protected function createPreparedStatements()
  {
    if ($this->prepared_statements_created == false)
    {
      $this->preparedInsertStatement = $this->PDO->prepare("INSERT INTO document_infos (document_info) VALUES (?);");
      $this->preparedSelectStatement = $this->PDO->prepare("SELECT * FROM document_infos limit 1;");
      
      $this->prepared_statements_created = true;
    }
  }