Stop referring to 2.9.0, there is more than one database upgrade step.
Recommend several indexes that improve MySQL performance.

Without these, an upgrade on MySQL of more than a few thousand tickets
may never finish.  The New/Old Reference Transaction indexes have the
greatest impact.  The Type and OCFV indexes 'help' but those updates
will still be slow.
Remove an ORDER BY

We don't need an ordered set of ids for the Ticket update and removing
it improves MySQL 5.5's UPDATE FROM SELECT performance.
Merge branch '2.9/2.9.0-performance-on-mysql' into 2.9-trunk
Trim upgrading docs

Point to docs/UPGRADING-* and point people at make upgrade-database
rather than having them run rt-setup-database manually.