这个问题

MySQL中有最大键长度限制。

InnoDB -最大密钥长度为1536字节(8kb页面大小)和768字节(4kb页面大小)(来源:Dev.MySQL.com) MyISAM -最大密钥长度是1000字节(来源Dev.MySQL.com)。

这些都是以字节为单位计算的!因此，一个UTF-8字符可能需要一个以上的字节才能存储到密钥中。

因此，你只有两个直接的解决方案:

只索引文本类型的前n个字符。 创建一个全文搜索-所有内容都可以在文本中搜索，以一种类似ElasticSearch的方式

索引文本类型的前N个字符

如果您正在创建一个表，请使用以下语法来索引某些字段的前255个字符:KEY sometextkey (SomeText(255))。像这样:

CREATE TABLE `MyTable` (
    `id` int(11) NOT NULL auto_increment,
    `SomeText` TEXT NOT NULL,
    PRIMARY KEY  (`id`),
    KEY `sometextkey` (`SomeText`(255))
);

如果你已经有了这个表，那么你可以用add unique (ConfigValue(20));为一个字段添加一个唯一键。像这样:

ALTER TABLE
MyTable
ADD UNIQUE(`ConfigValue`(20));

如果字段名不是保留的MySQL关键字，则在字段名周围不需要反引号(' ' ')。

创建全文搜索

全文搜索将允许您搜索文本字段的全部值。如果你使用自然语言模式，它会进行全词匹配，如果你使用其他模式之一，它会进行部分词匹配。查看更多关于全文本的选项:Dev.MySQL.com

创建您的文本表，并添加全文索引…

ALTER TABLE
        MyTable
ADD FULLTEXT INDEX
        `SomeTextKey` (`SomeTextField` DESC);

然后像这样搜索你的桌子……

SELECT
        MyTable.id, MyTable.Title,
MATCH
        (MyTable.Text)
AGAINST
        ('foobar' IN NATURAL LANGUAGE MODE) AS score
FROM
        MyTable
HAVING
        score > 0
ORDER BY
        score DESC;

5解决方法:

在5.7.7中提高了限制(MariaDB 10.2.2?)。并且可以通过5.6(10.1)中的一些工作来增加它。

如果你因为试图使用字符集utf8mb4而达到极限。然后做以下其中一种(每一种都有缺点)来避免错误:

⚈  Upgrade to 5.7.7 for 3072 byte limit -- your cloud may not provide this;
⚈  Change 255 to 191 on the VARCHAR -- you lose any values longer than 191 characters (unlikely?);
⚈  ALTER .. CONVERT TO utf8 -- you lose Emoji and some of Chinese;
⚈  Use a "prefix" index -- you lose some of the performance benefits.
⚈  Or... Stay with older version but perform 4 steps to raise the limit to 3072 bytes:

SET GLOBAL innodb_file_format=Barracuda;
SET GLOBAL innodb_file_per_table=1;
SET GLOBAL innodb_large_prefix=1;
logout & login (to get the global values);
ALTER TABLE tbl ROW_FORMAT=DYNAMIC;  -- (or COMPRESSED)

——http://mysql.rjweb.org/doc.php/limits 767 _limit_in_innodb_indexes

您使用的是什么字符编码?有些字符集(如UTF-16等等)每个字符使用一个以上的字节。

Laravel框架解决方案

根据Laravel 5.4。*文档;你必须在app/Providers/AppServiceProvider.php文件的引导方法中设置默认字符串长度，如下所示:

use Illuminate\Support\Facades\Schema;

public function boot() 
{
    Schema::defaultStringLength(191); 
}

Laravel 5.4对这个修复的解释。*文档:

Laravel uses the utf8mb4 character set by default, which includes support for storing "emojis" in the database. If you are running a version of MySQL older than the 5.7.7 release or MariaDB older than the 10.2.2 release, you may need to manually configure the default string length generated by migrations in order for MySQL to create indexes for them. You may configure this by calling the Schema::defaultStringLength method within your AppServiceProvider. Alternatively, you may enable the innodb_large_prefix option for your database. Refer to your database's documentation for instructions on how to properly enable this option.

更改排序规则。您可以使用支持几乎所有功能的utf8_general_ci

我认为varchar(20)只需要21个字节，而varchar(500)只需要 需要501字节。所以总字节数是522，小于767。那么,为什么 我收到错误信息了吗?

UTF8每个字符需要3个字节来存储字符串，因此在您的情况下，20 +500字符= 20*3+500*3 = 1560字节，这超过了允许的767字节。

UTF8的限制是767/3 = 255个字符，对于每个字符使用4个字节的UTF8mb4，它是767/4 = 191个字符。

如果您需要使用比限制更长的列，有两种解决方案:

Use "cheaper" encoding (the one that requires less bytes per character) In my case, I needed to add Unique index on column containing SEO string of article, as I use only [A-z0-9\-] characters for SEO, I used latin1_general_ci which uses only one byte per character and so column can have 767 bytes length. Create hash from your column and use unique index only on that The other option for me was to create another column which would store hash of SEO, this column would have UNIQUE key to ensure SEO values are unique. I would also add KEY index to original SEO column to speed up look up.