请问如果分组后取组内最后四条数据求平均的话应该怎么写效率比较高？
sym = rand(`A`B`C, 15)price= rand(49..51, 15)qty = rand(100..200, 15)time = 09:47:00 + 1..15t1 = table(timestamp, sym, qty, time, price);
select aggrTopN(avg, price, time, 4, false) from t1 group by sym
