Di centos 5.5, default bawaan sewaktu install Postgresql itu versi 8.4.7. Cara-cara untuk upgrade ke versi 9, bisa diliat disini ^^ 
Untuk cek versi sekarang (setelah `yum install postgresql` tentunya) 
[root@localhost ~]# psql -V
psql (PostgreSQL) 8.4.7
tambahkan RPM 
[root@localhost ~]# cd /etc/yum.repos.d/
[root@localhost ~]# wget http://yum.pgrpms.org/reporpms/9.0/pgdg-centos-9.0-2.noarch.rpm
[root@localhost ~]# rpm -i pgdg-centos-9.0-2.noarch.rpm
Edit file CentOS-Base.repo dan tambah 'exclude=postgresql*' di bagian [base] dan [updates]
[root@localhost /]# nano /etc/yum.repos.d/CentOS-Base.repo
# [base]  
# name=CentOS-$releasever - Base  
# mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os  
# #baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/  
# gpgcheck=1  
# gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5  
# exclude=postgresql*  
#  
# #released updates  
# [updates]  
# name=CentOS-$releasever - Updates  
# mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates  
# #baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/  
# gpgcheck=1  
# gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5  
# exclude=postgresql*  
Setelah itu, instal deh psql9 nya :D 
[root@localhost /]# yum install postgresql90-server
Sumbber : http://www.davidghedini.com/pg/entry/install_postgresql_9_on_centos