code: func (e *MergeJoinExec) fetchNextInnerRows() (err error) {
  	e.innerRows, err = e.innerTable.rowsWithSameKey()
  	if err != nil {
  		return err
  	}
  	e.innerIter4Row = chunk.NewIterator4Slice(e.innerRows)
  	e.innerIter4Row.Begin()
  	return nil
  }
docs: // fetchNextInnerRows fetches the next join group, within which all the rows
  // have the same join key, from the inner table.