Except a few special cases, optimizing SQL is about minimizing I/O. And by “I/O” we normally mean “physical I/O”, because everybody knows that logical I/O (LIO) is much, much faster. But how much faster exactly? For a long time, this question has been bothering me. It looks like there has been little research in this area. Basically the only thorough investigation I managed to find on the subject was one by Cary Millsap and co-authors. So I conducted some research on my own. Continue reading “Logical I/O” →