def upsert(self, table, data_list, on_conflict_fields, on_conflict_action='update',
               update_fields=None, return_cols='id'):
        """
        Create a bulk upsert statement which is much faster (~6x in tests with 10k & 100k rows and n cols)
        for upserting data then executemany()

        TODO: Is there a limit of length the query can be? If so handle it.
        """
        data_list = copy.deepcopy(data_list)  # Create deepcopy so the original list does not get modified
        # Make sure that `data_list` is a list
        if not isinstance(data_list, list):
            data_list = [data_list]
        # Make sure data_list has content
        if len(data_list) == 0:
            # No need to continue
            return []
        # Data in the list must be dicts (just check the first one)
        if not isinstance(data_list[0], dict):
            logger.critical("Data must be a list of dicts")
            # TODO: raise some error here rather then returning None
            return None

        # Make sure on_conflict_fields is a list
        if not isinstance(on_conflict_fields, list):
            on_conflict_fields = [on_conflict_fields]
        # Make sure on_conflict_fields has data
        if len(on_conflict_fields) == 0 or on_conflict_fields[0] is None:
            # No need to continue
            logger.critical("Must pass in `on_conflict_fields` argument")
            # TODO: raise some error here rather then returning None
            return None

        # Make sure return_cols is a list
        if return_cols is None or len(return_cols) == 0 or return_cols[0] is None:
            return_cols = ''
        elif not isinstance(return_cols, list):
            return_cols = [return_cols]

        if len(return_cols) > 0:
            return_cols = 'RETURNING ' + ','.join(return_cols)

        # Make sure update_fields is a list/valid
        if on_conflict_action == 'update':
            if not isinstance(update_fields, list):
                update_fields = [update_fields]
            # If noting is passed in, set `update_fields` to all (data_list-on_conflict_fields)
            if len(update_fields) == 0 or update_fields[0] is None:
                update_fields = list(set(data_list[0].keys()) - set(on_conflict_fields))
                # If update_fields is empty here that could only mean that all fields are set as conflict_fields
                if len(update_fields) == 0:
                    logger.critical("Not all the fields can be `on_conflict_fields` when doing an update")
                    # TODO: raise some error here rather then returning None
                    return None

            # If everything is good to go with the update fields
            fields_update_tmp = []
            for key in data_list[0].keys():
                fields_update_tmp.append('"{0}"="excluded"."{0}"'.format(key))
            conflict_action_sql = 'UPDATE SET {update_fields}'\
                                  .format(update_fields=', '.join(fields_update_tmp))
        else:
            # Do nothing on conflict
            conflict_action_sql = 'NOTHING'

        try:
            with self.getcursor() as cur:
                query = """INSERT INTO {table} ({insert_fields})
                           VALUES {values}
                           ON CONFLICT ({on_conflict_fields}) DO
                           {conflict_action_sql}
                           {return_cols}
                        """.format(table=table,
                                   insert_fields='"{0}"'.format('","'.join(data_list[0].keys())),
                                   values=','.join(['%s'] * len(data_list)),
                                   on_conflict_fields=','.join(on_conflict_fields),
                                   conflict_action_sql=conflict_action_sql,
                                   return_cols=return_cols,
                                   )
                # Get all the values for each row and create a lists of lists
                values = []
                for row in [list(v.values()) for v in data_list]:
                    values.append(_check_values(row))

                query = cur.mogrify(query, values)

                cur.execute(query)

                try:
                    return cur.fetchall()
                except Exception:
                    return None

        except Exception as e:
            logger.exception("Error upserting data")
            logger.debug("Error upserting data: {data}".format(data=data_list))
            raise e.with_traceback(sys.exc_info()[2])