1、测试环境：MySQL 5.7.17

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2、测试表结构

mysql> show create table a; +-------+--------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table                                                                                                                         | +-------+--------------------------------------------------------------------------------------------------------------------------------------+ | a     | CREATE TABLE `a` (   `id` int(11) NOT NULL,   `name` char(20) DEFAULT NULL,   PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=gbk | +-------+--------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)mysql> show create table b; +-------+------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table                                                                                                                       | +-------+------------------------------------------------------------------------------------------------------------------------------------+ | b     | CREATE TABLE `b` (   `id` int(11) NOT NULL,   `tx` char(20) DEFAULT NULL,   PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=gbk | +-------+------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)

3、两张表的数据量

mysql> select count(*) from a; +----------+ | count(*) | +----------+ |        7 | +----------+ 1 row in set (0.00 sec) mysql> select count(*) from b; +----------+ | count(*) | +----------+ |       10 | +----------+ 1 row in set (0.00 sec)

4、查看执行计划

mysql> explain select name from a,b where a.id=b.id; +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ | id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ |  1 | SIMPLE      | a     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL      |    7 |   100.00 | NULL        | |  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.a.id |    1 |   100.00 | Using index | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)mysql> explain select name from a,b where b.id=a.id; +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ | id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ |  1 | SIMPLE      | a     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL      |    7 |   100.00 | NULL        | |  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.a.id |    1 |   100.00 | Using index | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)mysql> explain select name from b,a where b.id=a.id; +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ | id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ |  1 | SIMPLE      | a     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL      |    7 |   100.00 | NULL        | |  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.a.id |    1 |   100.00 | Using index | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)

5、向a表插入3数据，使两表数据量一样，查看执行计划，发现第三条语句的执行计划发生了变化

mysql> insert into a values(8,'test'); Query OK, 1 row affected (0.00 sec) mysql> insert into a values(9,'test'); Query OK, 1 row affected (0.00 sec) mysql> insert into a values(10,'test'); Query OK, 1 row affected (0.01 sec) mysql> select count(*) from a; +----------+ | count(*) | +----------+ |       10 | +----------+ 1 row in set (0.00 sec)mysql> explain select name from a,b where a.id=b.id; +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ | id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ |  1 | SIMPLE      | a     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL      |   10 |   100.00 | NULL        | |  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.a.id |    1 |   100.00 | Using index | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)mysql> explain select name from a,b where b.id=a.id; +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ | id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ |  1 | SIMPLE      | a     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL      |   10 |   100.00 | NULL        | |  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.a.id |    1 |   100.00 | Using index | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)mysql> explain select name from b,a where b.id=a.id; +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ | id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ |  1 | SIMPLE      | b     | NULL       | index  | PRIMARY       | PRIMARY | 4       | NULL      |   10 |   100.00 | Using index | |  1 | SIMPLE      | a     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.b.id |    1 |   100.00 | NULL        | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)

6、向a表插入1条数据，使a表数据量大于b表，查看执行计划，三条语句执行计划都发现了变化

mysql> insert into a values(11,'test'); Query OK, 1 row affected (0.01 sec) mysql> select count(*) from a; +----------+ | count(*) | +----------+ |       11 | +----------+ 1 row in set (0.00 sec)mysql> explain select name from a,b where a.id=b.id; +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ | id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ |  1 | SIMPLE      | b     | NULL       | index  | PRIMARY       | PRIMARY | 4       | NULL      |   10 |   100.00 | Using index | |  1 | SIMPLE      | a     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.b.id |    1 |   100.00 | NULL        | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)mysql> explain select name from a,b where b.id=a.id; +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ | id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ |  1 | SIMPLE      | b     | NULL       | index  | PRIMARY       | PRIMARY | 4       | NULL      |   10 |   100.00 | Using index | |  1 | SIMPLE      | a     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.b.id |    1 |   100.00 | NULL        | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)mysql> explain select name from b,a where b.id=a.id; +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ | id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ |  1 | SIMPLE      | b     | NULL       | index  | PRIMARY       | PRIMARY | 4       | NULL      |   10 |   100.00 | Using index | |  1 | SIMPLE      | a     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | apex.b.id |    1 |   100.00 | NULL        | +----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+ 2 rows in set, 1 warning (0.01 sec)

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