技術標籤：Flink

基本概念

stddev

這個stddev是Strandard Deviation的縮寫

下面來分析一個FLINK SQL 執行Rolling Aggregation的例子

如下:

SELECT 
  measurement_time,
  city, 
  temperature,
  AVG(CAST(temperature AS FLOAT))    OVER last_minute AS   avg_temperature_minute,
  MAX(temperature)                   OVER last_minute AS   min_temperature_minute,
  MIN(temperature)                   OVER last_minute AS   max_temperature_minute,
  STDDEV(CAST(temperature AS FLOAT)) OVER last_minute AS stdev_temperature_minute
FROM temperature_measurements 
WINDOW last_minute AS (
  PARTITION BY city
  ORDER BY measurement_time
  RANGE BETWEEN INTERVAL '1' MINUTE PRECEDING AND CURRENT ROW 
);
 

具體解析如下:

總結:

①根據上面的這個解析,我們也就知道了Rolling Aggregation是啥意思呢?

也就是說要統計過去的一分鐘內,關於某個變數(temperature)的最新的統計值(AVG/MAX/MIN/STDDEV)

②SQL中哪裡體現Rolling呢?

last_minute

不斷計算最近一分鐘(Rolling)的統計值(聚合操作),體現了Rolling

Reference:

[1]Rows Over Window與Range Over Window的區別