Hierarchical Queries in MySQL (Function substitute for CONNECT BY)

Good point, but using nested sets means a lot of work when you update. Doing the work in a UDF is a lot quicker than putting it into the code that calls the SQL. I've done that. [sigh] Maybe I shouldn't admit to it.

Sure, especially if you nest the sets using a datatype indexable with an R-Tree index: http://explainextended.com/2009/09/29/adjacency-list-vs-nest...

However, nested sets is still very expensive to update which may be a deciding factor if your hierarchy is subject to frequent change (like, a folder-like structure with the folders being moved here and there) and you don't need to query for "all descendants" often.