code: func (ev *evaluator) vectorSelector(node *VectorSelector, ts int64) Vector {
  	checkForSeriesSetExpansion(ev.ctx, node)
  
  	var (
  		vec = make(Vector, 0, len(node.series))
  	)
  
  	it := storage.NewBuffer(durationMilliseconds(LookbackDelta))
  	for i, s := range node.series {
  		it.Reset(s.Iterator())
  
  		t, v, ok := ev.vectorSelectorSingle(it, node, ts)
  		if ok {
  			vec = append(vec, Sample{
  				Metric: node.series[i].Labels(),
  				Point:  Point{V: v, T: t},
  			})
  			ev.currentSamples++
  		}
  
  		if ev.currentSamples >= ev.maxSamples {
  			ev.error(ErrTooManySamples(env))
  		}
  	}
  	return vec
  }
docs: // vectorSelector evaluates a *VectorSelector expression.