Mysql查询语句中的引号对索引的影响

http://www.ideawu.net/blog/archives/775

2014-01-16

MySQL查询语句中的引号对索引使用的影响.

高性能Web架构

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# 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.

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