背景
最近在一个Stack Overflow的帖子中,我发现使用withColumn
可以改善DAG,特别是处理堆叠/链式列表达式与distinct窗口规范相结合的情况。然而,在这个例子中,withColumn
实际上会使DAG更糟,并且与使用select
的结果不同。
可重现的示例
首先,一些测试数据(PySpark 2.4.4 standalone):
import pandas as pd
import numpy as np
from pyspark.sql import SparkSession, Window
from pyspark.sql import functions as F
spark = SparkSession.builder.getOrCreate()
dfp = pd.DataFrame(
{
"col1": np.random.randint(0, 5, size=100),
"col2": np.random.randint(0, 5, size=100),
"col3": np.random.randint(0, 5, size=100),
"col4": np.random.randint(0, 5, size=100),
"col5": np.random.randint(0, 5, size=100),
}
)
df = spark.createDataFrame(dfp)
df.show(5)
+----+----+----+----+----+
|col1|col2|col3|col4|col5|
+----+----+----+----+----+
| 0| 3| 2| 2| 2|
| 1| 3| 3| 2| 4|
| 0| 0| 3| 3| 2|
| 3| 0| 1| 4| 4|
| 4| 0| 3| 3| 3|
+----+----+----+----+----+
only showing top 5 rows
这个例子很简单。它包含了两个窗口规范和基于它们的四个独立列表达式:
w1 = Window.partitionBy("col1").orderBy("col2")
w2 = Window.partitionBy("col3").orderBy("col4")
col_w1_1 = F.max("col5").over(w1).alias("col_w1_1")
col_w1_2 = F.sum("col5").over(w1).alias("col_w1_2")
col_w2_1 = F.max("col5").over(w2).alias("col_w2_1")
col_w2_2 = F.sum("col5").over(w2).alias("col_w2_2")
expr = [col_w1_1, col_w1_2, col_w2_1, col_w2_2]
使用withColumn会产生4次Shuffle操作
如果withColumn
与交替的窗口规范一起使用,DAG将产生不必要的Shuffle操作:
df.withColumn("col_w1_1", col_w1_1)\
.withColumn("col_w2_1", col_w2_1)\
.withColumn("col_w1_2", col_w1_2)\
.withColumn("col_w2_2", col_w2_2)\
.explain()
== Physical Plan ==
Window [sum(col5#92L) windowspecdefinition(col3#90L, col4#91L ASC NULLS FIRST, specifiedwindowframe(RangeFrame, unboundedpreceding$(), currentrow$())) AS col_w2_2#147L], [col3#90L], [col4#91L ASC NULLS FIRST]
+- *(4) Sort [col3#90L ASC NULLS FIRST, col4#91L ASC NULLS FIRST], false, 0
+- Exchange hashpartitioning(col3#90L, 200)
+- Window [sum(col5#92L) windowspecdefinition(col1#88L, col2#89L ASC NULLS FIRST, specifiedwindowframe(RangeFrame, unboundedpreceding$(), currentrow$())) AS col_w1_2#143L], [col1#88L], [col2#89L ASC NULLS FIRST]
+- *(3) Sort [col1#88L ASC NULLS FIRST, col2#89L ASC NULLS FIRST], false, 0
+- Exchange hashpartitioning(col1#88L, 200)
+- Window [max(col5#92L) windowspecdefinition(col3#90L, col4#91L ASC NULLS FIRST, specifiedwindowframe(RangeFrame, unboundedpreceding$(), currentrow$())) AS col_w2_1#145L], [col3#90L], [col4#91L ASC NULLS FIRST]
+- *(2) Sort [col3#90L ASC NULLS FIRST, col4#91L ASC NULLS FIRST], false, 0
+- Exchange hashpartitioning(col3#90L, 200)
+- Window [max(col5#92L) windowspecdefinition(col1#88L, col2#89L ASC NULLS FIRST, specifiedwindowframe(RangeFrame, unboundedpreceding$(), currentrow$())) AS col_w1_1#141L], [col1#88L], [col2#89L ASC NULLS FIRST]
+- *(1) Sort [col1#88L ASC NULLS FIRST, col2#89L ASC NULLS FIRST], false, 0
+- Exchange hashpartitioning(col1#88L, 200)
+- Scan ExistingRDD[col1#88L,col2#89L,col3#90L,col4#91L,col5#92L]
选择 - 2 洗牌
如果所有列都使用 select
传递,则 DAG 是正确的。
df.select("*", *expr).explain()
== Physical Plan ==
Window [max(col5#92L) windowspecdefinition(col3#90L, col4#91L ASC NULLS FIRST, specifiedwindowframe(RangeFrame, unboundedpreceding$(), currentrow$())) AS col_w2_1#119L, sum(col5#92L) windowspecdefinition(col3#90L, col4#91L ASC NULLS FIRST, specifiedwindowframe(RangeFrame, unboundedpreceding$(), currentrow$())) AS col_w2_2#121L], [col3#90L], [col4#91L ASC NULLS FIRST]
+- *(2) Sort [col3#90L ASC NULLS FIRST, col4#91L ASC NULLS FIRST], false, 0
+- Exchange hashpartitioning(col3#90L, 200)
+- Window [max(col5#92L) windowspecdefinition(col1#88L, col2#89L ASC NULLS FIRST, specifiedwindowframe(RangeFrame, unboundedpreceding$(), currentrow$())) AS col_w1_1#115L, sum(col5#92L) windowspecdefinition(col1#88L, col2#89L ASC NULLS FIRST, specifiedwindowframe(RangeFrame, unboundedpreceding$(), currentrow$())) AS col_w1_2#117L], [col1#88L], [col2#89L ASC NULLS FIRST]
+- *(1) Sort [col1#88L ASC NULLS FIRST, col2#89L ASC NULLS FIRST], false, 0
+- Exchange hashpartitioning(col1#88L, 200)
+- Scan ExistingRDD[col1#88L,col2#89L,col3#90L,col4#91L,col5#92L]
问题
有一些现有信息解释了为什么应该避免使用 withColumn
,然而它们主要涉及多次调用 withColumn
并且没有解决 DAG 偏差问题(参见 这里 和 这里)。
有人知道为什么 withColumn
和 select
之间的DAG不同吗?Spark的优化算法应该适用于任何情况,并且不应受到表达相同操作方式的不同方式的影响。
提前致谢。