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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。
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