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mysql索引长度tips innodb和myisam引擎

 

由于开发人员对索引认识不深或忽略,还有版本不同等问题,在生产环境中创建表失败,引发了一些问题。归纳了一下

    测试环境

 

       mysql> select version();

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

| version()  |

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

| 5.5.31-log | 

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

1 row in set (0.01 sec)

   innodb 引擎

mysql> CREATE TABLE `meta_topic_scan` (   `domain` varchar(257) NOT NULL,   `topic_name` varchar(200) NOT NULL,   `topic_url` varchar(200) NOT NULL,   `topic_pv` int(11) DEFAULT'0',   `topic_uv` int(11) DEFAULT '0',   PRIMARY KEY (`domain`,`topic_url`) ) ENGINE=innodb  DEFAULT CHARSET=utf8;

ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes

 

innodb 单列索引长度不能超过767 bytes,联合索引限制是3072 bytes 。对于创建innodb的组合索引中,如果各个列中的长度有单个超过767 bytes,也会创建失败;

 

myisam 引擎 

创建复合索引:

 

mysql> CREATE TABLE `meta_topic_scan` (

    ->   `domain` varchar(200) NOT NULL,

    ->   `topic_name` varchar(200) NOT NULL,

    ->   `topic_url` varchar(200) NOT NULL,

    ->   `topic_pv` int(11) DEFAULT '0',

    ->   `topic_uv` int(11) DEFAULT '0',

    ->   PRIMARY KEY (`domain`,`topic_name`,`topic_url`)

    -> ) ENGINE=myisam DEFAULT CHARSET=utf8 ;

ERROR 1071 (42000): Specified key was too long; max key length is 1000 bytes

创建单列索引:

mysql> CREATE TABLE `meta_topic_scan` (

    ->   `domain` varchar(334) NOT NULL,

    ->   `topic_name` varchar(200) NOT NULL,

    ->   `topic_url` varchar(200) NOT NULL,

    ->   `topic_pv` int(11) DEFAULT '0',

    ->   `topic_uv` int(11) DEFAULT '0',

    ->   PRIMARY KEY (`domain`)

    -> ) ENGINE=myisam DEFAULT CHARSET=utf8 ;

ERROR 1071 (42000): Specified key was too long; max key length is 1000 bytes

由此可知:myisam 单列索引长度、所创建的复合索引长度和都不能超过1000 bytes,否则会报错,创建失败。

另外不同字符集占用不同字节:latin一个字符占1 bytes,utf8存储一个字符占3 bytes, gbk存储一个字符2 bytes

扩展: innodb复合索引长度为什么是3072 

 我们知道InnoDB一个page的默认大小是16k。由于是Btree组织,要求叶子节点上一个page至少要包含两条记录(否则就退化链表了)。

 

        所以一个记录最多不能超过8k。

        又由于InnoDB的聚簇索引结构,一个二级索引要包含主键索引,因此每个单个索引不能超过4k (极端情况,pk和某个二级索引都达到这个限制)。

         由于需要预留和辅助空间,扣掉后不能超过3500,取个“整数”就是(1024*3)。 

 

单列索引限制

 

         上面有提到单列索引限制767,起因是256×3-1。这个3是字符最大占用空间(utf8)。但是在5.5以后,开始支持4个字节的uutf8。255×4>767, 于是增加了一个参数叫做 innodb_large_prefix。

 

         这个参数默认值是OFF。当改为ON时,允许列索引最大达到3072。

 

               如下效果(5.5):


mysql索引长度tips innodb和myisam引擎_MySQL


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