This is an extremely dangerous query, because it includes the user
table twice, along-side two other potentially large tables,
role_assignments and user_enrolments.

The solution is to rewrite the query so that:

1. The subquery is JOINed, not WHERE ... INed. Typically query
   optimisers handle the JOIN case better.

2. Before the join was role-assignments <-> users <-> subquery.
   That is, everything was linked to u.id.

   Now the linking is role-assignments <-> subquery <-> users,
   so the SELECT DISTINT eu1_u.id FROM {enrolled users} is central.
   That seems to send a strong hint to the query optimiser about
   a good order to execute the query.