IMO, we need to introduce a migration which fix any bad schema. But first we need to be sure of what is the exact criteria (Hibernate/XWiki version, connector versions, database engine version ?) to indicate it as a requirement on https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Installation/InstallationWAR/InstallationMySQL/ (and maybe https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Installation/InstallationWAR/InstallationMariaDB/).