Avoiding shuffle on GROUP BY in Spark SQL [duplicate]

Question

I want to know if Spark knows the partitioning key of the parquet file and uses this information to avoid shuffles.

Context:

Running Spark 2.0.1 running local SparkSession. I have a csv dataset that I am saving as parquet file on my disk like so:

val df0 = spark
  .read
  .format("csv")
  .option("header", true)
  .option("delimiter", ";")
  .option("inferSchema", false)
  .load("SomeFile.csv"))


val df = df0.repartition(partitionExprs = col("numerocarte"), numPartitions = 42)

df.write
  .mode(SaveMode.Overwrite)
  .format("parquet")
  .option("inferSchema", false)
  .save("SomeFile.parquet")

I am creating 42 partitions by column numerocarte. This should group multiple numerocarte to same partition. I don't want to do partitionBy("numerocarte") at the write time because I don't want one partition per card. It would be millions of them.

After that in another script I read this SomeFile.parquet parquet file and do some operations on it. In particular I am running a window function on it where the partitioning is done on the same column that the parquet file was repartitioned by.

import org.apache.spark.sql.expressions.Window
import org.apache.spark.sql.functions._

val df2 = spark.read
  .format("parquet")
  .option("header", true)
  .option("inferSchema", false)
  .load("SomeFile.parquet")

val w = Window.partitionBy(col("numerocarte"))
.orderBy(col("SomeColumn"))

df2.withColumn("NewColumnName",
      sum(col("dollars").over(w))

After read I can see that the repartition worked as expected and DataFrame df2 has 42 partitions and in each of them are different cards.

Questions:

1. Does Spark know that the dataframe df2 is partitioned by column numerocarte?
2. If it knows, then there will be no shuffle in the window function. True?
3. If it does not know, It will do a shuffle in the window function. True?
4. If it does not know, how do I tell Spark the data is already partitioned by the right column?
5. How can I check a partitioning key of DataFrame? Is there a command for this? I know how to check number of partitions but how to see partitioning key?
6. When I print number of partitions in a file after each step, I have 42 partitions after read and 200 partitions after withColumn which suggests that Spark repartitioned my DataFrame.
7. If I have two different tables repartitioned with the same column, would the join use that information?

Answer 1

Does Spark know that the dataframe df2 is partitioned by column numerocarte?

It does not.

If it does not know, how do I tell Spark the data is already partitioned by the right column?

You don't. Just because you save data which has been shuffled, it does not mean, that it will be loaded with the same splits.

How can I check a partitioning key of DataFrame?

There is no partitioning key once you loaded data, but you can check queryExecution for Partitioner.

---

In practice:

• If you want to support efficient pushdowns on the key, use partitionBy method of DataFrameWriter.
• If you want a limited support for join optimizations use bucketBy with metastore and persistent tables.

See How to define partitioning of DataFrame? for detailed examples.

Answer 2

I am answering my own question for future reference what worked.

Following suggestion of @user8371915, bucketBy works!

I am saving my DataFrame df:

df.write
  .bucketBy(250, "userid")
  .saveAsTable("myNewTable")

Then when I need to load this table:

val df2 = spark.sql("SELECT * FROM myNewTable")

val w = Window.partitionBy("userid")

val df3 = df2.withColumn("newColumnName", sum(col("someColumn")).over(w)
df3.explain

I confirm that when I do window functions on df2 partitioned by userid there is no shuffle! Thanks @user8371915!

Some things I learned while investigating it

• myNewTable looks like a normal parquet file but it is not. You could read it normally with spark.read.format("parquet").load("path/to/myNewTable") but the DataFrame created this way will not keep the original partitioning! You must use spark.sql select to get correctly partitioned DataFrame.
• You can look inside the table with spark.sql("describe formatted myNewTable").collect.foreach(println). This will tell you what columns were used for bucketing and how many buckets there are.
• Window functions and joins that take advantage of partitioning often require also sort. You can sort data in your buckets at the write time using .sortBy() and the sort will be also preserved in the hive table. df.write.bucketBy(250, "userid").sortBy("somColumnName").saveAsTable("myNewTable")
• When working in local mode the table myNewTable is saved to a spark-warehouse folder in my local Scala SBT project. When saving in cluster mode with mesos via spark-submit, it is saved to hive warehouse. For me it was located in /user/hive/warehouse.
• When doing spark-submit you need to add to your SparkSession two options: .config("hive.metastore.uris", "thrift://addres-to-your-master:9083") and .enableHiveSupport(). Otherwise the hive tables you created will not be visible.
• If you want to save your table to specific database, do spark.sql("USE your database") before bucketing.

Update 05-02-2018

I encountered some problems with spark bucketing and creation of Hive tables. Please refer to question, replies and comments in Why is Spark saveAsTable with bucketBy creating thousands of files?