Our mdl_role_capabilities table has half a million rows in it and the login process can only handle about 5 students logging in at once...

Our experiments show that changing UNION to UNION ALL here:

https://github.com/moodle/moodle/blob/master/lib/accesslib.php#L890

Makes the query run 40% faster. Could this be considered for upcoming Moodle point releases?

The final ORDER BY should be removed too for another 40% improvement but related code claims the order is actually important somehow :/