using the transaction executor to get retry behavior, this method becomes:

    def __getitem__(self, key):
        try:
            record = self._execute_db_transaction(
                single_row_sql,
                """ -- didn't want to retype the SQL """,
                (key,)
            )
        except SQLDidNotReturnSingleRow:
            raise KeyError(key)
        columns = ( ... )  # I didn't want to type it all        
        row = dict(zip(columns, record)
        row['last_error'] = json.loads(row['last_error'])
        return row