Apache spark 如何最大化值并保留所有列（对于每个组的最大记录）？

给定以下数据帧：

+----+-----+---+-----+
| uid|    k|  v|count|
+----+-----+---+-----+
|   a|pref1|  b|  168|
|   a|pref3|  h|  168|
|   a|pref3|  t|   63|
|   a|pref3|  k|   84|
|   a|pref1|  e|   84|
|   a|pref2|  z|  105|
+----+-----+---+-----+

如何从

uid

，

k

中获取最大值，但包括

v

+----+-----+---+----------+
| uid|    k|  v|max(count)|
+----+-----+---+----------+
|   a|pref1|  b|       168|
|   a|pref3|  h|       168|
|   a|pref2|  z|       105|
+----+-----+---+----------+

我可以这样做，但它会删除列“v”：

---

您可以使用窗口功能：

from pyspark.sql.functions import max as max_
from pyspark.sql.window import Window

w = Window.partitionBy("uid", "k")

df.withColumn("max_count", max_("count").over(w))

---

以下是我迄今为止提出的最佳解决方案：

val w = Window.partitionBy("uid","k").orderBy(col("count").desc)

df.withColumn("rank", dense_rank().over(w)).select("uid", "k","v","count").where("rank == 1").show

---

这是窗口操作符（使用

over

函数）或

join

的完美示例

既然您已经了解了如何使用windows，我只关注

join

scala> val inventory = Seq(
     |   ("a", "pref1", "b", 168),
     |   ("a", "pref3", "h", 168),
     |   ("a", "pref3", "t",  63)).toDF("uid", "k", "v", "count")
inventory: org.apache.spark.sql.DataFrame = [uid: string, k: string ... 2 more fields]

scala> val maxCount = inventory.groupBy("uid", "k").max("count")
maxCount: org.apache.spark.sql.DataFrame = [uid: string, k: string ... 1 more field]

scala> maxCount.show
+---+-----+----------+
|uid|    k|max(count)|
+---+-----+----------+
|  a|pref3|       168|
|  a|pref1|       168|
+---+-----+----------+

scala> val maxCount = inventory.groupBy("uid", "k").agg(max("count") as "max")
maxCount: org.apache.spark.sql.DataFrame = [uid: string, k: string ... 1 more field]

scala> maxCount.show
+---+-----+---+
|uid|    k|max|
+---+-----+---+
|  a|pref3|168|
|  a|pref1|168|
+---+-----+---+

scala> maxCount.join(inventory, Seq("uid", "k")).where($"max" === $"count").show
+---+-----+---+---+-----+
|uid|    k|max|  v|count|
+---+-----+---+---+-----+
|  a|pref3|168|  h|  168|
|  a|pref1|168|  b|  168|
+---+-----+---+---+-----+

---

几乎，它添加了一个具有最大值的列，但保留了所有行。这不会有性能问题吗？它始终取决于数据的大小+可读性。如果大小不是那么大，而您只是喜欢Spark API，那么这只是您想使用的个人体验，不是吗？倾向于同意Sir，有人能给我展示一下上面使用Java API的示例吗？我是Spark（使用Java）的初学者，在使用column alias方面有困难

scala> val inventory = Seq(
     |   ("a", "pref1", "b", 168),
     |   ("a", "pref3", "h", 168),
     |   ("a", "pref3", "t",  63)).toDF("uid", "k", "v", "count")
inventory: org.apache.spark.sql.DataFrame = [uid: string, k: string ... 2 more fields]

scala> val maxCount = inventory.groupBy("uid", "k").max("count")
maxCount: org.apache.spark.sql.DataFrame = [uid: string, k: string ... 1 more field]

scala> maxCount.show
+---+-----+----------+
|uid|    k|max(count)|
+---+-----+----------+
|  a|pref3|       168|
|  a|pref1|       168|
+---+-----+----------+

scala> val maxCount = inventory.groupBy("uid", "k").agg(max("count") as "max")
maxCount: org.apache.spark.sql.DataFrame = [uid: string, k: string ... 1 more field]

scala> maxCount.show
+---+-----+---+
|uid|    k|max|
+---+-----+---+
|  a|pref3|168|
|  a|pref1|168|
+---+-----+---+

scala> maxCount.join(inventory, Seq("uid", "k")).where($"max" === $"count").show
+---+-----+---+---+-----+
|uid|    k|max|  v|count|
+---+-----+---+---+-----+
|  a|pref3|168|  h|  168|
|  a|pref1|168|  b|  168|
+---+-----+---+---+-----+