In WordPress 4.2, we’re upgrading tables to utf8mb4 , when we can. Your site will only upgrade when the following conditions are met:

You’re currently using the utf8 character set.

character set. Your MySQL MySQL is a relational database management system. A database is a structured collection of data where content, configuration and other options are stored. https://www.mysql.com/ MySQL

Your MySQL client libraries are version 5.5.3 or higher. If you’re using mysqlnd , 5.0.9 or higher.

The difference between utf8 and utf8mb4 is that the former can only store 3 byte characters, while the latter can store 4 byte characters. In Unicode terms, utf8 can only store characters in the Basic Multilingual Plane, while utf8mb4 can store any Unicode character. This greatly expands the language usability of WordPress, especially in countries that use Han character sets. Unicode isn’t without its problems, but it’s the best option available.

utf8mb4 is 100% backwards compatible with utf8 .

Due to index size restrictions in MySQL, this does mean we need to re-create a handful of indexes to fit within MySQL’s rules. Using a standard configuration, MySQL allows 767 bytes per index, which for utf8 means 767 bytes / 3 bytes = 255 characters. For utf8mb4 , that means 767 bytes / 4 bytes = 191 characters. The indexes that will be resized are:



wp_usermeta.meta_key

wp_terms.slug

wp_terms.name

wp_commentmeta.meta_key

wp.postmeta.meta_key

wp_posts.post_name



And from Multisitemultisite Used to describe a WordPress installation with a network of multiple blogs, grouped by sites. This installation type has shared users tables, and creates separate database tables for each blog (wp_posts becomes wp_0_posts). See also network, blog, site:



wp_site.domain

wp_sitemeta.meta_key

wp_signups.domain



Of course, the Multisite (and wp_usermeta ) keys obey the DO_NOT_UPGRADE_GLOBAL_TABLES setting. The upgrade will only be attempted once, though we’ll probably add a check in a future WordPress version to see if we can upgrade now (say, if you’ve upgraded your MySQL server since upgrading to WordPress 4.2).

If you’re a pluginPlugin A plugin is a piece of software containing a group of functions that can be added to a WordPress website. They can extend functionality or add new features to your WordPress websites. WordPress plugins are written in the PHP programming language and integrate seamlessly with WordPress. These can be free in the WordPress.org Plugin Directory https://wordpress.org/plugins/ or can be cost-based plugin from a third-party developer and your plugin includes custom tables, please test that your indexes fit within MySQL’s limits. MySQL won’t always produce an error when the index is too big, so you’ll need to manually check the size of each index, instead of relying on automated testing.

EDIT: One more thing…

If you’d like to upgrade your custom tables to utf8mb4 (and your indexes are all in order), you can do it really easily with the shiny new maybe_convert_table_to_utf8mb4( $tablename ) function. It’s available in `wp-adminadmin (and super admin)/includes/upgrade.php`, and will sanity check that your tables are entirely utf8 before upgrading.