现象描述
在更新 Prod 环境的数据库时，运行了一个 alter table 语句，MySQL 报错：1071 - Specified key was too long; max key length is 767 bytes , 经过排查数据里的数据、字段和索引，并没有发现太明显的异常。
从错误信息来看，和索引是有关系的；但是本地数据库、test 环境数据库、uat 环境数据库都是正常的，只有 Prod 是有问题的；进过查询资料，定位到一个 MySQL 的配置参数可能和这个问题有关系
排错
在每个环境中查询 MySQL 参数：innodb_large_prefix
复制 -- Test
show variables like 'innodb_large_prefix';
-- result: ON
-- UAT
show variables like 'innodb_large_prefix';
-- result: ON
-- Prod
show variables like 'innodb_large_prefix';
-- Result: OFF 可见不同环境的配置是不一样，这个参数的作用是控制单列索引的最大长度的
对于 Innodb 存储引擎，innodb_large_prefix = OFF 时最大的单列索引长度是 767 Bytes，这里要注意是字节，我们在指定列的类型时一般是 char 或者 varchar，在 utf8 编码下，一个字符 3 Bytes，在 utf8mb4 编码下，一个字符是 4 bytes
对于 MyISAM 存储引擎，单列索引的最大长度是 1000 Bytes
而在我们自己的表定义中，有一个 uniqueId varchar(200) 的字段被用作了索引，而且编码是 utf8mb4，那么索引的理论长度是 800，超过了 767 Bytes。在 Prod 环境中发生这个问题，可能的原因：
innodb_large_prefix 以前是 ON 的，但后来被改成了 OFF，这样就会导致再次修改表结构时报错，增删改查数据不受影响
原理分析
已知结论：
对于 Innodb 存储引擎，当 innodb_large_prefix = OFF 时，单列索引的最大长度时 767 Bytes；当 innodb_large_prefix = ON 时，单列索引的最当长度可以到 3072 Bytes （还有地方说开启 innodb_large_prefix 后的索引长度增大，只针对 row format 是 DYNAMIC 和 COMPRESSED
对于 Innodb 存储引擎，联合索引的最大长度都是 3072 Bytes
由于最大长度限制的是字节数，所以不同编码下，索引列的最大长度是不一样的，比如 utf8 编码下，innodb_large_prefix = OFF 时，最大的 varchar 长度是 255，而 utf8mb4 最大的 varchar 长度就是 191
innodb_large_prefix 是 MySQL 5.5 以后引入的，如果想要在开启 innodb_large_prefix 后索引长度能支持到 3072 Bytes，还必须保证 MySQL 的 file format 设置（使用 Barracuda）和 row format 设置 （使用 DYNAMIC 和 COMPRESSED）
小结一下，innodb_large_prefix=1 并且 innodb_file_format=BARRACUDA 时，对于 row_format 为 dynamic 或 compressed 的表可以指定索引列长度大于 767 bytes。但是索引列总长度的不能大于 3072 bytes的限制仍然存在
MySQL 官方文档的描述：
if innodb_large_prefix is enabled (the default), the index key prefix limit is 3072 bytes for InnoDB tables that use the DYNAMICCOMPRESSEDinnodb_large_prefix is disabled, the index key prefix limit is 767 bytes for tables of any row format.
innodb_large_prefix is deprecated; expect it to be removed in a future MySQL release. innodb_large_prefix was introduced in MySQL 5.5 to disable large index key prefixes for compatibility with earlier versions of InnoDB that do not support large index key prefixes.
The index key prefix length limit is 767 bytes for InnoDB tables that use the REDUNDANTCOMPACT
For example, you might hit this limit with a column prefix index of more than 255 characters on a TEXT or VARCHAR column, assuming a utf8mb3 character set and the maximum of 3 bytes for each character.Attempting to use an index key prefix length that exceeds the limit returns an error.
To avoid such errors in replication configurations, avoid enabling innodb_large_prefix on the source if it cannot also be enabled on replicas.If you reduce the InnoDB page size to 8KB or 4KB by specifying the innodb_page_size option when creating the MySQL instance, the maximum length of the index key is lowered proportionally, based on the limit of 3072 bytes for a 16KB page size. That is, the maximum index key length is 1536 bytes when the page size is 8KB, and 768 bytes when the page size is 4KB.
The limits that apply to index key prefixes also apply to full-column index keys.
