Uninstalling PostgreSQL may leave some undesired files. Here’s how to purge it:
First, uninstall all packages from PostgreSQL from the system using
sudo apt-get --purge remove postgresql\*
Then remove all the configuration and library stuff
sudo rm -r /etc/postgresql/
sudo rm -r /etc/postgresql-common/
sudo rm -r /var/lib/postgresql/
sudo rm -r /etc/postgresql-common/
sudo rm -r /var/lib/postgresql/
And finally, remove the user and group
sudo userdel -r postgres
sudo groupdel postgres
sudo groupdel postgres