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https://issues.apache.org/jira/browse/SPARK-21056?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16050426#comment-16050426
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Bertrand Bossy commented on SPARK-21056:
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cc [~michael]
> InMemoryFileIndex.listLeafFiles should create at most one spark job when
> listing files in parallel
> --------------------------------------------------------------------------------------------------
>
> Key: SPARK-21056
> URL: https://issues.apache.org/jira/browse/SPARK-21056
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 2.1.1, 2.2.0
> Reporter: Bertrand Bossy
>
> Given partitioned file relation (e.g. parquet):
> {code}
> root/a=../b=../c=..
> {code}
> InMemoryFileIndex.listLeafFiles runs numberOfPartitions(a) times
> numberOfPartitions(b) spark jobs sequentially to list leaf files, if both
> numberOfPartitions(a) and numberOfPartitions(b) are below
> {{spark.sql.sources.parallelPartitionDiscovery.threshold}} and
> numberOfPartitions(c) is above
> {{spark.sql.sources.parallelPartitionDiscovery.threshold}}
> Since the jobs are run sequentially, the overhead of the jobs dominates and
> the file listing operation can become significantly slower than listing the
> files from the driver.
> I propose that InMemoryFileIndex.listLeafFiles should launch at most one
> spark job for listing leaf files.
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