My gripe with using a relational database as a priority queue is tends not to scale well. (Similarly as if you tried to use a relational database as a queue).

My gripe with using a relational database as a priority queue is tends not to scale well. (Similarly as if you tried to use a relational database as a queue). Once you get to the 10s of millions of messages per day (for example ~8M at priority X and 2M at priority Y), performance tends to go down the drain (due to contention), disk space bloats (due to vacumming/deletes). I've yet to come across a good task scheduling system that will take into the account resources available and the priority of the tasks, while still usable on a largish dataset that isn't a pain to maintain.