The idea how to speed up nested-loops join a little I’ve mentioned earlier has now been promoted into a worklog entry. It is now known as WL#3724 “Short-Cutting Join Execution: Speeding up star queries” and its text is publicly available at MySQLForge.
At the moment there is only a short description, but hopefully Martin Hansson (the assigned developer) will add more content there.
Sergei Petrunia's blog 