Yes you can improve on that performance - if you are using an object oriented database or something like DB_File. (With some sort of threading of keys - MLDBM is not the best of approaches either.)

Otherwise the fact that hierarchical data structures do not map well onto relational databases will kill you. And firing repeated queries against the database (a very obvious guess) is really bad.

You can beat what they do though, but not portably. And the way you do it is to create and populate a temp table on the fly with a stored procedure. Works except that all of that is mumbo jumbo that some databases (very specifically MYSQL) doesn't understand, and which tends to be non-portable from database to database.