Performance Impact: Bookmark Lookup is Expensive - Even in Memory

(Linchi Shea) Its well known that bookmark lookup (aka key lookup in case of a clustered index) is not cheap, especially when it comes to retrieving a lot of data. So Im not going to rehash the pros and cons of bookmark lookup or why bookmark lookup is expensive. But Ive noticed that when it comes to discussing bookmark lookup, all the literature seems to being focusing on their implications on storage I/Os. There is nothing wrong with that. In practice, it ultimately does come down to the fact that when reading a large amount of data, random I/Os are much more expensive than sequential I/Os, and bookmark lookup tends to incur random I/Os.