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https://issues.apache.org/jira/browse/SPARK-11840?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Apache Spark reassigned SPARK-11840:
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Assignee: Apache Spark (was: Yin Huai)
> Restore the 1.5's behavior of planning a single distinct aggregation.
> ---------------------------------------------------------------------
>
> Key: SPARK-11840
> URL: https://issues.apache.org/jira/browse/SPARK-11840
> Project: Spark
> Issue Type: Sub-task
> Components: SQL
> Reporter: Yin Huai
> Assignee: Apache Spark
>
> The impact of this change is for a query that has a single distinct column
> and does not have any grouping expression like
> {{SELECT COUNT(DISTINCT a) FROM table}}
> The plan will be changed from
> {code}
> AGG-2 (count distinct)
> Shuffle to a single reducer
> Partial-AGG-2 (count distinct)
> AGG-1 (grouping on a)
> Shuffle by a
> Partial-AGG-1 (grouping on 1)
> {code}
> to the following one (1.5 uses this)
> {code}
> AGG-2
> AGG-1 (grouping on a)
> Shuffle to a single reducer
> Partial-AGG-1(grouping on a)
> {code}
> The first plan is more robust. However, to better benchmark the impact of
> this change, we should use 1.5's plan and use the conf of
> {{spark.sql.specializeSingleDistinctAggPlanning}} to control the plan.
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