Github user dilipbiswal commented on a diff in the pull request:

    https://github.com/apache/spark/pull/17491#discussion_r109191711
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/subquery.scala
 ---
    @@ -498,3 +498,31 @@ object RewriteCorrelatedScalarSubquery extends 
Rule[LogicalPlan] {
           }
       }
     }
    +
    +/**
    + * This rule rewrites a EXISTS predicate sub-queries into an Aggregate 
with count.
    + * So it doesn't be converted to a JOIN later.
    + */
    +object RewriteEmptyExists extends Rule[LogicalPlan] with PredicateHelper {
    +  private def containsAgg(plan: LogicalPlan): Boolean = {
    +    plan.collect {
    +      case a: Aggregate => a
    +    }.nonEmpty
    +  }
    +
    +  def apply(plan: LogicalPlan): LogicalPlan = plan transform {
    +    case Filter(condition, child) =>
    +      val (withSubquery, withoutSubquery) =
    +        
splitConjunctivePredicates(condition).partition(SubqueryExpression.hasInOrExistsSubquery)
    +      val newWithSubquery = withSubquery.map(_.transform {
    +        case e @ Exists(sub, conditions, exprId) if conditions.isEmpty && 
!containsAgg(sub) =>
    --- End diff --
    
    @viirya @nsyca I am still looking at the code. If we do go the route to 
turn this into a scalar subquery, would a "limit" help prevent a scan of the 
inner table (assuming its very large). So something like "select Count(1) from 
(original subquery + limit 1)" ?


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