示例代码如下：

symbol = take(`APPL, 6) join take(`FB, 5)
time = 2019.02.27T09:45:01 2019.02.27T09:45:01 2019.02.27T09:45:04 2019.02.27T09:45:04 2019.02.27T09:45:05 2019.02.27T09:45:06 2019.02.27T09:45:01 2019.02.27T09:45:02 2019.02.27T09:45:03 2019.02.27T09:45:04 2019.02.27T09:45:05
price = (170 + rand(5, 6)) join (64 + rand(2, 5))
quotes = table(symbol, time, price)
weights = dict(`APPL`FB, 0.6 0.4)
ETF = select symbol, time, price * weights[symbol] as price from quotes
t = select rowSum(ffill(last(price))) from ETF pivot by time, symbol

请问，上述的select rowSum(ffill(last(price))) from ETF pivot by time, symbol具体执行流程是怎么样的？