[jira] [Created] (SPARK-16208) Add `CollapseEmptyPlan` optimizer

["Dongjoon Hyun (JIRA)" <ji...@apache.org>]

Dongjoon Hyun created SPARK-16208:
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             Summary: Add `CollapseEmptyPlan` optimizer
                 Key: SPARK-16208
                 URL: https://issues.apache.org/jira/browse/SPARK-16208
             Project: Spark
          Issue Type: Improvement
          Components: SQL
            Reporter: Dongjoon Hyun
            Priority: Minor


This issue adds a new logical optimizer, `CollapseEmptyPlan`, to collapse a logical plain consisting of only empty LocalRelations.

**Before**
{code}
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 <empty>
{code}

**After**
{code}
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 <empty>, [a#15]
{code}



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