Bar fights do occur. In the centre of the city and around Nevsky Prospekt, they are unlikely to happen. However, in the suburbs and local cheaper pubs, fights occur almost daily. If you are staying with locals living in these areas, it might be a good idea to avoid these bars. Police are unlikely to show up as they consider fights as small, unimportant, regular and a waste of time, and they will probably laugh at you for calling.