Patrick Woody created SPARK-22408:
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Summary: RelationalGroupedDataset's distinct pivot value
calculation can launch many stages
Key: SPARK-22408
URL: https://issues.apache.org/jira/browse/SPARK-22408
Project: Spark
Issue Type: Improvement
Components: SQL
Affects Versions: 2.2.0
Reporter: Patrick Woody
When calculating the distinct values for a pivot in RelationalGroupedDataset
(https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/RelationalGroupedDataset.scala#L322),
we sort before doing a take(maxValues + 1).
We should be able to improve this by adding a global limit before the sort,
which should reduce the work of the sort, and by simply doing a collect to
avoid multiple launching multiple stages as a part of the take.
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