> The first time they guess wrong, you have to spend a lot of time understanding not only how the lower layers work, and not only why they did the "wrong" thing

> The first time they guess wrong, you have to spend a lot of time understanding not only how the lower layers work, and not only why they did the "wrong" thing in this one instance, but how to fiddle correctly with the layer you're operating at to get the lower layers to behave properly. SQL in a nutshell.