Scan records starting with index i.

def _IndexedScan(self, i, max_records=None):
    """Scan records starting with index i."""
    self._ReadIndex()

    # The record number that we will read next.
    idx = 0
    # The timestamp that we will start reading from.
    start_ts = 0
    if i >= self._max_indexed:
      start_ts = max((0, 0), (self._index[self._max_indexed][0],
                              self._index[self._max_indexed][1] - 1))
      idx = self._max_indexed
    else:
      try:
        possible_idx = i - i % self.INDEX_SPACING
        start_ts = (max(0, self._index[possible_idx][0]),
                    self._index[possible_idx][1] - 1)
        idx = possible_idx
      except KeyError:
        pass

    if max_records is not None:
      max_records += i - idx

    with data_store.DB.GetMutationPool() as mutation_pool:
      for (ts, value) in self.Scan(
          after_timestamp=start_ts,
          max_records=max_records,
          include_suffix=True):
        self._MaybeWriteIndex(idx, ts, mutation_pool)
        if idx >= i:
          yield (idx, ts, value)
        idx += 1