字段类型是:
`enterpriseId` int(10) unsigned DEFAULT NULL,
`email` char(255) NOT NULL DEFAULT '',
表的索引是:
UNIQUE KEY `emailent` (`email`,`enterpriseId`),
KEY `edf` (`enterpriseId`,`departId`,`flag`),
有这么两条sql语句,分别表现是:
- explain select email from email where enterpriseId= and (email like 'aaa%');
- +----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
- | | SIMPLE | email | ref | emailent,edf | edf | | const | | Using where |
看到key_len的长度是5 ,可以知道使用的是edf这个索引 , 因为edf索引中的enterpriseId是int类型4个字节 ,默认null 加1个字节,总共5个字节
也就是先使用enterpriseId查到索引,在索引中使用where过滤数据
- explain select email from email where enterpriseId= and (email like 'aaas%');
- +----+-------------+-------+-------+---------------+----------+---------+------+------+--------------------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+-------+-------+---------------+----------+---------+------+------+--------------------------+
- | | SIMPLE | email | range | emailent,edf | emailent | | NULL | | Using where; Using index |
- +----+-------------+-------+-------+---------------+----------+---------+------+------+--------------------------+
在like的时候比上面多了一个字符,这个时候的索引情况是key_len是770,可以知道使用的是emailent这个索引,因为这个的索引长度是
255*3+5=770 varchar是255个字符,utf8下是*3, 加上int 5个字节
like两边都有%的情况,只会使用第一个条件的edf索引
- mysql> explain select * from email where enterpriseId= and (email like '%shihanasas%');
- +----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
- | | SIMPLE | email | ref | edf | edf | | const | | Using where |
- +----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
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