MySQLl数据量不一样，导致走不同的索引

1、测试环境：Mysql 5.7.17

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)