emp_no,(select max(s.salary) from salaries s where s.emp_no=e.emp_no) from employees e limit 10;
+--------+----------------------------------------------------------------+
| emp_no | (select max(s.salary) from salaries s where s.emp_no=e.emp_no) |
+--------+----------------------------------------------------------------+
|  10001 |                                                          88958 |
|  10002 |                                                          72527 |
|  10003 |                                                          43699 |
|  10004 |                                                          74057 |
|  10005 |                                                          94692 |
|  10006 |                                                          60098 |
|  10007 |                                                          88070 |
|  10008 |                                                          52668 |
|  10009 |                                                          94443 |
|  10010 |                                                          80324 |
+--------+----------------------------------------------------------------+
10 rows in set (0.00 sec)

mysql> select VARIABLE_VALUE into @b from performance_schema.session_status where variable_name = 'Innodb_rows_read';
Query OK, 1 row affected (0.00 sec)

mysql> select @b-@a;
+-------+
| @b-@a |
+-------+
|   122 |
+-------+
1 row in set (0.00 sec)

2、将这个关联SQL，做成视图，再次查询会非常慢，实际扫描了 314W 行。

mysql> create view v_test as select e.emp_no,(select max(s.salary) from salaries s where s.emp_no=e.emp_no) from employees e;
Query OK, 0 rows affected (0.01 sec)

mysql> select VARIABLE_VALUE into @a from performance_schema.session_status where variable_name = 'Innodb_rows_read';
Query OK, 1 row affected (0.00 sec)

mysql> select * from v_test limit 10;
+--------+----------------------------------------------------------------+
| emp_no | (select max(s.salary) from salaries s where s.emp_no=e.emp_no) |
+--------+----------------------------------------------------------------+
|  10001 |                                                          88958 |
|  10002 |                                                          72527 |
|  10003 |                                                          43699 |
|  10004 |                                                          74057 |
|  10005 |                                                          94692 |
|  10006 |                                                          60098 |
|  10007 |                                                          88070 |
|  10008 |                                                          52668 |
|  10009 |                                                          94443 |
|  10010 |                                                          80324 |
+--------+----------------------------------------------------------------+
10 rows in set (1.34 sec)

mysql> select VARIABLE_VALUE into @b from performance_schema.session_status where variable_name = 'Innodb_rows_read';
Query OK, 1 row affected (0.00 sec)

mysql> select @b-@a;
+---------+
| @b-@a   |
+---------+
| 3144071 |
+---------+
1 row in set (0.00 sec)

3、分别查看执行计划

mysql> explain select e.emp_no,(select max(s.salary) from salaries s where s.emp_no=e.emp_no) from employees e limit 10;
+----+--------------------+-------+------------+-------+---------------+---------+---------+--------------------+--------+----------+-------------+
| id | select_type        | table | partitions | type  | possible_keys | key     | key_len | ref                | rows   | filtered | Extra       |
+----+--------------------+-------+------------+-------+---------------+---------+---------+--------------------+--------+----------+-------------+
|  1 | PRIMARY            | e     | NULL       | index | NULL          | PRIMARY | 4       | NULL               | 299556 |   100.00 | Using index |
|  2 | DEPENDENT SUBQUERY | s     | NULL       | ref   | PRIMARY       | PRIMARY | 4       | employees.e.emp_no |      9 |   100.00 | NULL        |
+----+--------------------+-------+------------+-------+---------------+---------+---------+--------------------+--------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)

mysql> explain select * from v_test limit 10;
+----+--------------------+------------+------------+-------+---------------+---------+---------+--------------------+--------+----------+-------------+
| id | select_type        | table      | partitions | type  | possible_keys | key     | key_len | ref                | rows   | filtered | Extra       |
+----+--------------------+------------+------------+-------+---------------+---------+---------+--------------------+--------+----------+-------------+
|  1 | PRIMARY            | <derived2> | NULL       | ALL   | NULL          | NULL    | NULL    | NULL               | 299556 |   100.00 | NULL        |
|  2 | DERIVED            | e          | NULL       | index | NULL          | PRIMARY | 4       | NULL               | 299556 |   100.00 | Using index |
|  3 | DEPENDENT SUBQUERY | s          | NULL       | ref   | PRIMARY       | PRIMARY | 4       | employees.e.emp_no |      9 |   100.00 | NULL        |
+----+--------------------+------------+------------+-------+---------------+---------+---------+--------------------+--------+----------+-------------+
3 rows in set, 2 warnings (0.00 sec)

4、分析执行计划：

两个执行计划中，唯一不同的是使用视图后，多了一个派生表。

关于派生表说明如下：

https://dev.mysql.com/doc/refman/5.7/en/derived-tables.html

关于派生表官方优化

https://dev.mysql.com/doc/refman/5.7/en/derived-table-optimization.html

由于加了limit 10；第一个 SQL 虽然显示 e 表扫描行数很多，但实际并没有进行全表扫描，只统计了前10条记录便停止了。

第二个 SQL 虽然也加了 limit 10，但因为优化器产生了派生表，也就是将统计SQL结果都写入到一个临时表中，再到这个临时表中去读10条记录。

官方虽然有派生表合并优化功能，但对于派生表中包含聚合函数，group by  ，having ， count ，limit 等，就无法进行优化。

目前解决这种问题，应该只有一个办法 ，就是别用视图。