See http://tracker.moodle.org/browse/MDL-35088 for details of the reported problem and http://tracker.moodle.org/browse/MDL-34864 for details on the solution.
My question is I would like to know how to recover from when the category enrollments query uses up all the resources on the database servers; besides, upgrading which I will eventually do once 2.3.x is released. Thank you in advance.