Random data point: I have a hierarchal authentication system under MySQL (no recursive query support). My schema does the most obvious thing of having user.pare

Random data point: I have a hierarchal authentication system under MySQL (no recursive query support). My schema does the most obvious thing of having user.parent as a user_id (adjacency model). My most common query is to find whether user X is a child of user Y. My solution was to use application-level triggers to maintain a separate lookup table, so i can simply do `user_id IN (SELECT child_id FROM lookup_table WHERE parent_id = Y)` as an additional search clause. With appropriate indexes it's very fast to query, and i can do partial updates to maintain the lookup table. If my most common tree query was something else (e.g. enumerate children in sorted order) then i'd need some other data structure.