In a modern datacenter, network is significantly faster than disk. See Jeff Dean's numbers everyone should know. http://www.google.com/imgres?q=google+numbers+e

In a modern datacenter, network is significantly faster than disk. See Jeff Dean's numbers everyone should know. http://www.google.com/imgres?q=google+numbers+everyone+should+know&um=1&hl=en&sa=N&biw=1862&bih=1083&tbm=isch&tbnid=Z-o-Fiwd8NGEMM:&imgrefurl=http://doubleclix.wordpress.com/2010/11/11/google-a-study-in-scalability-and-a-little-systems-horse-sense/&docid=h6N4kIPchU34oM&imgurl=http://doubleclix.files.wordpress.com/2010/11/numberseveryoneshouldknow.png&w=595&h=444&ei=jeGVTrtJrdSIAsmhuMQN&zoom=1&iact=rc&dur=313&sig=107667830273554061378&page=1&tbnh=135&tbnw=181&start=0&ndsp=56&ved=1t:429,r:0,s:0&tx=143&ty=81 http://www.google.com/imgres?q=google+numbers+everyone+shoul... Databases are slow for many reasons. The most common is that your dataset doesn't fit into memory.