欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页  >  数据库

mysql隐形转换造成索引失效的事故总结

程序员文章站 2022-04-05 09:10:22
...

隐式转换导致索引失效.这一点应当引起重视.也是开发中经常会犯的错误.由于表的字段tu_mdn定义为varchar2(20),但在查询时把该字段作为number类型以where条件传给m

隐式转换导致索引失效.这一点应当引起重视.也是开发中经常会犯的错误. 由于表的字段tu_mdn定义为varchar2(20),但在查询时把该字段作为number类型以where条件传给mysql,这样会导致索引失效. 错误的例子:select * from test where tu_mdn=13333333333; 正确的例子:select * from test where tu_mdn='13333333333';

看一下下面的案例,这个案例是开发过程中经常犯的一个错误,这种索引在大表的查询中是很致命的,直接能把数据库拖死:

mysql> show create table shipping\G;

*************************** 1. row ***************************

Table: shipping

Create Table: CREATE TABLE `shipping` (

`id` int(11) unsigned NOT NULL AUTO_INCREMENT,

`shipping_no` int(11) NOT NULL,

PRIMARY KEY (`id`),

UNIQUE KEY `shipping_no` (`shipping_no`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1

1 row in set (0.00 sec)

mysql> explain SELECT shipping_no FROM `shipping` WHERE `shipping_no` IN (62487941,62653594,62952180,63556576,63684186,99097538006,100433005006,100433006006);

+----+-------------+----------------------+-------+---------------+-------------+---------+------+------+-------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+----------------------+-------+---------------+-------------+---------+------+------+-------------+

| 1 | SIMPLE | shipping | range | shipping_no | shipping_no | 4 | NULL | 6 | Using where |

+----+-------------+----------------------+-------+---------------+-------------+---------+------+------+-------------+

1 row in set (0.00 sec)


mysql> explain SELECT shipping_no FROM `shipping` WHERE (`shipping_no` IN ('62487941','62653594','62952180','63556576','63684186','99097538006','100433005006','100433006006'));

+----+-------------+----------------------+------+---------------+------+---------+------+----------+-------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+----------------------+------+---------------+------+---------+------+----------+-------------+

| 1 | SIMPLE | shipping | ALL | shipping_no | NULL | NULL | NULL | 12803696 | Using where |

+----+-------------+----------------------+------+---------------+------+---------+------+----------+-------------+

1 row in set (0.00 sec)

很蛋疼的东西,希望开发者在开发的时候注意字段不要越界,最主要的是不要使用隐形转换,有些是转换不了的,DBA的同行们注意这种隐形转换带来的危害,,一定要给开发者提供规范。