code: func (r *Reader) PostingsRanges() (map[labels.Label]Range, error) {
  	m := map[labels.Label]Range{}
  
  	for k, e := range r.postings {
  		for v, start := range e {
  			d := encoding.NewDecbufAt(r.b, int(start), castagnoliTable)
  			if d.Err() != nil {
  				return nil, d.Err()
  			}
  			m[labels.Label{Name: k, Value: v}] = Range{
  				Start: int64(start) + 4,
  				End:   int64(start) + 4 + int64(d.Len()),
  			}
  		}
  	}
  	return m, nil
  }
docs: // PostingsRanges returns a new map of byte range in the underlying index file
  // for all postings lists.