10.1.11. Upgrading from Previous to Current Unicode Support

This section describes issues pertaining to Unicode support that you may face when upgrading to MySQL 5.7 from an older MySQL release. It also provides guidelines for downgrading from MySQL 5.7 to an older release.

In most respects, upgrading to MySQL 5.7 should present few problems with regard to Unicode usage, although there are some potential areas of incompatibility. These are the primary areas of concern:

For the variable-length character data types (VARCHAR and the TEXT types), the maximum length in characters is less for utf8mb4 columns than for utf8 columns.
For all character data types (CHAR, VARCHAR, and the TEXT types), the maximum number of characters that can be indexed is less for utf8mb4 columns than for utf8 columns.

Consequently, if you want to upgrade tables from utf8 to utf8mb4 to take advantage of supplementary-character support, it may be necessary to change some column or index definitions.

Tables can be converted from utf8 to utf8mb4 by using ALTER TABLE. Suppose that a table was originally defined as follows:

CREATE TABLE t1 (  col1 CHAR(10) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,  col2 CHAR(10) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL) CHARACTER SET utf8;

The following statement converts t1 to use utf8mb4:

ALTER TABLE t1  DEFAULT CHARACTER SET utf8mb4,  MODIFY col1 CHAR(10)    CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL,  MODIFY col2 CHAR(10)    CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NOT NULL;

In terms of table content, conversion from utf8 to utf8mb4 presents no problems:

For a BMP character, utf8 and utf8mb4 have identical storage characteristics: same code values, same encoding, same length.
For a supplementary character, utf8 cannot store the character at all, while utf8mb4 requires four bytes to store it. Since utf8 cannot store the character at all, you do not have any supplementary characters in utf8 columns and you need not worry about converting characters or losing data when upgrading utf8 data from older versions of MySQL.

In terms of table structure, the catch when converting from utf8 to utf8mb4 is that the maximum length of a column or index key is unchanged in terms of bytes. Therefore, it is smaller in terms of characters because the maximum length of a character is four bytes instead of three. For the CHAR, VARCHAR, and TEXT data types, watch for these things when converting your MySQL tables:

Check all definitions of utf8 columns and make sure they will not exceed the maximum length for the storage engine.
Check all indexes on utf8 columns and make sure they will not exceed the maximum length for the storage engine. Sometimes the maximum can change due to storage engine enhancements.

If the preceding conditions apply, you must either reduce the defined length of columns or indexes, or continue to use utf8 rather than utf8mb4.

Here are some examples where structural changes may be needed:

A TINYTEXT column can hold up to 255 bytes, so it can hold up to 85 3-byte or 63 4-byte characters. Suppose that you have a TINYTEXT column that uses utf8 but must be able to contain more than 63 characters. You cannot convert it to utf8mb4 unless you also change the data type to a longer type such as TEXT.

Similarly, a very long VARCHAR column may need to be changed to one of the longer TEXT types if you want to convert it from utf8 to utf8mb4.
InnoDB has a maximum index length of 767 bytes, so for utf8 or utf8mb4 columns, you can index a maximum of 255 or 191 characters, respectively. If you currently have utf8 columns with indexes longer than 191 characters, you will need to index a smaller number of characters. In an InnoDB table, these column and index definitions are legal:
```
col1 VARCHAR(500) CHARACTER SET utf8, INDEX (col1(255))
```
To use utf8mb4 instead, the index must be smaller:
```
col1 VARCHAR(500) CHARACTER SET utf8mb4, INDEX (col1(191))
```

The preceding types of changes are most likely to be required only if you have very long columns or indexes. Otherwise, you should be able to convert your tables from utf8 to utf8mb4 without problems. You can do this by using ALTER TABLE as described earlier in this section after upgrading in place to 5.7.

The following items summarize other potential areas of incompatibility:

Performance of 4-byte UTF-8 (utf8mb4) is slower than for 3-byte UTF-8 (utf8). If you do not want to incur this penalty, continue to use utf8.
SET NAMES 'utf8mb4' causes use of the 4-byte character set for connection character sets. As long as no 4-byte characters are sent from the server, there should be no problems. Otherwise, applications that expect to receive a maximum of three bytes per character may have problems. Conversely, applications that expect to send 4-byte characters must ensure that the server understands them.
Applications cannot send utf16, utf16le, or utf32 character data to an older server that does not understand them.
For replication, if the character sets that support supplementary characters are going to be used on the master, all slaves must understand them as well. If you attempt to replicate from a MySQL 5.7 master to an older slave, utf8 data will be seen as utf8 by the slave and should replicate correctly. But you cannot send utf8mb4, utf16, utf16le, or utf32 data.

Also, keep in mind the general principle that if a table has different definitions on the master and slave, this can lead to unexpected results. For example, the differences in limitations on index key length makes it risky to use utf8 on the master and utf8mb4 on the slave.

If you have upgraded to MySQL 5.7, and then decide to downgrade back to an older release, these considerations apply:

ucs2 and utf8 data should present no problems.
Any definitions that refer to the utf8mb4, utf16, utf16le, or utf32 character sets will not be recognized by the older server.
For object definitions that refer to the utf8mb4 character set, you can dump them with mysqldump in MySQL 5.7, edit the dump file to change instances of utf8mb4 to utf8, and reload the file in the older server, as long as there are no 4-byte characters in the data. The older server will see utf8 in the dump file object definitions and create new objects that use the (3-byte) utf8 character set.