重新分区pyspark数据框失败以及如何避免初始分区大小

Question

I'm trying to tune the performance of spark, by the use of partitioning on a spark dataframe. Here is the code:

file_path1 = spark.read.parquet(*paths[:15])
df = file_path1.select(columns) \
    .where((func.col("organization") == organization)) 
df = df.repartition(10)
#execute an action just to make spark execute the repartition step
df.first()

During the execution of first() I check the job stages in Spark UI and here what I find:

• Why there is no repartition step in the stage?
• Why there is also stage 8? I only requested one action of first() . Is it because of the shuffle caused by the repartition ?
• Is there a way to change the repartition of the parquet files without having to occur to such operations? As initially when I read the df you can see that it's partitioned over 43k partitions which is really a lot (compared to its size when I save it to a csv file: 4 MB with 13k rows) and creating problems in further steps, that's why I wanted to repartition it.
• Should I use cache() after repartition? df = df.repartition(10).cache() ? As when I executed df.first() the second time, I also get a scheduled stage with 43k partitions, despite df.rdd.getNumPartitions() which returned 10. EDIT: the number of partitions is just to try. my questions are directed to help me understand how to do the right repartition.

Note: initially the Dataframe is read from a selection of parquet files in Hadoop.

I already read this as reference How does Spark partition(ing) work on files in HDFS?

Answer 1

• Whenever there is shuffling, there is a new stage. and the
  repartition causes shuffling that"s why you have two stages.
• the caching is used when you'll use the dataframe multiple times to avoid reading it twice.

Use coalesce instead of repartiton. I think it causes less shuffling since it only reduces the number of partitions.