Starting with RT 5.0.0, the minimum supported MySQL version is 5.7.7
because this is the first version to provide full support for 4 byte
utf8 characters in tables and indexes. Read on for details on this
change.

Note that MySQL 8 is not yet supported because of changes to the group
keyword.

RT 5.0.0 now defaults MySQL tables to utf8mb4, which is available in
versions before 5.7.7. However, before MySQL version 5.7.7, utf8mb4
tables could not have indexes with type VARCHAR(255): the default size
for index entries was 767 bytes, which is enough for 255 chars stored
as at most 3 chars (the utf8 format), but not as 4 bytes (utf8mb4).
5.7.7 sets the default index size to 3072 for InnoDB tables, resolving
that issue.

https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-7.html#mysqld-5-7-7-feature

In MySQL, RT uses the utf8mb4 character set to support all
unicode characters, including the ones that are encoded with 4 bytes in
utf8 (some Kanji characters and a good number of emojis). The DB tables
and the RT are both set to this character set.

If your MySQL database is used only for RT, you can consider
setting the default character set to utf8mb4. This will
ensure that backups and other database access outside of RT have the
correct character set.

This is done by adding the following lines to the MySQL configuration:

[mysqld]
character-set-server = utf8mb4

[client]
default-character-set = utf8mb4

You can check the values your server is using by running this command:

    mysqladmin variables | grep -i character_set

Setting the default is particularly important for mysqldump, to avoid
backups to be silently corrupted.

If the MySQL DB is shared with other applications and the default
character set cannot be set to utf8mb4, the command to backup the
database can be set explicitly:

    ( mysqldump --default-character-set=utf8mb4 rt5 --tables sessions --no-data --single-transaction; \
      mysqldump --default-character-set=utf8mb4 rt5 --ignore-table rt5.sessions --single-transaction ) \
        | gzip > rt-`date +%Y%m%d`.sql.gz

Restoring a backup is done the usual way, since the character set for
all tables is set to utf8mb4, there is no further need to tell MySQL
about it:

    gunzip -c rt-20191125.sql.gz | mysql -uroot -p rt5

These character set updates now allow RT on MySQL to accept and store 4-byte
characters like emojis. However, searches can still be inconsistent. You may be
able to get different or better results by experimenting with different collation
settings. For more information:

https://stackoverflow.com/a/41148052
https://dev.mysql.com/doc/refman/5.7/en/charset-unicode-sets.html
