MySQL 异常：max key length is 767 bytes
前言
最近迁移几张表，又遇到 767 异常，迁移前只检查了 sql_mode 忽略对比了这个参数，导致几张表创建失败，其实解决方法也很简单，开启 innodb_large_prefix 参数重新导入即可。该 ERROR 常发生在字符串类型的大字段上，本篇文章简单记录下该问题。
Specified key was too long; max key length is 767 bytes
1. 767 bytes 解读
如何知道某张表字符串字段占多少 bytes 呢？需要结合表的字符集来看：
CREATE TABLE `test_index` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`name` varchar(800) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
上表的字符类型是 utf8 我们可以通过查表来判断字符最大可占用的空间：
show character set;
| Charset | Description | Default collation | Maxlen |
|---|---|---|---|
| big5 | Big5 Traditional Chinese | big5_chinese_ci | 2 |
| dec8 | DEC West European | dec8_swedish_ci | 1 |
| cp850 | DOS West European | cp850_general_ci | 1 |
| hp8 | HP West European | hp8_english_ci | 1 |
| koi8r | KOI8-R Relcom Russian | koi8r_general_ci | 1 |
| latin1 | cp1252 West European | latin1_swedish_ci | 1 |
| latin2 | ISO 8859-2 Central European | latin2_general_ci | 1 |
| swe7 | 7bit Swedish | swe7_swedish_ci | 1 |
| ascii | US ASCII | ascii_general_ci | 1 |
| ujis | EUC-JP Japanese | ujis_japanese_ci | 3 |
| sjis | Shift-JIS Japanese | sjis_japanese_ci | 2 |
| hebrew | ISO 8859-8 Hebrew | hebrew_general_ci | 1 |
| tis620 | TIS620 Thai | tis620_thai_ci | 1 |
| euckr | EUC-KR Korean | euckr_korean_ci | 2 |
| koi8u | KOI8-U Ukrainian | koi8u_general_ci | 1 |
| gb2312 | GB2312 Simplified Chinese | gb2312_chinese_ci | 2 |
| greek | ISO 8859-7 Greek | greek_general_ci | 1 |
| cp1250 | Windows Central European | cp1250_general_ci | 1 |
| gbk | GBK Simplified Chinese | gbk_chinese_ci | 2 |
| latin5 | ISO 8859-9 Turkish | latin5_turkish_ci | 1 |
| armscii8 | ARMSCII-8 Armenian | armscii8_general_ci | 1 |
| utf8 | UTF-8 Unicode | utf8_general_ci | 3 |
| ucs2 | UCS-2 Unicode | ucs2_general_ci | 2 |
| cp866 | DOS Russian | cp866_general_ci | 1 |
| keybcs2 | DOS Kamenicky Czech-Slovak | keybcs2_general_ci | 1 |
| macce | Mac Central European | macce_general_ci | 1 |
| macroman | Mac West European | macroman_general_ci | 1 |
| cp852 | DOS Central European | cp852_general_ci | 1 |
| latin7 | ISO 8859-13 Baltic | latin7_general_ci | 1 |
| utf8mb4 | UTF-8 Unicode | utf8mb4_general_ci | 4 |
| cp1251 | Windows Cyrillic | cp1251_general_ci | 1 |
| utf16 | UTF-16 Unicode | utf16_general_ci | 4 |
| utf16le | UTF-16LE Unicode | utf16le_general_ci | 4 |
| cp1256 | Windows Arabic | cp1256_general_ci | 1 |
| cp1257 | Windows Baltic | cp1257_general_ci | 1 |
| utf32 | UTF-32 Unicode | utf32_general_ci | 4 |
| binary | Binary pseudo charset | binary | 1 |
| geostd8 | GEOSTD8 Georgian | geostd8_general_ci | 1 |
| cp932 | SJIS for Windows Japanese | cp932_japanese_ci | 2 |
| eucjpms | UJIS for Windows Japanese | eucjpms_japanese_ci | 3 |
| gb18030 | China National Standard GB18030 | gb18030_chinese_ci | 4 |
例如 utf8 的 Maxlen = 3 表示一个字符最大占用 3 bytes。那么 varchar 类型小于 255 字符，就不会触发 767 异常。
2. 异常原因
为什么会有这个异常呢？其实该异常属于 MySQL Innodb 引擎的限制，例如一张表最多有 1017 个字段，一张表最大有 64 个二级索引一样，如果索引列的字符长度很大的话，二级索引占用的空间也会非常大，一个页面存储的索引字段减少，索引深度会更容易变深，增加 IO 次数，搜索的性能也不会理想。
If innodb_large_prefix is enabled (the default), the index key prefix limit is 3072 bytes for InnoDB tables that use the DYNAMIC or COMPRESSED row format. If innodb_large_prefix is disabled, the index key prefix limit is 767 bytes for tables of any row format.
Innodb 引擎的限制：
3. 5.7 和 8.0 版本的区别
在 5.7 版本中，对于使用 DYNAMIC 或 COMPRESSED 行格式的 InnoDB 表，索引前缀的限制为 767 bytes，如果开启 innodb_large_prefix 参数，限制阈值上调为 3072 bytes。
在 8.0 版本中，参数 innodb_large_prefix 被删除，索引前缀最大为 3072 bytes 不再有 767 bytes 的限制。
