code: func (s *Schema) ColumnsIndices(cols []*Column) (ret []int) {
  	ret = make([]int, 0, len(cols))
  	for _, col := range cols {
  		pos := s.ColumnIndex(col)
  		if pos != -1 {
  			ret = append(ret, pos)
  		} else {
  			return nil
  		}
  	}
  	return
  }
docs: // ColumnsIndices will return a slice which contains the position of each column in schema.
  // If there is one column that doesn't match, nil will be returned.