I've thought about this problem a little as the underlying acs_privilege_hierarchy table forms a convergent directed-graph instead of a tree structure, so I had to jump through some hoops to make this work in openacs. One thing I'm planning on doing when I get some time is to rewrite acs_privilege_descendant_map as a table that is populated by an insert trigger on acs_privilege_hierarchy. This might help some, but I would think that acs_privilege_hierarchy table is quite small, so I'm thinking there must be some other problems here besides acs_privilege_descendant_map view.
Probably, as was pointed out by others, the nesting of views is causing problems with optimizing the queries, though I'm not sure that postgresql has the same problem in this respect. It would to see how the two db's compare.
