postgresql - psql: fatal role "root" does not exist -

gone through many question answer problem not solved..

method tried,

user@user-latitude-xt3:~$ sudo su postgres no passwd entry user 'postgres' user@user-latitude-xt3:~$  

and many..

am keep on getting fatal error user not exist

any working solution?

please note before marking duplicate, tried similar wasnt solved yet...

this how login postgres:

sudo -i sudo -i -u postgres