#mysql 笔记#索引长度限制

http://dev.mysql.com/doc/refman/5.5/en/create-index.html
Prefix support and lengths of prefixes (where supported) are storage engine dependent. For example, a prefix can be up to 1000 bytes long for MyISAM tables, and 767 bytes for InnoDB tables。

1 character：latin1->1 byte，gbk-> 2 bytes，utf8 -> 3 bytes;

不同引擎的前缀长度限制不同
myisam:1000 bytes，索引总长度不能超过1000byte
innodb:767 bytes，索引中单列的前缀长度不能超过767byte


DROP TABLE `tb_index_myisam`;
CREATE TABLE tb_index_myisam (
id INT NOT NULL AUTO_INCREMENT,
url VARCHAR(255) NOT NULL,
inode VARCHAR(512) NOT NULL,
dt DATETIME,
PRIMARY KEY (id),
UNIQUE (url, inode(78)) COMMENT 'inode,3*(255+78)=999<1000,  more than78 is error'
)ENGINE=MYISAM DEFAULT CHARSET=utf8;

DROP TABLE `tb_index_inno`;
CREATE TABLE tb_index_inno (
id INT NOT NULL AUTO_INCREMENT,
url VARCHAR(255) NOT NULL,
inode VARCHAR(512) NOT NULL,
dt DATETIME,
PRIMARY KEY (id),
UNIQUE (url, inode(255)) COMMENT 'inode,3*255=765<767 ,  more than 255 is error'
)ENGINE=INNODB DEFAULT CHARSET=utf8;