kørende gennemsnit i mysql

Din forespørgsel er en måde at lave et løbende gennemsnit på:

SELECT t.*,
       (select avg(speed) from tbl tt where tt.timestamp <= t.timestamp) as avg
FROM tbl t;

Alternativet er at bruge variabler:

select t.*, (sum_speed / cnt) as running_avg_speed
from (select t.*, (@rn := @rn + 1) as cnt, (@s := @s + speed) as sum_speed
      from tbl t cross join
           (select @rn := 0, @s := 0) params
      order by timestamp
     ) t;

Et indeks på tbl(timestamp) bør forbedre ydeevnen yderligere.