presto-main/src/main/java/io/prestosql/sql/planner/iterative/rule/TransformExistsApplyToCorrelatedJoin.java/TransformExistsApplyToCorrelatedJoin/apply
class TransformExistsApplyToCorrelatedJoin:    @Override
    public Result apply(ApplyNode parent, Captures captures, Context context)
    {
        if (parent.getSubqueryAssignments().size() != 1) {
            return Result.empty();
        }

        Expression expression = getOnlyElement(parent.getSubqueryAssignments().getExpressions());
        if (!(expression instanceof ExistsPredicate)) {
            return Result.empty();
        }

        /*
        Empty correlation list indicates that the subquery contains no correlation symbols from the
        immediate outer scope. The subquery might be either not correlated at all, or correlated with
        symbols from further outer scope.
        Currently, the two cases are indistinguishable.
        To support the latter case, the ApplyNode with empty correlation list is rewritten to default
        aggregation, which is inefficient in the rare case of uncorrelated EXISTS subquery,
        but currently allows to successfully decorrelate a correlated EXISTS subquery.

        TODO: remove this condition when exploratory optimizer is implemented or support for decorrelating joins is implemented in PlanNodeDecorrelator
        */
        if (parent.getCorrelation().isEmpty()) {
            return Result.ofPlanNode(rewriteToDefaultAggregation(parent, context));
        }

        Optional<PlanNode> nonDefaultAggregation = rewriteToNonDefaultAggregation(parent, context);
        return nonDefaultAggregation
                .map(Result::ofPlanNode)
                .orElseGet(() -> Result.ofPlanNode(rewriteToDefaultAggregation(parent, context)));
    }