pyspark：在窗口上计数不同

Question

我刚刚尝试在窗口上执行countDistinct 并收到此错误：

AnalysisException: u'Distinct 窗口函数不受支持： 计数（不同颜色#1926）

有没有办法对 pyspark 中的窗口进行不同的计数？

下面是一些示例代码：

from pyspark.sql.window import Window    
from pyspark.sql import functions as F

#function to calculate number of seconds from number of days
days = lambda i: i * 86400

df = spark.createDataFrame([(17, "2017-03-10T15:27:18+00:00", "orange"),
                    (13, "2017-03-15T12:27:18+00:00", "red"),
                    (25, "2017-03-18T11:27:18+00:00", "red")],
                    ["dollars", "timestampGMT", "color"])
                    
df = df.withColumn('timestampGMT', df.timestampGMT.cast('timestamp'))

#create window by casting timestamp to long (number of seconds)
w = (Window.orderBy(F.col("timestampGMT").cast('long')).rangeBetween(-days(7), 0))

df = df.withColumn('distinct_color_count_over_the_last_week', F.countDistinct("color").over(w))

df.show()

这是我希望看到的输出：

+-------+--------------------+------+---------------------------------------+
|dollars|        timestampGMT| color|distinct_color_count_over_the_last_week|
+-------+--------------------+------+---------------------------------------+
|     17|2017-03-10 15:27:...|orange|                                      1|
|     13|2017-03-15 12:27:...|   red|                                      2|
|     25|2017-03-18 11:27:...|   red|                                      1|
+-------+--------------------+------+---------------------------------------+

Answer 1

编辑：

正如 noleto 在下面的回答中提到的，现在有一个 approx_count_distinct 函数，因为 pyspark 2.1 可以在窗口上工作。

---

原答案

我发现我可以结合使用 collect_set 和 size 函数来模拟窗口上的 countDistinct 功能：

from pyspark.sql.window import Window
from pyspark.sql import functions as F

#function to calculate number of seconds from number of days
days = lambda i: i * 86400

#create some test data
df = spark.createDataFrame([(17, "2017-03-10T15:27:18+00:00", "orange"),
                    (13, "2017-03-15T12:27:18+00:00", "red"),
                    (25, "2017-03-18T11:27:18+00:00", "red")],
                    ["dollars", "timestampGMT", "color"])

#convert string timestamp to timestamp type             
df = df.withColumn('timestampGMT', df.timestampGMT.cast('timestamp'))

#create window by casting timestamp to long (number of seconds)
w = (Window.orderBy(F.col("timestampGMT").cast('long')).rangeBetween(-days(7), 0))

#use collect_set and size functions to perform countDistinct over a window
df = df.withColumn('distinct_color_count_over_the_last_week', F.size(F.collect_set("color").over(w)))

df.show()

这会导致前一周记录的颜色计数不同：

+-------+--------------------+------+---------------------------------------+
|dollars|        timestampGMT| color|distinct_color_count_over_the_last_week|
+-------+--------------------+------+---------------------------------------+
|     17|2017-03-10 15:27:...|orange|                                      1|
|     13|2017-03-15 12:27:...|   red|                                      2|
|     25|2017-03-18 11:27:...|   red|                                      1|
+-------+--------------------+------+---------------------------------------+

Answer 2

@Bob Swain 的回答很好而且有效！从那时起，Spark version 2.1，Spark 提供了与countDistinct 等效的函数approx_count_distinct，它的使用效率更高，最重要的是，它支持对窗口进行计数。

下面是替换代码：

#approx_count_distinct supports a window
df = df.withColumn('distinct_color_count_over_the_last_week', F.approx_count_distinct("color").over(w))

对于基数较小的列，结果应该与“countDistinct”相同。当数据集增长很多时，您应该考虑调整参数rsd - 允许的最大估计误差，这可以让您调整权衡精度/性能。