How important are intersections for non-existent
categories? Without
we could have something like (page_id int, cat_intersect bigint) or
(page_id int, cat1 int, cat2 int) to get two cat intersection without
collisions; and maybe even scale up by defining n-intersections
recursively, without collisions.
How fast are ANDs in SELECT WHEREs? I would guess it's quicker to
search by hash than by 2 ints.