title: Keeping a unused column in Spark when using group by?
body: <p>So I have a dataframe of usernames what threads they have posted in and the timestamp of those posts. What I am trying to do if figure out who was the first user for a thread and what time it was. I know to figure out the  first post is to do a group by on a thread and then a min on the timestamp. But that removes the username. how do I use the group by and keep the usernames?</p>