避免在sql的where中对字段进行转换（函数转换/操作符转换/隐式转换）------索引会失效从而导致慢查询

        看例子：

mysql> show create table tb_test;
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                                                                                                                                                             |
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tb_test | CREATE TABLE `tb_test` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'test',
  `name` varchar(32) NOT NULL COMMENT 'test',
  `score` int(10) unsigned NOT NULL DEFAULT '0' COMMENT 'test',
  PRIMARY KEY (`id`),
  KEY `id_score` (`score`)
) ENGINE=InnoDB AUTO_INCREMENT=10005 DEFAULT CHARSET=utf8 COMMENT='测试表， 无实际作用'          |
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

        可见，  score是索引， 我们继续看：

mysql> select * from tb_test where score = 1;        
+----+------+-------+
| id | name | score |
+----+------+-------+
|  1 | n1   |     1 |
+----+------+-------+
1 row in set (0.00 sec)

mysql> select * from tb_test where score + 1 = 2;        
+----+------+-------+
| id | name | score |
+----+------+-------+
|  1 | n1   |     1 |
+----+------+-------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> show profiles;
+----------+------------+---------------------------------------------------+
| Query_ID | Duration   | Query                                             |
+----------+------------+---------------------------------------------------+

|    10061 | 0.00076248 | select * from tb_test where score = 1             |
|    10062 | 0.00316482 | select * from tb_test where score + 1 = 2         |
+----------+------------+---------------------------------------------------+
15 rows in set (0.00 sec)

        可以看到， 时间差距较大， 为什么呢？ 因为对sql的where字段进行转换（函数转换/操作符转换/隐式转换）， 会导致索引失效。来explain看看：

mysql> explain select * from tb_test where score = 1;    
+------+-------------+---------+------+---------------+----------+---------+-------+------+-------+
| id   | select_type | table   | type | possible_keys | key      | key_len | ref   | rows | Extra |
+------+-------------+---------+------+---------------+----------+---------+-------+------+-------+
|    1 | SIMPLE      | tb_test | ref  | id_score      | id_score | 4       | const |    1 |       |
+------+-------------+---------+------+---------------+----------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain select * from tb_test where score + 1 = 2;
+------+-------------+---------+------+---------------+------+---------+------+-------+-------------+
| id   | select_type | table   | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+------+-------------+---------+------+---------------+------+---------+------+-------+-------------+
|    1 | SIMPLE      | tb_test | ALL  | NULL          | NULL | NULL    | NULL | 10224 | Using where |
+------+-------------+---------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.01 sec)

       看type, 很显然， 后者进行了全表搜索， 也就是索引失效了。 看看 key和rows也能获知结果。

       要避免， 杜绝。