How to unstack dataset (using pivot)?

问题

I tried the new "pivot" function of 1.6 on a larger stacked dataset. It has 5,656,458 rows and the IndicatorCode column has 1344 different codes.

The idea was to use pivot to "unstack" (in pandas terms) this data set and have a column for each IndicatorCode.

schema = StructType([ \
   StructField("CountryName", StringType(), True), \
   StructField("CountryCode", StringType(), True), \
   StructField("IndicatorName", StringType(), True), \
   StructField("IndicatorCode", StringType(), True), \
   StructField("Year", IntegerType(), True), \
   StructField("Value", DoubleType(), True)  \
])

data = sqlContext.read.load('hdfs://localhost:9000/tmp/world-development-indicators/Indicators.csv', 
                            format='com.databricks.spark.csv', 
                            header='true', 
                            schema=schema)

data2 = indicators_csv.withColumn("IndicatorCode2", regexp_replace("indicatorCode", "\.", "_"))\
                      .select(["CountryCode", "IndicatorCode2", "Year", "Value"])

columns = [row.IndicatorCode2 for row in data2.select("IndicatorCode2").distinct().collect()]

data3 = data2.groupBy(["Year", "CountryCode"])\
             .pivot("IndicatorCode2", columns)\
             .max("Value")

While this returned successfully, data3.first() never returned a result (I interrupted on my standalone using 3 cores after 10 min).

My approach using RDD and aggregateByKey worked well, so I'm not searching for a solution about how to do it, but whether pivot with DataFrames can also do the trick.

回答1:

Well, pivoting is not a very efficient operation in general and there is not much you can do about it using DataFrame API. One thing you can try though is to repartition your data:

(data2
  .repartition("Year", "CountryCode")
  .groupBy("Year", "CountryCode")
  .pivot("IndicatorCode2", columns)
  .max("Value"))

or even aggregate:

from pyspark.sql.functions import max

(df
    .groupBy("Year", "CountryCode", "IndicatorCode")
    .agg(max("Value").alias("Value"))
    .groupBy("Year", "CountryCode")
    .pivot("IndicatorCode", columns)
    .max("Value"))

before applying pivot. The idea behind both solutions is the same. Instead of moving large expanded Rows move narrow dense data and expand locally.

回答2:

Spark 2.0 introduced SPARK-13749 an implementation of pivot that is faster for a large number of pivot column values.

Testing with Spark 2.1.0 on my computer, your example now runs in 48 seconds.

来源：https://stackoverflow.com/questions/35427812/how-to-unstack-dataset-using-pivot

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