The hint of a change is in here (search for "secondary"):
http://jorgenloland.blogspot.co.uk/2012/12/favorite-mysql-56-features-optimizer.html
> -Original Message-
> From: Rick James
> Sent: Wednesday, January 30, 2013 1:08 PM
> To: 'Jeremy Chase'; mysql@lists.mysql.com
> Subject: RE: Inno
secondarykey and redundantkey are redundant with each other -- in all versions
of InnoDB.
One "expert" said that redundant key would have two copies of `1`,`2`. I think
he is wrong. I believe the two are the same in size.
There is a subtle change in 5.6 that _may_ make a _few_ queries work be
