Yin Huai created SPARK-11840:
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Summary: 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: Yin Huai
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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