ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes I have a databases with latin1 charachter set, I need to change tables character set to utf8, but I get error. the current character set is : 

mysql&gt; show variables like "%character%";
+--------------------------+----------------------------+
| Variable_name            | Value                      |
+--------------------------+----------------------------+
| character_set_client     | utf8                       |
| character_set_connection | utf8                       |
| character_set_database   | latin1                     |
| character_set_filesystem | binary                     |
| character_set_results    | utf8                       |
| character_set_server     | latin1                     |
| character_set_system     | utf8                       |
| character_sets_dir       | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+


the table that I want to change the character set is : 

mysql&gt; describe spool;
+------------+---------------------+------+-----+-------------------+----------------+
| Field      | Type                | Null | Key | Default           | Extra          |
+------------+---------------------+------+-----+-------------------+----------------+
| username   | varchar(250)        | NO   | MUL | NULL              |                |
| xml        | text                | NO   |     | NULL              |                |
| seq        | bigint(20) unsigned | NO   | PRI | NULL              | auto_increment |
| created_at | timestamp           | NO   |     | CURRENT_TIMESTAMP |                |
+------------+---------------------+------+-----+-------------------+----------------+
4 rows in set (0.00 sec)


when I want to change the character set I see bellow error : 

mysql&gt; ALTER TABLE spool CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes


It's a company functioning database and I don't want to disturb the functionality, so I don't want to change the size of the varchar(250) type, I wonder if there is a solution to change this encoding.

mysql&gt; show create table spool\G;
*************************** 1. row ***************************
       Table: spool
Create Table: CREATE TABLE `spool` (
  `username` varchar(250) NOT NULL,
  `xml` text NOT NULL,
  `seq` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  UNIQUE KEY `seq` (`seq`),
  KEY `i_despool` (`username`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=23490775 DEFAULT CHARSET=utf8
Solved : 

I solve this problem, by creating new table like this spool. when we want to create a table with utf8mb4 COLLATE utf8mb4_unicode_ci character set we can not use varchar(250) because 250 * 4 = 1000 (4 byte per character in this new encoding). 1000 is more than max key 767 byte for this encoding. so I create a table spoolll with varchar(100) :

CREATE TABLE `spoollll` (
  `username` varchar(100) NOT NULL,
  `xml` text NOT NULL,
  `seq` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  UNIQUE KEY `seq` (`seq`),
  KEY `i_despool` (`username`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=23492667 DEFAULT CHARSET=utf8 


and now I change the character set : 

mysql&gt; ALTER TABLE spoollll CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0


I would appreciate if someone comment on it and complete this issue.