Support

Your old database is using plain old UTF8 instead of the modern UTF8MB4 encoding. One of the extensions you are using is either stuck at a version published before Joomla! 3.5.0 (that's be about two years now) or the developer didn't get the memo about UTF8MB4. As a result, the database structure is such that the indices are against columns which are too big to be indexed under the UTF8MB4 encoding*. Meanwhile, you are restoring your site using the options which tell ANGIE to convert all your database tables to UTF8MB4, thus causing this MySQL error.

During restoration you'll eventually reach the Database Restoration page. You need to uncheck "Allow UTF8MB4 auto-detection", "Force UTF-8 collation on database" and "Force UTF-8 collation on tables".

If that causes a problem on the restored site retry the restoration with "Allow UTF8MB4 auto-detection" unchecked BUT do check the "Force UTF-8 collation on database" and "Force UTF-8 collation on tables" options this time (they work around the database collation being other than utf8_general_ci).

Alternatively, if you are sure you don't need that table (e.g. you no longer use the extension) you can delete the table(s) from that extensions and then take a new backup. If that's not an option see above :)

* Sidebar / technical notes: You'll note that the index is against a VARCHAR column with a length of 255 characters. So where does 767 bytes come from in the error message? UTF8 uses up to 3 bytes to store a glyph. Therefore MySQL reserves 255 characters x 3 bytes / character = 765 bytes for each index record. With UTF8MB4 we can use up to 4 bytes to store a glyph. Therefore MySQL needs to reserve 255 characters x 4 bytes / character = 1020 bytes for each index record. However, indices in MySQL are implemented as binary trees with a maximum leaf node size of 767 bytes. This number has to do with the internal data structures and the efficiency of the algorithm. Anyway. 1020 is larger than the limit of 767, hence the problem. The solution is for the developer to either reduce the size of the column or tell MySQL to index up to the first 191 characters.