psql asks for password despite trust authentication

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http://serverfault.com – In /var/lib/pgsql/9.1/data/pg_hba.conf, I have the following: # TYPE DATABASE USER ADDRESS METHOD host all all trust But when I execute this: su -c "psql -d postgres" - postgres it asks me for a password. My expectation is that users shoud not be prompted for a password. (HowTos)