Mysql查询语句中的引号对索引的影响

来源：动视网责编：小采时间：2020-11-09 07:41:09

Mysql查询语句中的引号对索引的影响

Mysql查询语句中的引号对索引的影响:# explain select * from t1 where user_ip=123 limit 1;-------------------------------------------------------------------------------------------| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |-------

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导读Mysql查询语句中的引号对索引的影响:# explain select * from t1 where user_ip=123 limit 1;-------------------------------------------------------------------------------------------| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |-------

# explain select * from t1 where user_ip=123 limit 1;-------------------------------------------------------------------------------------------| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |-------

# 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.