字符类型的数据没有使用引号，索引将被抑制，下边看一个案例：
查看表结构：

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mysql> show create table test06 \G
*************************** 1. row ***************************
Table: test06
Create Table: CREATE TABLE `test06` (
`id` bigint(11) NOT NULL DEFAULT '0',
`u_id` bigint(11) NOT NULL,
`openid` varchar(100) DEFAULT NULL,
`unionid` varchar(100) DEFAULT NULL,
`username` varchar(100) NOT NULL,
`password` varchar(100) NOT NULL,
`create_time` datetime NOT NULL,
KEY `idx_test03_id` (`id`),
KEY `idx_test03_name` (`username`),
KEY `idx_test06_crea_time` (`create_time`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)username字段不加引号：

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mysql> select * from test06 where username=13499770088;
Empty set, 8208 warnings (5.77 sec)

mysql> explain select * from test06 where username=13499770088;
+----+-------------+--------+------+-----------------+------+---------+------+---------+-------------+
| id | select_type | table| type | possible_keys | key| key_len | ref | rows | Extra |
+----+-------------+--------+------+-----------------+------+---------+------+---------+-------------+
| 1 | SIMPLE | test06 | ALL | idx_test03_name | NULL | NULL | NULL | 2009559 | Using where |
+----+-------------+--------+------+-----------------+------+---------+------+---------+-------------+username字段加引号：

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mysql> select * from test06 where username='13499770088';
Empty set (0.07 sec)

mysql> explain select * from test06 where username='13499770088';
+----+-------------+--------+------+-----------------+-----------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+-----------------+-----------------+---------+-------+------+-----------------------+
| 1 | SIMPLE | test06 | ref | idx_test03_name | idx_test03_name | 302 | const | 1 | Using index condition |
+----+-------------+--------+------+-----------------+-----------------+---------+-------+------+-----------------------+查询速度明显变快，执行计划走了索引，这样是正常的写法。