From this thread https://groups.google.com/forum/#!topic/python-tornado/mgj18... (posting here because Google Groups sometimes require login):

  We experimented with different async DB approaches, but settled on 
  synchronous at FriendFeed because generally if our DB queries were 
  backlogging our requests, our backends couldn't scale to the load 
  anyway. Things that were slow enough were abstracted to separate 
  backend services which we fetched asynchronously via the async HTTP 
  module. 

  I may open source the async MySQL client I wrote, but I am still 
  skeptical of the long term value given the code complexity it 
  introduces. 

  Bret

The impact of an occasional slow query is mitigated by the fact that you're running multiple processes. You need multiple processes anyway because of the GIL, so just run a few more to make sure you're able to keep the CPU busy. An async DB interface would be nice, but as long as you keep your database performance under control it's not crucial, so it hasn't been written yet.