rails/rails — pull request #17601 by kamipo (2014-11-12 21:17:54+00:00)
Title: Support any unicode charsets for MySQL.

MySQL unicode support is not only `utf8mb4`.
Then, The index length problem is not only `utf8mb4`.

http://dev.mysql.com/doc/refman/5.6/en/charset-unicode.html

SELECT * FROM information_schema.character_sets WHERE maxlen > 3;
+--------------------+----------------------+------------------+--------+
| CHARACTER_SET_NAME | DEFAULT_COLLATE_NAME | DESCRIPTION | MAXLEN |
+--------------------+----------------------+------------------+--------+
| utf8mb4 | utf8mb4_general_ci | UTF-8 Unicode | 4 |
| utf16 | utf16_general_ci | UTF-16 Unicode | 4 |
| utf16le | utf16le_general_ci | UTF-16LE Unicode | 4 |
| utf32 | utf32_general_ci | UTF-32 Unicode | 4 |
+--------------------+----------------------+------------------+--------+

In addition, the charset of `version` column in `schema_migrations`
table is depend on the database default charset and collation
rather than encoding of connection.