People wonder whether they should do their best to prevent exceptions, or just let the system handle them. I've seen several discussions where folks debate whether they should do whatever they can to prevent an exception, because error handling is "expensive." There is no doubt that error handling isn't free, but I would predict that a constraint violation is at least as efficient as checking for a potential violation first. This may be different for a key violation than a static constraint violation, for example, but in this post I'm going to focus on the former.
