It’d be super helpful if you could write about your current experience on the ticket: what size tables are you needing to convert, and how long does that take? What versions of MySQL are you upgrading from/to? Are there particular plugins that generate these failing JOIN queries?

I’ll try to extract the exact query, anyway the collate mismatch is on the order items meta value which is a string with unicode_520 collation.

I want to get deeper in this blog to understand is it’s a general case. It seems the standard wp-config.php which contains by default utf-8 as charset and and empty string for collation, could generate that when there is an update of the mysql version and subsequent new plugins installation which create their own tables.

Just a question: shouldn’t WP migrate all tables to the unicode_520 collation when it detects support for that feature? Like the old one “maybe_convert_to_utf8mb4()”?

Or the wp-config.php, once created, could be completed with the chosen first installation collation to keep it static.

Thank you, Stefano.