执行以下命令时:
ALTER TABLE `mytable` ADD UNIQUE (
`column1` ,
`column2`
);
我得到了这个错误信息:
#1071 - Specified key was too long; max key length is 767 bytes
columnn1和column2的信息:
column1 varchar(20) utf8_general_ci
column2 varchar(500) utf8_general_ci
我认为varchar(20)只需要21个字节,而varchar(500)只需要501个字节。所以总字节数是522,小于767。为什么我得到了错误消息?
#1071 - Specified key was too long; max key length is 767 bytes
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.
根据下面给出的列,这两个变量字符串列使用utf8_general_ci排序规则(隐含utf8字符集)。
在MySQL中,utf8字符集每个字符最多使用3个字节。因此,它需要分配500*3=1500字节,这比MySQL允许的767字节要大得多。这就是为什么您会得到1071错误。
换句话说,您需要基于字符集的字节表示来计算字符数,因为并非每个字符集都是一个字节表示(正如您所假设的那样)。例如,MySQL中的utf8每个字符最多使用3个字节,767/3≈255个字符,而对于utf8mb4,最多使用4个字节表示,767/4≈191个字符。
众所周知,MySQL
column1 varchar(20) utf8_general_ci
column2 varchar(500) utf8_general_ci
我发现这个查询在检测哪些列的索引违反了最大长度方面很有用:
SELECT
c.TABLE_NAME As TableName,
c.COLUMN_NAME AS ColumnName,
c.DATA_TYPE AS DataType,
c.CHARACTER_MAXIMUM_LENGTH AS ColumnLength,
s.INDEX_NAME AS IndexName
FROM information_schema.COLUMNS AS c
INNER JOIN information_schema.statistics AS s
ON s.table_name = c.TABLE_NAME
AND s.COLUMN_NAME = c.COLUMN_NAME
WHERE c.TABLE_SCHEMA = DATABASE()
AND c.CHARACTER_MAXIMUM_LENGTH > 191
AND c.DATA_TYPE IN ('char', 'varchar', 'text')