5 subtle ways you're using MySQL as a queue, and why it'll bite you
Many people realize the difficulties in building a good job queue or batch processing system, and try not to create one inside MySQL. Although the job queue is a great design pattern from the developer’s point of view, they know it’s often hard to implement well in a relational database. However, experience shows me that job queues sneak up in unexpected ways, even if you’re a seasoned developer.
Source: blog.engineyard.com