GitHub user yhuai opened a pull request:
https://github.com/apache/spark/pull/9828
[SQL] Restore the 1.5's behavior of planning a single distinct aggregation.
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
```
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)
```
to the following one (1.5 uses this)
```
AGG-2
AGG-1 (grouping on a)
Shuffle to a single reducer
Partial-AGG-1(grouping on a)
```
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.
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/yhuai/spark distinctRewriter
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/spark/pull/9828.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #9828
----
commit 19699f7829323b1e561e1a0ef671c7877128af92
Author: Yin Huai <[email protected]>
Date: 2015-11-19T01:35:11Z
Restore the 1.5's behavior of planning a single distinct aggregation.
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