比如说，我有一张表：

create table test (
    id    int primary key auto_increment,
    c_no  varchar(11),
    c2    varchar(2),
    
    key c_no(c_no)
);

然后基于这个表，我有一个sql查询，它是：

select
    *
from
    test
where
    c_no = 100

但是当我使用explain分析sql时，我发现它没有使用c_no上的索引：

explain  select *  from test2 where c_no=100

+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | test | NULL       | ALL  | c_no          | NULL | NULL    | NULL |    7 |    14.29 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 3 warnings (0.00 sec)

为甚么？