There was an extra acs_users_all assignee join in the first part of the query which caused the set of returned rows to be the cartesian product of the bugs X keywords X users on system = very very slow for anything more than a trivial number of users.
It still returns bugs x keywords rows which is not great but seems tolerable.
merge changes on oacs-5-1 branch to head, changes between jcd-merge-20040420 and jcd-merge-5-1-20040517 were merged, head pre changes is jcd-merge-pre-20040517 and post is jcd-merge-post-20040517