Download repo
wget http://yum.pgrpms.org/reporpms/9.0/pgdg-centos-9.0-2.noarch.rpm
Install repo
rpm -i pgdg-centos-9.0-2.noarch.rpm
Install Postgres
yum install postgresql90-server
Initialise DB
service postgresql-9.0 initdb
Start Service
service postgresql-9.0 start
