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
Start the service:
service postgresql start
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`
Plesk Postgresql Support