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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