将 MySQL 数据库从 latin1 更新为 utf8mb4：最大密钥长度为 767 字节

Question

我正在尝试将 MySQL 数据库（Rails 3 应用程序的一部分）中的字符编码从 latin1 升级到 utfmb8。我已经完成了some reading，但我被困在最后两张桌子上：users 和 trigrams。

尝试更新users 表会得到可怕的ERROR 1071：

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

按照this answer，我可以通过在有问题的VARCHAR(255) 列上删除并重新创建索引来解决这个问题：

mysql> DROP INDEX index_users_on_remember_token ON users;
mysql> CREATE INDEX index_users_on_remember_token ON users (remember_token(191));

我担心这会把事情搞砸，因为我不太了解索引。不过暂且不说……

我的主要问题是如何为我的最后一张桌子做类似的事情：trigrams。以下是trigrams 表的一些信息：

mysql> SHOW CREATE TABLE trigrams;
CREATE TABLE `trigrams` (
    `id` int(11) NOT NULL AUTO_INCREMENT,
    `trigram` varchar(3) DEFAULT NULL,
    `score` smallint(6) DEFAULT NULL,
    `owner_id` int(11) DEFAULT NULL,
    `owner_type` varchar(255) DEFAULT NULL,
    `fuzzy_field` varchar(255) DEFAULT NULL,
    PRIMARY KEY (`id`),
    KEY `index_for_match` (`owner_id`,`owner_type`,`fuzzy_field`,`trigram`,`score`),
    KEY `index_by_owner` (`owner_id`,`owner_type`)
    ) ENGINE=InnoDB AUTO_INCREMENT=41531 DEFAULT CHARSET=latin1

mysql> SHOW INDEXES IN trigrams;
+----------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table    | Non_unique | Key_name        | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| trigrams |          0 | PRIMARY         |            1 | id          | A         |       35502 |     NULL | NULL   |      | BTREE      |         |               |
| trigrams |          1 | index_for_match |            1 | owner_id    | A         |       35502 |     NULL | NULL   | YES  | BTREE      |         |               |
| trigrams |          1 | index_for_match |            2 | owner_type  | A         |       35502 |     NULL | NULL   | YES  | BTREE      |         |               |
| trigrams |          1 | index_for_match |            3 | fuzzy_field | A         |       35502 |     NULL | NULL   | YES  | BTREE      |         |               |
| trigrams |          1 | index_for_match |            4 | trigram     | A         |       35502 |     NULL | NULL   | YES  | BTREE      |         |               |
| trigrams |          1 | index_for_match |            5 | score       | A         |       35502 |     NULL | NULL   | YES  | BTREE      |         |               |
| trigrams |          1 | index_by_owner  |            1 | owner_id    | A         |       35502 |     NULL | NULL   | YES  | BTREE      |         |               |
| trigrams |          1 | index_by_owner  |            2 | owner_type  | A         |       35502 |     NULL | NULL   | YES  | BTREE      |         |               |
+----------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

我的问题是：如何解决以下错误？

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

我要对trigrams 进行哪些更改？进行这些更改有哪些风险？

（如果相关：trigrams 表是由某些第三方代码自动创建的，fuzzily gem。）

Answer 1

试试

CREATE TABLE tmp LIKE triggrams;
SET GLOBAL innodb_large_prefix=1;
ALTER TABLE tmp CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
LOCK TABLE trigrams, tmp WRITE;
INSERT INTO tmp SELECT * FROM trigrams;
RENAME TABLE trigrams to trigrams_old, tmp to trigrams;
UNLOCK TABLES;

第二个选项是减少 varchar 列的索引前缀。