code: func (e *indexScanExec) getRowFromPoint(ran kv.KeyRange) ([][]byte, error) {
  	val, err := e.mvccStore.Get(ran.StartKey, e.startTS, e.isolationLevel)
  	if err != nil {
  		return nil, errors.Trace(err)
  	}
  	if len(val) == 0 {
  		return nil, nil
  	}
  	return e.decodeIndexKV(Pair{Key: ran.StartKey, Value: val})
  }
docs: // getRowFromPoint is only used for unique key.