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经过字段类型转化后的查询不走索引
表结构如下：
mysql> show create table tag_item_list\G
*************************** 1. row ***************************
Table: tag_item_list
Create Table: CREATE TABLE `tag_item_list` (
`tag_item_list_id` bigint(20) unsigned NOT NULL auto_increment,
`tag_id` bigint(20) unsigned NOT NULL,
`item_type` bigint(20) unsigned NOT NULL,
`item_id` varchar(100) NOT NULL,
`list_id` bigint(20) unsigned NOT NULL,
`deleted` tinyint(1) NOT NULL,
`item_ext1` bigint(20) NOT NULL default ‘0‘,
`item_ext2` bigint(20) NOT NULL default ‘0‘,
`item_ext3` bigint(20) NOT NULL default ‘0‘,
`relation_ext1` bigint(20) NOT NULL default ‘0‘,
`relation_ext2` bigint(20) NOT NULL default ‘0‘,
`relation_ext3` bigint(20) NOT NULL default ‘0‘,
PRIMARY KEY (`tag_item_list_id`),
KEY `idx_iid_itp` (`item_id`,`item_type`),
KEY `idx_tid_itp` (`tag_id`,`item_type`)
) ENGINE=InnoDB AUTO_INCREMENT=231210 DEFAULT CHARSET=utf8 ROW_FORMAT=DYNAMIC
1 row in set (0.00 sec)
索引字段上有发生int到varchare、varbinary的类型转换，不会使用到索引
mysql> explain SELECT sql_no_cache * from tag_item_list where item_id = 122 and item_type = 1 ;
+----+-------------+---------------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------+------+---------------+------+---------+------+--------+-------------+
| 1 | SIMPLE | tag_item_list | ALL | idx_iid_itp | NULL | NULL | NULL | 231852 | Using where |
+----+-------------+---------------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)
mysql> explain SELECT sql_no_cache * from tag_item_list where item_id = ‘122‘ and item_type = 1 ;
+----+-------------+---------------+------+---------------+-------------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------+------+---------------+-------------+---------+-------------+------+-------------+
| 1 | SIMPLE | tag_item_list | ref | idx_iid_itp | idx_iid_itp | 310 | const,const | 1 | Using where |
+----+-------------+---------------+------+---------------+-------------+---------+-------------+------+-------------+
1 row in set (0.00 sec)
而如在索引字段上发生varchar向int的类型转化，仍然会使用到索引
mysql> show create table tag_item_list\G
*************************** 1. row ***************************
Table: tag_item_list
Create Table: CREATE TABLE `tag_item_list` (
`tag_item_list_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`tag_id` bigint(20) unsigned NOT NULL,
`item_type` bigint(20) unsigned NOT NULL,
`item_id` int(10) NOT NULL,
`list_id` bigint(20) unsigned NOT NULL,
`deleted` tinyint(1) NOT NULL,
`item_ext1` bigint(20) NOT NULL DEFAULT ‘0‘,
`item_ext2` bigint(20) NOT NULL DEFAULT ‘0‘,
`item_ext3` bigint(20) NOT NULL DEFAULT ‘0‘,
`relation_ext1` bigint(20) NOT NULL DEFAULT ‘0‘,
`relation_ext2` bigint(20) NOT NULL DEFAULT ‘0‘,
`relation_ext3` bigint(20) NOT NULL DEFAULT ‘0‘,
PRIMARY KEY (`tag_item_list_id`),
KEY `idx_itemid_type` (`item_id`,`item_type`),
KEY `idx_tayid_type` (`tag_id`,`item_type`)
) ENGINE=InnoDB AUTO_INCREMENT=226322 DEFAULT CHARSET=utf8 ROW_FORMAT=DYNAMIC
mysql> explain SELECT sql_no_cache * from tag_item_list where item_id = ‘122‘ and item_type = 1 ;
+----+-------------+---------------+------+-----------------+-----------------+---------+-------------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------+------+-----------------+-----------------+---------+-------------+------+-------+
| 1 | SIMPLE | tag_item_list | ref | idx_itemid_type | idx_itemid_type | 12 | const,const | 1 | |
+----+-------------+---------------+------+-----------------+-----------------+---------+-------------+------+-------+
测试mysql版本 5.0.51b，5.1.45
经过字段类型转化后的查询不走索引