* @return A complete plan tree for the entire SQl.
*/
private AbstractPlanNode connectChildrenBestPlans(AbstractPlanNode parentPlan) {
if (parentPlan instanceof AbstractScanPlanNode) {
AbstractScanPlanNode scanNode = (AbstractScanPlanNode) parentPlan;
StmtTableScan tableScan = scanNode.getTableScan();
if (tableScan instanceof StmtSubqueryScan) {
CompiledPlan betsCostPlan = ((StmtSubqueryScan)tableScan).getBestCostPlan();
assert (betsCostPlan != null);
AbstractPlanNode subQueryRoot = betsCostPlan.rootPlanGraph;
subQueryRoot.disconnectParents();