Optimize SELECT?

[mrhappiness]

SELECT
  COUNT(DISTINCT IF(user_id = 1, user_session, user_id)) usercount,
  CONCAT(LEFT(user_lastaction, 8), '000000') the_day
FROM stats_users
GROUP BY the_day
ORDER BY 
  anzahl DESC, 
  the_day DESC
LIMIT 1

the table stats_memberzones stores a history uf users's activities

user_id is a refernce to my user table, user_session hiólds the session-id and user_lastaction holds the timestamp that clicked something or hit refresh or...

i want to know the day with the highes count of visitors (guests and members) and how much people this have been

the members are identified by their user_id which is > 1
guests will have a user_id of 1 so i have to distinguish them via user_session

1. any ideas how to improve this?

2. does mysql use indices i set on user_id and user_session even if i use them in SELECT only and not in WHERE clause?

3. tia 🙂