code: protected void getTablesAndIndexesFromSubqueries(Map<String, StmtTargetTableScan> tablesRead,
              Collection<String> indexes) {
          for(AbstractExpression expr : findAllSubquerySubexpressions()) {
              assert(expr instanceof AbstractSubqueryExpression);
              AbstractSubqueryExpression subquery = (AbstractSubqueryExpression) expr;
              AbstractPlanNode subqueryNode = subquery.getSubqueryNode();
              assert(subqueryNode != null);
              subqueryNode.getTablesAndIndexes(tablesRead, indexes);
          }
      }
docs: Collect read tables read and index names used in the current node subquery expressions.
  
  @param tablesRead Set of table aliases read potentially added to at each recursive level.
  @param indexes Set of index names used in the plan tree
  Only the current node is of interest.