[simonw]

Sounds to me like you're an expert on relational database optimization. That's an extremely valuable specialty. Don't talk yourself down as "not the best coder"! You've been thinking about this problem for seven years.

Don't be too worried about the puzzle interview stuff. Many companies don't use them any more... and if you do want to apply someone that uses them I've heard there are excellent books that could get you fully up to speed in a few weeks.