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How do I get a user's default role?
Good question. only by querying mysql.user table, so it seems. Looks like a serious omission. Could you create an issue in Jira for it and we'll fix it? And feel free to suggest there how and where one should be able to see user's default role.
Have added to JIRA: https://mariadb.atlassian.net/browse/MDEV-6918