def _derive_distinct_intervals(self, rows):
ret = set()<EOL>for row in rows:<EOL><INDENT>self._add_interval(ret, (row[self._key_start_date], row[self._key_end_date]))<EOL><DEDENT>return ret<EOL>
Returns the set of distinct intervals in a row set.

:param list[dict[str,T]] rows: The rows set.

:rtype: set[(int,int)]