docs: def _execute_select_commands(self, source, commands):
          """Execute select queries for all of the tables from a source database."""
          rows = {}
          for tbl, command in tqdm(commands, total=len(commands), desc='Executing {0} select queries'.format(source)):
              # Add key to dictionary
              if tbl not in rows:
                  rows[tbl] = []
              rows[tbl].extend(self.fetch(command, commit=True))
          self._commit()
          return rows