Error 1104:
The SELECT would examine more than MAX_JOIN_SIZE rows; check your WHERE and use SET SQL_BIG_SELECTS=1 or SET MAX_JOIN_SIZE=# if the SELECT is okay

On query:
SELECT SQL_CALC_FOUND_ROWS u.*, uc.*, (u.last_date > (NOW() - INTERVAL '15' MINUTE)) as u_online FROM users u , users_counts uc, feeds_users fu WHERE u.id=uc.uid AND u.id = fu.id_user AND fu.id_serial = 98 LIMIT 0, 10