If I'm generous, that "database" stores a single number. If you transform a more realistic database (one that can store many arbitrary values) into a Boolean circuit, you'll generally end up with at least one operation per stored value. And when you evaluate the circuit on encrypted data, you have to evaluate all those operations every time. That's why I wrote "without doing at least as much work as computing E(y)" instead of just "without computing E(y)." Yes, you do not necessarily explicitly encrypt all stored values. But you'll end up performing a corresponding amount of computation anyways.