I’ve started poking around the MySQL 5.7.1 source tree (although just from tarball as I don’t see a BZR tree yet). I thought I’d share a few thoughts:

• InnoDB temporary tables. Not REDO logged. What does this mean? It’s a huge step in removing the dependency on MEMORY and MyISAM engines for temporary tables used in query execution. With InnoDB temporary tables there is no reason for MEMORY engine to continue to exist, there is absolutely no way in which it is better.
• InnoDB temp tables aren’t insert buffered
  This probably doesn’t really matter as you’re not going to be doing REDO logging for them (plus things are generally short lived)… but it could be a future area for performance improvement
• The NO_REDO log mode appears to be implemented fairly neatly.
• Improvements in innodb read only mode. What does this mean? Maybe we can finally get rid of the oddity of compressed read only MyISAM tables on read only media. (on the other hand, CDs and DVDs aren’t exactly a modern form of software distribution).
• Some of the source code comments have improved.. it’s getting easier to understand InnoDB. I’d still make the argument that if you need source code comments you’re code isn’t clear enough… but any step is an improvement. (that being said, InnoDB was always easier than the server)
• There is some pretty heavy refactoring of lock0lock.cc – I really need to sit down and poke at it a bunch.
• The shared tablespace code (innodb system tablespace) has been heavily refactored. This also introduces tablespaces for temporary tables – and it appears to be implemented in the correct way.

I need to look into things a bunch more, and it’ll be really useful to see a bzr tree to better understand some of the changes.

More to come later, but that’s my quick look.