Installing Postgresql and having Plesk recognize it as a valid database option is almost as easy as installing it outside Plesk but with one additional step. You have to tell plesk postgresql is installed on the server.
# yum install postgresql postgresql-server
After yum downloads and installs the server you have to start the service:
# /sbin/service postgresql start
The above command only starts the service. When you reboot the server you will need to manually start the service. To have the service start when the server reboots issue the following command:
# chkconfig --level 35 postgresql on
The final step to get Postgresql to be recognized by Plesk is to to tell Plesk that we have installed Postgresql
# /usr/local/psa/bin/database-server --update-server localhost:5432 -type postgresql -admin admin -passwd `cat /etc/psa/.psa.shadow`