GitHub user dongjoon-hyun opened a pull request:
https://github.com/apache/spark/pull/13905
[SPARK-16208][SQL] Add `CollapseEmptyPlan` optimizer
## What changes were proposed in this pull request?
This PR adds a new logical optimizer, `CollapseEmptyPlan`, to collapse a
logical plain consisting of only empty LocalRelations.
**Before**
```scala
scala> sql("select a from (select explode(array(1,2,3)) a) where 1=0 group
by a having count(*)>1 order by a").explain
== Physical Plan ==
*Sort [a#15 ASC], true, 0
+- Exchange rangepartitioning(a#15 ASC, 200)
+- *Project [a#15]
+- *Filter (count(1)#17L > 1)
+- *HashAggregate(keys=[a#15], functions=[count(1)])
+- Exchange hashpartitioning(a#15, 200)
+- *HashAggregate(keys=[a#15], functions=[partial_count(1)])
+- Generate explode([1,2,3]), false, false, [a#15]
+- LocalTableScan
```
**After**
```scala
scala> sql("select a from (select explode(array(1,2,3)) a) where 1=0 group
by a having count(*)>1 order by a").explain
== Physical Plan ==
LocalTableScan , [a#15]
```
## How was this patch tested?
Pass the Jenkins tests (including a new testsuite).
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/dongjoon-hyun/spark SPARK-16208
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/spark/pull/13905.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 #13905
commit db0aeb9fa9a38b1deb44399f434b1e549a79be6e
Author: Dongjoon Hyun
Date: 2016-06-25T09:18:44Z
[SPARK-16208][SQL] Add `CollapseEmptyPlan` optimizer
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