2014-01-16

MySQL查询语句中的引号对索引使用的影响.

Views: 16248 | 2 Comments
# explain select * from t1 where user_ip=123 limit 1;
+----+-------------+-------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table       | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1 | ALL  | user_ip       | NULL | NULL    | NULL |  262 | Using where |
+----+-------------+-------------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

# explain select * from t1 where user_ip='123' limit 1;
+----+-------------+-------------+------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table       | type | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+-------------+-------------+------+---------------+---------+---------+-------+------+-------------+
|  1 | SIMPLE      | t1 | ref  | user_ip       | user_ip | 47      | const |    1 | Using where |
+----+-------------+-------------+------+---------------+---------+---------+-------+------+-------------+
1 row in set (0.01 sec)

可以看到, user_ip 字段是 varchar 类型. 查询时, 如果这个字段的值没有加引号, MySQL 就不会使用索引. MySQL 为什么会有这种致命的策略?!

----- 更新: -----

相关的文档已经找到: http://dev.mysql.com/doc/refman/5.5/en/type-conversion.html, 这里面提到和几种类型转换的情况, 字符串和数字比较都被统一转成浮点数再比较.

Related posts:

  1. MySQL”海量数据”查询性能分析
  2. MySQL 查询使用 Group By 的注意点
  3. 史上最强大的PHP MySQL操作类
  4. MySQL 建立外键约束
  5. 如何使用SSDB的zscan命令
Posted by ideawu at 2014-01-16 13:47:15

2 Responses to "MySQL查询语句中的引号对索引使用的影响."

Leave a Comment