gistfile1.txt

-- pg 9.2 (should also work with 9.1)
tanga_dev=# select users.id, users.username from users inner join points on points.user_id = users.id group by users.id;
 id |    username     
----+-----------------
  6 | active_user
  4 | normal


-- pg 9.0 
tanga_dev=# select users.id, users.username from users inner join points on points.user_id = users.id group by users.id;
ERROR:  column "users.username" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: select users.id, users.username from users inner join points...