-- via: https://dev.mysql.com/doc/refman/5.7/en/innodb-limits.html
-- via: https://dev.mysql.com/doc/refman/5.7/en/innodb-parameters.html#sysvar_innodb_large_prefix
至于为什么最大限制是 767 Bytes，为什么和 innodb file format 与 row format 有关，有兴趣的可以再研究一下，和 Innodb 的存储结构、表空间、行记录格式等有关。
复制 # 查看数据库的默认编码配置
mysql> show variables like 'char%';
+--------------------------+---------------------------------------------------------------------------+
| Variable_name | Value |
+--------------------------+---------------------------------------------------------------------------+
| character_set_client | utf8 |
| character_set_connection | utf8 |
| character_set_database | utf8mb4 |
| character_set_filesystem | binary |
| character_set_results | utf8 |
| character_set_server | utf8mb4 |
| character_set_system | utf8 |
| character_sets_dir | /Users/dfg/workspace/middleware/mysql/mysql-5.7.30-master/share/charsets/ |
+--------------------------+---------------------------------------------------------------------------+
8 rows in set (0.01 sec)
# 查看数据库 test 的编码
mysql> show create database test;
+----------+---------------------------------------------------------------------------------------------+
| Database | Create Database |
+----------+---------------------------------------------------------------------------------------------+
| test | CREATE DATABASE `test` /*!40100 DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci */ |
+----------+---------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> CREATE TABLE `events_2` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`uniqueId` varchar(200) COLLATE utf8mb4_unicode_ci NOT NULL,
`type` varchar(32) COLLATE utf8mb4_unicode_ci NOT NULL,
`key1` varchar(200) COLLATE utf8mb4_unicode_ci NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
mysql> SET GLOBAL innodb_large_prefix = 0;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> show variables like 'innodb_large_prefix';
+---------------------+-------+
| Variable_name | Value |
+---------------------+-------+
| innodb_large_prefix | OFF |
+---------------------+-------+
1 row in set (0.01 sec)
mysql> alter table events_2 add unique key uni_unique_id (uniqueId);
ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes
mysql> show create table events_3\G;
*************************** 1. row ***************************
Table: events_3
Create Table: CREATE TABLE `events_3` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`uniqueId` varchar(200) COLLATE utf8_unicode_ci NOT NULL,
`type` varchar(32) COLLATE utf8_unicode_ci NOT NULL,
`key1` varchar(200) COLLATE utf8_unicode_ci NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
mysql> alter table events_3 add unique key uni_unique_id (uniqueId);
Query OK, 0 rows affected (0.02 sec)
mysql> show index from events_3;
+----------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| events_3 | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| events_3 | 0 | uni_unique_id | 1 | uniqueId | A | 0 | NULL | NULL | | BTREE | | |
+----------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
mysql> alter table events_3 character set utf8mb4 collate utf8mb4_unicode_ci;
Query OK, 0 rows affected (0.02 sec)
mysql> show create table events_3\G;
*************************** 1. row ***************************
Table: events_3
Create Table: CREATE TABLE `events_3` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`uniqueId` varchar(200) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
`type` varchar(32) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
`key1` varchar(200) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `uni_unique_id` (`uniqueId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
mysql> show variables like 'innodb_file_format';
+--------------------+-----------+
| Variable_name | Value |
+--------------------+-----------+
| innodb_file_format | Barracuda |
+--------------------+-----------+
1 row in set (0.16 sec)
mysql> show variables like '%row_format%';
+---------------------------+---------+
| Variable_name | Value |
+---------------------------+---------+
| innodb_default_row_format | dynamic |
+---------------------------+---------+
mysql> show variables like 'innodb_file_per_table';
+-----------------------+-------+
| Variable_name | Value |
+-----------------------+-------+
| innodb_file_per_table | ON |
+-----------------------+-------+
1 row in set (0.26 sec)
复制 CREATE TABLE `events_1` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`uniqueId` varchar(200) COLLATE utf8mb4_unicode_ci NOT NULL,
`type` varchar(32) COLLATE utf8mb4_unicode_ci NOT NULL,
`key1` varchar(200) COLLATE utf8mb4_unicode_ci NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
CREATE TABLE `events_2` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`uniqueId` varchar(200) COLLATE utf8mb4_unicode_ci NOT NULL,
`type` varchar(32) COLLATE utf8mb4_unicode_ci NOT NULL,
`key1` varchar(200) COLLATE utf8mb4_unicode_ci NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
CREATE TABLE `events_3` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`uniqueId` varchar(200) NOT NULL,
`type` varchar(32) NOT NULL,
`key1` varchar(200) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
alter table events_1 add unique key uni_unique_id (uniqueId);
show variables like 'innodb_large_prefix';
SET GLOBAL innodb_large_prefix = 0; 参考链接
