[GitHub] [spark] ueshin opened a new pull request #28730: [SPARK-31903][PYSPARK][R] Fix toPandas with Arrow enabled to show metrics in Query UI.

[GitBox]

ueshin opened a new pull request #28730:
URL: https://github.com/apache/spark/pull/28730


   ### What changes were proposed in this pull request?
   
   In `Dataset.collectAsArrowToR` and `Dataset.collectAsArrowToPython`, since 
the code block for `serveToStream` is run in the separate thread, `withAction` 
finishes as soon as it starts the thread. As a result, it doesn't collect the 
metrics of the actual action and Query UI shows the plan graph without metrics.
   
   We should call `serveToStream` first, then `withAction` in it.
   
   ### Why are the changes needed?
   
   When calling toPandas, usually Query UI shows each plan node's metric and 
corresponding Stage ID and Task ID:
   
   ```py
   >>> df = spark.createDataFrame([(1, 10, 'abc'), (2, 20, 'def')], 
schema=['x', 'y', 'z'])
   >>> df.toPandas()
      x   y    z
   0  1  10  abc
   1  2  20  def
   ```
   
   ![Screen Shot 2020-06-03 at 4 47 07 
PM](https://user-images.githubusercontent.com/506656/83815735-bec22380-a675-11ea-8ecc-bf2954731f35.png)
   
   but if Arrow execution is enabled, it shows only plan nodes and the duration 
is not correct:
   
   ```py
   >>> spark.conf.set('spark.sql.execution.arrow.pyspark.enabled', True)
   >>> df.toPandas()
      x   y    z
   0  1  10  abc
   1  2  20  def
   ```
   
   ![Screen Shot 2020-06-03 at 4 47 27 
PM](https://user-images.githubusercontent.com/506656/83815804-de594c00-a675-11ea-933a-d0ffc0f534dd.png)
   
   ### Does this PR introduce _any_ user-facing change?
   
   Yes, the Query UI will show the plan with the correct metrics.
   
   ### How was this patch tested?
   
   I checked it manually in my local.
   
   ![Screen Shot 2020-06-04 at 3 19 41 
PM](https://user-images.githubusercontent.com/506656/83816265-d77f0900-a676-11ea-84b8-2a8d80428bc6.png)
   
   


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