After you have installed PostgreSQL 9.1 on your Linux Mint 12/13/14, created a user login, a database with that user as the owner and try to connect to that database using that user, you may run into the following error:
Password for user someuser:
psql: FATAL: Peer authentication failed for user “someuser”
psql: FATAL: Peer authentication failed for user “someuser”
If this is the case, you need to open up this file /etc/postgresql/9.1/main/pg_hba.conf in a text editor (with root privileges) and change this line:
local all all peer
to this:
local all all md5
After changing and saving the file, restart the PostgreSQL server:
sudo service postgresql restart
You should not have no issue connecting to your databases using the associated user.