code: def make_db_data_fetcher(postgresql_conn_info, template_path, reload_templates,
                           query_cfg, io_pool):
      
  
      sources = parse_source_data(query_cfg)
      queries_generator = make_queries_generator(
          sources, template_path, reload_templates)
      return DataFetcher(
          postgresql_conn_info, queries_generator, io_pool)
docs: Returns an object which is callable with the zoom and unpadded bounds and
      which returns a list of rows.