左外连接在反转连接顺序时不使用键

Question

正如您所看到的，两个表具有相同的结构，但是一个左外连接使用键而另一个没有，因此在第二种情况下性能太差。任何解释为什么第二个左连接不使用键？

mysql> show create table t1;
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                          |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `certificateNo` varchar(32) DEFAULT NULL,
  UNIQUE KEY `certificateNo` (`certificateNo`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show create table t2;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                             |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t2    | CREATE TABLE `t2` (
  `certificateNo` varchar(32) CHARACTER SET utf8 DEFAULT NULL,
  UNIQUE KEY `certificateNo` (`certificateNo`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select count(1) from t1 left outer join t2 on (t1.certificateNo = t2.certificateNo) where t2.certificateNo is null;
+----------+
| count(1) |
+----------+
|    10839 |
+----------+
1 row in set (0.00 sec)

mysql> explain select count(1) from t1 left outer join t2 on (t1.certificateNo = t2.certificateNo) where t2.certificateNo is null;
+----+-------------+-------+-------+---------------+---------------+---------+------+-------+--------------------------+
| id | select_type | table | type  | possible_keys | key           | key_len | ref  | rows  | Extra                    |
+----+-------------+-------+-------+---------------+---------------+---------+------+-------+--------------------------+
|  1 | SIMPLE      | t1    | index | NULL          | certificateNo | 35      | NULL | 35691 | Using index              |
|  1 | SIMPLE      | t2    | ref   | certificateNo | certificateNo | 99      | func |   138 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------------+---------+------+-------+--------------------------+
2 rows in set (0.01 sec)

mysql> explain select count(1) from t2 left outer join t1 on (t1.certificateNo = t2.certificateNo) where t1.certificateNo is null;
+----+-------------+-------+-------+---------------+---------------+---------+------+-------+--------------------------+
| id | select_type | table | type  | possible_keys | key           | key_len | ref  | rows  | Extra                    |
+----+-------------+-------+-------+---------------+---------------+---------+------+-------+--------------------------+
|  1 | SIMPLE      | t2    | index | NULL          | certificateNo | 99      | NULL | 27785 | Using index              |
|  1 | SIMPLE      | t1    | index | NULL          | certificateNo | 35      | NULL | 35691 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------------+---------+------+-------+--------------------------+
2 rows in set (0.00 sec)

mysql> select count(1) from t1 inner join t2  on (t1.certificateNo = t2.certificateNo) ;
+----------+
| count(1) |
+----------+
|    21603 |
+----------+
1 row in set (0.16 sec)

mysql> select count(1) from t1;
+----------+
| count(1) |
+----------+
|    32442 |
+----------+
1 row in set (0.01 sec)

mysql> select count(1) from t2;
+----------+
| count(1) |
+----------+
|    26837 |
+----------+
1 row in set (0.01 sec)