I have an important announcement for 9.0 RPM users.
Because of packaging related issues, I released 9.0.1-6 sets. However, these packages don't actually solve the main problem: An upgrade from previous releases (9.0.1-5 and below) will remove all alternatives links. A workaround is running "yum reinstall postgresql90". (and other 9.0 packages you want). Please note that it does not affect fresh 9.0.1-6 installs. This issue is also valid for 8.4.5+RHEL 6. RHEL 6 RPMs also have parallel installation feature
Please note that yum in RHEL 4 does not have reinstall parameter, so you may end up go on with a fresh install
So, the solution, if you already have 9.0.1 RPMs, is: