diff --git a/rules/alerting.go b/rules/alerting.go
index f1961e42a..a779d6959 100644
--- a/rules/alerting.go
+++ b/rules/alerting.go
@@ -80,7 +80,7 @@ type Alert struct {
 }
 
 // sample returns a Sample suitable for recording the alert.
-func (a Alert) sample(timestamp clientmodel.Timestamp, value clientmodel.SampleValue) *clientmodel.Sample {
+func (a Alert) sample(timestamp clientmodel.Timestamp, value clientmodel.SampleValue) *ast.Sample {
 	recordedMetric := clientmodel.Metric{}
 	for label, value := range a.Labels {
 		recordedMetric[label] = value
@@ -90,8 +90,11 @@ func (a Alert) sample(timestamp clientmodel.Timestamp, value clientmodel.SampleV
 	recordedMetric[AlertNameLabel] = clientmodel.LabelValue(a.Name)
 	recordedMetric[AlertStateLabel] = clientmodel.LabelValue(a.State.String())
 
-	return &clientmodel.Sample{
-		Metric:    recordedMetric,
+	return &ast.Sample{
+		Metric: clientmodel.COWMetric{
+			Metric: recordedMetric,
+			Copied: true,
+		},
 		Value:     value,
 		Timestamp: timestamp,
 	}
@@ -145,18 +148,17 @@ func (rule *AlertingRule) Eval(timestamp clientmodel.Timestamp, storage local.St
 	// or update the expression value for existing elements.
 	resultFingerprints := utility.Set{}
 	for _, sample := range exprResult {
-		fp := new(clientmodel.Fingerprint)
-		fp.LoadFromMetric(sample.Metric)
-		resultFingerprints.Add(*fp)
+		fp := sample.Metric.Metric.Fingerprint()
+		resultFingerprints.Add(fp)
 
-		if alert, ok := rule.activeAlerts[*fp]; !ok {
+		if alert, ok := rule.activeAlerts[fp]; !ok {
 			labels := clientmodel.LabelSet{}
-			labels.MergeFromMetric(sample.Metric)
+			labels.MergeFromMetric(sample.Metric.Metric)
 			labels = labels.Merge(rule.Labels)
 			if _, ok := labels[clientmodel.MetricNameLabel]; ok {
 				delete(labels, clientmodel.MetricNameLabel)
 			}
-			rule.activeAlerts[*fp] = &Alert{
+			rule.activeAlerts[fp] = &Alert{
 				Name:        rule.name,
 				Labels:      labels,
 				State:       Pending,
diff --git a/rules/ast/ast.go b/rules/ast/ast.go
index 79cc768f2..b36385ab9 100644
--- a/rules/ast/ast.go
+++ b/rules/ast/ast.go
@@ -34,17 +34,30 @@ var stalenessDelta = flag.Duration("query.staleness-delta", 300*time.Second, "St
 // ----------------------------------------------------------------------------
 // Raw data value types.
 
+// SampleStream is a stream of Values belonging to an attached COWMetric.
+type SampleStream struct {
+	Metric clientmodel.COWMetric
+	Values metric.Values
+}
+
+// Sample is a single sample belonging to a COWMetric.
+type Sample struct {
+	Metric    clientmodel.COWMetric
+	Value     clientmodel.SampleValue
+	Timestamp clientmodel.Timestamp
+}
+
 // Vector is basically only an alias for clientmodel.Samples, but the
 // contract is that in a Vector, all Samples have the same timestamp.
-type Vector clientmodel.Samples
+type Vector []*Sample
 
-// Matrix is a slice of SampleSets that implements sort.Interface and
+// Matrix is a slice of SampleStreams that implements sort.Interface and
 // has a String method.
 // BUG(julius): Pointerize this.
-type Matrix []metric.SampleSet
+type Matrix []SampleStream
 
 type groupedAggregation struct {
-	labels     clientmodel.Metric
+	labels     clientmodel.COWMetric
 	value      clientmodel.SampleValue
 	groupCount int
 }
@@ -191,7 +204,7 @@ type (
 		labelMatchers metric.LabelMatchers
 		// The series iterators are populated at query analysis time.
 		iterators map[clientmodel.Fingerprint]local.SeriesIterator
-		metrics   map[clientmodel.Fingerprint]clientmodel.Metric
+		metrics   map[clientmodel.Fingerprint]clientmodel.COWMetric
 		// Fingerprints are populated from label matchers at query analysis time.
 		fingerprints clientmodel.Fingerprints
 	}
@@ -231,7 +244,7 @@ type (
 		labelMatchers metric.LabelMatchers
 		// The series iterators are populated at query analysis time.
 		iterators map[clientmodel.Fingerprint]local.SeriesIterator
-		metrics   map[clientmodel.Fingerprint]clientmodel.Metric
+		metrics   map[clientmodel.Fingerprint]clientmodel.COWMetric
 		// Fingerprints are populated from label matchers at query analysis time.
 		fingerprints clientmodel.Fingerprints
 		interval     time.Duration
@@ -400,44 +413,43 @@ func EvalVectorRange(node VectorNode, start clientmodel.Timestamp, end clientmod
 
 	// TODO implement watchdog timer for long-running queries.
 	evalTimer := queryStats.GetTimer(stats.InnerEvalTime).Start()
-	sampleSets := map[uint64]*metric.SampleSet{}
-	for t := start; t.Before(end); t = t.Add(interval) {
+	sampleStreams := map[uint64]*SampleStream{}
+	for t := start; !t.After(end); t = t.Add(interval) {
 		vector := node.Eval(t)
 		for _, sample := range vector {
 			samplePair := metric.SamplePair{
 				Value:     sample.Value,
 				Timestamp: sample.Timestamp,
 			}
-			groupingKey := labelsToKey(sample.Metric)
-			if sampleSets[groupingKey] == nil {
-				sampleSets[groupingKey] = &metric.SampleSet{
+			groupingKey := labelsToKey(sample.Metric.Metric)
+			if sampleStreams[groupingKey] == nil {
+				sampleStreams[groupingKey] = &SampleStream{
 					Metric: sample.Metric,
 					Values: metric.Values{samplePair},
 				}
 			} else {
-				sampleSets[groupingKey].Values = append(sampleSets[groupingKey].Values, samplePair)
+				sampleStreams[groupingKey].Values = append(sampleStreams[groupingKey].Values, samplePair)
 			}
 		}
 	}
 	evalTimer.Stop()
 
 	appendTimer := queryStats.GetTimer(stats.ResultAppendTime).Start()
-	for _, sampleSet := range sampleSets {
-		matrix = append(matrix, *sampleSet)
+	for _, sampleStream := range sampleStreams {
+		matrix = append(matrix, *sampleStream)
 	}
 	appendTimer.Stop()
 
 	return matrix, nil
 }
 
-func labelIntersection(metric1, metric2 clientmodel.Metric) clientmodel.Metric {
-	intersection := clientmodel.Metric{}
-	for label, value := range metric1 {
-		if metric2[label] == value {
-			intersection[label] = value
+func labelIntersection(metric1, metric2 clientmodel.COWMetric) clientmodel.COWMetric {
+	for label, value := range metric1.Metric {
+		if metric2.Metric[label] != value {
+			metric1.Delete(label)
 		}
 	}
-	return intersection
+	return metric1
 }
 
 func (node *VectorAggregation) groupedAggregationsToVector(aggregations map[uint64]*groupedAggregation, timestamp clientmodel.Timestamp) Vector {
@@ -451,7 +463,7 @@ func (node *VectorAggregation) groupedAggregationsToVector(aggregations map[uint
 		default:
 			// For other aggregations, we already have the right value.
 		}
-		sample := &clientmodel.Sample{
+		sample := &Sample{
 			Metric:    aggregation.labels,
 			Value:     aggregation.value,
 			Timestamp: timestamp,
@@ -467,7 +479,7 @@ func (node *VectorAggregation) Eval(timestamp clientmodel.Timestamp) Vector {
 	vector := node.vector.Eval(timestamp)
 	result := map[uint64]*groupedAggregation{}
 	for _, sample := range vector {
-		groupingKey := node.labelsToGroupingKey(sample.Metric)
+		groupingKey := node.labelsToGroupingKey(sample.Metric.Metric)
 		if groupedResult, ok := result[groupingKey]; ok {
 			if node.keepExtraLabels {
 				groupedResult.labels = labelIntersection(groupedResult.labels, sample.Metric)
@@ -493,14 +505,18 @@ func (node *VectorAggregation) Eval(timestamp clientmodel.Timestamp) Vector {
 				panic("Unknown aggregation type")
 			}
 		} else {
-			m := clientmodel.Metric{}
+			var m clientmodel.COWMetric
 			if node.keepExtraLabels {
 				m = sample.Metric
-				delete(m, clientmodel.MetricNameLabel)
+				m.Delete(clientmodel.MetricNameLabel)
 			} else {
+				m = clientmodel.COWMetric{
+					Metric: clientmodel.Metric{},
+					Copied: true,
+				}
 				for _, l := range node.groupBy {
-					if v, ok := sample.Metric[l]; ok {
-						m[l] = v
+					if v, ok := sample.Metric.Metric[l]; ok {
+						m.Set(l, v)
 					}
 				}
 			}
@@ -524,8 +540,8 @@ func (node *VectorSelector) Eval(timestamp clientmodel.Timestamp) Vector {
 		sampleCandidates := it.GetValueAtTime(timestamp)
 		samplePair := chooseClosestSample(sampleCandidates, timestamp)
 		if samplePair != nil {
-			samples = append(samples, &clientmodel.Sample{
-				Metric:    node.metrics[fp], // TODO: need copy here because downstream can modify!
+			samples = append(samples, &Sample{
+				Metric:    node.metrics[fp],
 				Value:     samplePair.Value,
 				Timestamp: timestamp,
 			})
@@ -737,7 +753,7 @@ func (node *VectorArithExpr) Eval(timestamp clientmodel.Timestamp) Vector {
 			if keep {
 				rhsSample.Value = value
 				if node.opType.shouldDropMetric() {
-					delete(rhsSample.Metric, clientmodel.MetricNameLabel)
+					rhsSample.Metric.Delete(clientmodel.MetricNameLabel)
 				}
 				result = append(result, rhsSample)
 			}
@@ -751,7 +767,7 @@ func (node *VectorArithExpr) Eval(timestamp clientmodel.Timestamp) Vector {
 			if keep {
 				lhsSample.Value = value
 				if node.opType.shouldDropMetric() {
-					delete(lhsSample.Metric, clientmodel.MetricNameLabel)
+					lhsSample.Metric.Delete(clientmodel.MetricNameLabel)
 				}
 				result = append(result, lhsSample)
 			}
@@ -762,12 +778,12 @@ func (node *VectorArithExpr) Eval(timestamp clientmodel.Timestamp) Vector {
 		rhs := node.rhs.(VectorNode).Eval(timestamp)
 		for _, lhsSample := range lhs {
 			for _, rhsSample := range rhs {
-				if labelsEqual(lhsSample.Metric, rhsSample.Metric) {
+				if labelsEqual(lhsSample.Metric.Metric, rhsSample.Metric.Metric) {
 					value, keep := evalVectorBinop(node.opType, lhsSample.Value, rhsSample.Value)
 					if keep {
 						lhsSample.Value = value
 						if node.opType.shouldDropMetric() {
-							delete(lhsSample.Metric, clientmodel.MetricNameLabel)
+							lhsSample.Metric.Delete(clientmodel.MetricNameLabel)
 						}
 						result = append(result, lhsSample)
 					}
@@ -788,21 +804,21 @@ func (node *MatrixSelector) Eval(timestamp clientmodel.Timestamp) Matrix {
 	}
 
 	//// timer := v.stats.GetTimer(stats.GetRangeValuesTime).Start()
-	sampleSets := []metric.SampleSet{}
+	sampleStreams := []SampleStream{}
 	for fp, it := range node.iterators {
 		samplePairs := it.GetRangeValues(*interval)
 		if len(samplePairs) == 0 {
 			continue
 		}
 
-		sampleSet := metric.SampleSet{
-			Metric: node.metrics[fp], // TODO: need copy here because downstream can modify!
+		sampleStream := SampleStream{
+			Metric: node.metrics[fp],
 			Values: samplePairs,
 		}
-		sampleSets = append(sampleSets, sampleSet)
+		sampleStreams = append(sampleStreams, sampleStream)
 	}
 	//// timer.Stop()
-	return sampleSets
+	return sampleStreams
 }
 
 // EvalBoundaries implements the MatrixNode interface and returns the
@@ -814,21 +830,21 @@ func (node *MatrixSelector) EvalBoundaries(timestamp clientmodel.Timestamp) Matr
 	}
 
 	//// timer := v.stats.GetTimer(stats.GetBoundaryValuesTime).Start()
-	sampleSets := []metric.SampleSet{}
+	sampleStreams := []SampleStream{}
 	for fp, it := range node.iterators {
 		samplePairs := it.GetBoundaryValues(*interval)
 		if len(samplePairs) == 0 {
 			continue
 		}
 
-		sampleSet := metric.SampleSet{
-			Metric: node.metrics[fp], // TODO: make copy of metric.
+		sampleStream := SampleStream{
+			Metric: node.metrics[fp],
 			Values: samplePairs,
 		}
-		sampleSets = append(sampleSets, sampleSet)
+		sampleStreams = append(sampleStreams, sampleStream)
 	}
 	//// timer.Stop()
-	return sampleSets
+	return sampleStreams
 }
 
 // Len implements sort.Interface.
@@ -874,7 +890,7 @@ func NewVectorSelector(m metric.LabelMatchers) *VectorSelector {
 	return &VectorSelector{
 		labelMatchers: m,
 		iterators:     map[clientmodel.Fingerprint]local.SeriesIterator{},
-		metrics:       map[clientmodel.Fingerprint]clientmodel.Metric{},
+		metrics:       map[clientmodel.Fingerprint]clientmodel.COWMetric{},
 	}
 }
 
@@ -967,7 +983,7 @@ func NewMatrixSelector(vector *VectorSelector, interval time.Duration) *MatrixSe
 		labelMatchers: vector.labelMatchers,
 		interval:      interval,
 		iterators:     map[clientmodel.Fingerprint]local.SeriesIterator{},
-		metrics:       map[clientmodel.Fingerprint]clientmodel.Metric{},
+		metrics:       map[clientmodel.Fingerprint]clientmodel.COWMetric{},
 	}
 }
 
diff --git a/rules/ast/functions.go b/rules/ast/functions.go
index 328d6b83d..3604cc87d 100644
--- a/rules/ast/functions.go
+++ b/rules/ast/functions.go
@@ -128,12 +128,12 @@ func deltaImpl(timestamp clientmodel.Timestamp, args []Node) interface{} {
 		intervalCorrection := clientmodel.SampleValue(targetInterval) / clientmodel.SampleValue(sampledInterval)
 		resultValue *= intervalCorrection
 
-		resultSample := &clientmodel.Sample{
+		resultSample := &Sample{
 			Metric:    samples.Metric,
 			Value:     resultValue,
 			Timestamp: timestamp,
 		}
-		delete(resultSample.Metric, clientmodel.MetricNameLabel)
+		resultSample.Metric.Delete(clientmodel.MetricNameLabel)
 		resultVector = append(resultVector, resultSample)
 	}
 	return resultVector
@@ -169,7 +169,7 @@ func (s vectorByValueHeap) Swap(i, j int) {
 }
 
 func (s *vectorByValueHeap) Push(x interface{}) {
-	*s = append(*s, x.(*clientmodel.Sample))
+	*s = append(*s, x.(*Sample))
 }
 
 func (s *vectorByValueHeap) Pop() interface{} {
@@ -254,7 +254,7 @@ func dropCommonLabelsImpl(timestamp clientmodel.Timestamp, args []Node) interfac
 		return Vector{}
 	}
 	common := clientmodel.LabelSet{}
-	for k, v := range vector[0].Metric {
+	for k, v := range vector[0].Metric.Metric {
 		// TODO(julius): Should we also drop common metric names?
 		if k == clientmodel.MetricNameLabel {
 			continue
@@ -264,7 +264,7 @@ func dropCommonLabelsImpl(timestamp clientmodel.Timestamp, args []Node) interfac
 
 	for _, el := range vector[1:] {
 		for k, v := range common {
-			if el.Metric[k] != v {
+			if el.Metric.Metric[k] != v {
 				// Deletion of map entries while iterating over them is safe.
 				// From http://golang.org/ref/spec#For_statements:
 				// "If map entries that have not yet been reached are deleted during
@@ -275,88 +275,15 @@ func dropCommonLabelsImpl(timestamp clientmodel.Timestamp, args []Node) interfac
 	}
 
 	for _, el := range vector {
-		for k := range el.Metric {
+		for k := range el.Metric.Metric {
 			if _, ok := common[k]; ok {
-				delete(el.Metric, k)
+				el.Metric.Delete(k)
 			}
 		}
 	}
 	return vector
 }
 
-// === sampleVectorImpl() Vector ===
-func sampleVectorImpl(timestamp clientmodel.Timestamp, args []Node) interface{} {
-	return Vector{
-		&clientmodel.Sample{
-			Metric: clientmodel.Metric{
-				clientmodel.MetricNameLabel: "http_requests",
-				clientmodel.JobLabel:        "api-server",
-				"instance":                  "0",
-			},
-			Value:     10,
-			Timestamp: timestamp,
-		},
-		&clientmodel.Sample{
-			Metric: clientmodel.Metric{
-				clientmodel.MetricNameLabel: "http_requests",
-				clientmodel.JobLabel:        "api-server",
-				"instance":                  "1",
-			},
-			Value:     20,
-			Timestamp: timestamp,
-		},
-		&clientmodel.Sample{
-			Metric: clientmodel.Metric{
-				clientmodel.MetricNameLabel: "http_requests",
-				clientmodel.JobLabel:        "api-server",
-				"instance":                  "2",
-			},
-			Value:     30,
-			Timestamp: timestamp,
-		},
-		&clientmodel.Sample{
-			Metric: clientmodel.Metric{
-				clientmodel.MetricNameLabel: "http_requests",
-				clientmodel.JobLabel:        "api-server",
-				"instance":                  "3",
-				"group":                     "canary",
-			},
-			Value:     40,
-			Timestamp: timestamp,
-		},
-		&clientmodel.Sample{
-			Metric: clientmodel.Metric{
-				clientmodel.MetricNameLabel: "http_requests",
-				clientmodel.JobLabel:        "api-server",
-				"instance":                  "2",
-				"group":                     "canary",
-			},
-			Value:     40,
-			Timestamp: timestamp,
-		},
-		&clientmodel.Sample{
-			Metric: clientmodel.Metric{
-				clientmodel.MetricNameLabel: "http_requests",
-				clientmodel.JobLabel:        "api-server",
-				"instance":                  "3",
-				"group":                     "mytest",
-			},
-			Value:     40,
-			Timestamp: timestamp,
-		},
-		&clientmodel.Sample{
-			Metric: clientmodel.Metric{
-				clientmodel.MetricNameLabel: "http_requests",
-				clientmodel.JobLabel:        "api-server",
-				"instance":                  "3",
-				"group":                     "mytest",
-			},
-			Value:     40,
-			Timestamp: timestamp,
-		},
-	}
-}
-
 // === scalar(node VectorNode) Scalar ===
 func scalarImpl(timestamp clientmodel.Timestamp, args []Node) interface{} {
 	v := args[0].(VectorNode).Eval(timestamp)
@@ -381,8 +308,8 @@ func aggrOverTime(timestamp clientmodel.Timestamp, args []Node, aggrFn func(metr
 			continue
 		}
 
-		delete(el.Metric, clientmodel.MetricNameLabel)
-		resultVector = append(resultVector, &clientmodel.Sample{
+		el.Metric.Delete(clientmodel.MetricNameLabel)
+		resultVector = append(resultVector, &Sample{
 			Metric:    el.Metric,
 			Value:     aggrFn(el.Values),
 			Timestamp: timestamp,
@@ -447,7 +374,7 @@ func absImpl(timestamp clientmodel.Timestamp, args []Node) interface{} {
 	n := args[0].(VectorNode)
 	vector := n.Eval(timestamp)
 	for _, el := range vector {
-		delete(el.Metric, clientmodel.MetricNameLabel)
+		el.Metric.Delete(clientmodel.MetricNameLabel)
 		el.Value = clientmodel.SampleValue(math.Abs(float64(el.Value)))
 	}
 	return vector
@@ -468,8 +395,11 @@ func absentImpl(timestamp clientmodel.Timestamp, args []Node) interface{} {
 		}
 	}
 	return Vector{
-		&clientmodel.Sample{
-			Metric:    m,
+		&Sample{
+			Metric: clientmodel.COWMetric{
+				Metric: m,
+				Copied: true,
+			},
 			Value:     1,
 			Timestamp: timestamp,
 		},
@@ -543,12 +473,6 @@ var functions = map[string]*Function{
 		returnType: VECTOR,
 		callFn:     rateImpl,
 	},
-	"sampleVector": {
-		name:       "sampleVector",
-		argTypes:   []ExprType{},
-		returnType: VECTOR,
-		callFn:     sampleVectorImpl,
-	},
 	"scalar": {
 		name:       "scalar",
 		argTypes:   []ExprType{VECTOR},
diff --git a/rules/ast/functions_test.go b/rules/ast/functions_test.go
index fa4d1da71..86add4eae 100644
--- a/rules/ast/functions_test.go
+++ b/rules/ast/functions_test.go
@@ -30,8 +30,10 @@ func (node emptyRangeNode) Children() Nodes            { return Nodes{} }
 
 func (node emptyRangeNode) Eval(timestamp clientmodel.Timestamp) Matrix {
 	return Matrix{
-		metric.SampleSet{
-			Metric: clientmodel.Metric{clientmodel.MetricNameLabel: "empty_metric"},
+		SampleStream{
+			Metric: clientmodel.COWMetric{
+				Metric: clientmodel.Metric{clientmodel.MetricNameLabel: "empty_metric"},
+			},
 			Values: metric.Values{},
 		},
 	}
@@ -39,8 +41,10 @@ func (node emptyRangeNode) Eval(timestamp clientmodel.Timestamp) Matrix {
 
 func (node emptyRangeNode) EvalBoundaries(timestamp clientmodel.Timestamp) Matrix {
 	return Matrix{
-		metric.SampleSet{
-			Metric: clientmodel.Metric{clientmodel.MetricNameLabel: "empty_metric"},
+		SampleStream{
+			Metric: clientmodel.COWMetric{
+				Metric: clientmodel.Metric{clientmodel.MetricNameLabel: "empty_metric"},
+			},
 			Values: metric.Values{},
 		},
 	}
diff --git a/rules/ast/printer.go b/rules/ast/printer.go
index 04068e8a9..0155a5838 100644
--- a/rules/ast/printer.go
+++ b/rules/ast/printer.go
@@ -90,20 +90,21 @@ func (vector Vector) String() string {
 
 func (matrix Matrix) String() string {
 	metricStrings := make([]string, 0, len(matrix))
-	for _, sampleSet := range matrix {
-		metricName, ok := sampleSet.Metric[clientmodel.MetricNameLabel]
-		if !ok {
-			panic("Tried to print matrix without metric name")
+	for _, sampleStream := range matrix {
+		metricName, hasName := sampleStream.Metric.Metric[clientmodel.MetricNameLabel]
+		numLabels := len(sampleStream.Metric.Metric)
+		if hasName {
+			numLabels--
 		}
-		labelStrings := make([]string, 0, len(sampleSet.Metric)-1)
-		for label, value := range sampleSet.Metric {
+		labelStrings := make([]string, 0, numLabels)
+		for label, value := range sampleStream.Metric.Metric {
 			if label != clientmodel.MetricNameLabel {
 				labelStrings = append(labelStrings, fmt.Sprintf("%s=%q", label, value))
 			}
 		}
 		sort.Strings(labelStrings)
-		valueStrings := make([]string, 0, len(sampleSet.Values))
-		for _, value := range sampleSet.Values {
+		valueStrings := make([]string, 0, len(sampleStream.Values))
+		for _, value := range sampleStream.Values {
 			valueStrings = append(valueStrings,
 				fmt.Sprintf("\n%v @[%v]", value.Value, value.Timestamp))
 		}
@@ -218,7 +219,7 @@ func EvalToVector(node Node, timestamp clientmodel.Timestamp, storage local.Stor
 	case SCALAR:
 		scalar := node.(ScalarNode).Eval(timestamp)
 		evalTimer.Stop()
-		return Vector{&clientmodel.Sample{Value: scalar}}, nil
+		return Vector{&Sample{Value: scalar}}, nil
 	case VECTOR:
 		vector := node.(VectorNode).Eval(timestamp)
 		evalTimer.Stop()
@@ -228,8 +229,16 @@ func EvalToVector(node Node, timestamp clientmodel.Timestamp, storage local.Stor
 	case STRING:
 		str := node.(StringNode).Eval(timestamp)
 		evalTimer.Stop()
-		return Vector{&clientmodel.Sample{
-			Metric: clientmodel.Metric{"__value__": clientmodel.LabelValue(str)}}}, nil
+		return Vector{
+			&Sample{
+				Metric: clientmodel.COWMetric{
+					Metric: clientmodel.Metric{
+						"__value__": clientmodel.LabelValue(str),
+					},
+					Copied: true,
+				},
+			},
+		}, nil
 	}
 	panic("Switch didn't cover all node types")
 }
diff --git a/rules/helpers_test.go b/rules/helpers_test.go
index aaf249056..2abf5adf6 100644
--- a/rules/helpers_test.go
+++ b/rules/helpers_test.go
@@ -41,10 +41,10 @@ func getTestValueStream(startVal clientmodel.SampleValue, endVal clientmodel.Sam
 
 func getTestVectorFromTestMatrix(matrix ast.Matrix) ast.Vector {
 	vector := ast.Vector{}
-	for _, sampleSet := range matrix {
-		lastSample := sampleSet.Values[len(sampleSet.Values)-1]
-		vector = append(vector, &clientmodel.Sample{
-			Metric:    sampleSet.Metric,
+	for _, sampleStream := range matrix {
+		lastSample := sampleStream.Values[len(sampleStream.Values)-1]
+		vector = append(vector, &ast.Sample{
+			Metric:    sampleStream.Metric,
 			Value:     lastSample.Value,
 			Timestamp: lastSample.Timestamp,
 		})
@@ -54,10 +54,10 @@ func getTestVectorFromTestMatrix(matrix ast.Matrix) ast.Vector {
 
 func storeMatrix(storage local.Storage, matrix ast.Matrix) {
 	pendingSamples := clientmodel.Samples{}
-	for _, sampleSet := range matrix {
-		for _, sample := range sampleSet.Values {
+	for _, sampleStream := range matrix {
+		for _, sample := range sampleStream.Values {
 			pendingSamples = append(pendingSamples, &clientmodel.Sample{
-				Metric:    sampleSet.Metric,
+				Metric:    sampleStream.Metric.Metric,
 				Value:     sample.Value,
 				Timestamp: sample.Timestamp,
 			})
@@ -69,96 +69,118 @@ func storeMatrix(storage local.Storage, matrix ast.Matrix) {
 
 var testMatrix = ast.Matrix{
 	{
-		Metric: clientmodel.Metric{
-			clientmodel.MetricNameLabel: "http_requests",
-			clientmodel.JobLabel:        "api-server",
-			"instance":                  "0",
-			"group":                     "production",
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "http_requests",
+				clientmodel.JobLabel:        "api-server",
+				"instance":                  "0",
+				"group":                     "production",
+			},
 		},
 		Values: getTestValueStream(0, 100, 10, testStartTime),
 	},
 	{
-		Metric: clientmodel.Metric{
-			clientmodel.MetricNameLabel: "http_requests",
-			clientmodel.JobLabel:        "api-server",
-			"instance":                  "1",
-			"group":                     "production",
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "http_requests",
+				clientmodel.JobLabel:        "api-server",
+				"instance":                  "1",
+				"group":                     "production",
+			},
 		},
 		Values: getTestValueStream(0, 200, 20, testStartTime),
 	},
 	{
-		Metric: clientmodel.Metric{
-			clientmodel.MetricNameLabel: "http_requests",
-			clientmodel.JobLabel:        "api-server",
-			"instance":                  "0",
-			"group":                     "canary",
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "http_requests",
+				clientmodel.JobLabel:        "api-server",
+				"instance":                  "0",
+				"group":                     "canary",
+			},
 		},
 		Values: getTestValueStream(0, 300, 30, testStartTime),
 	},
 	{
-		Metric: clientmodel.Metric{
-			clientmodel.MetricNameLabel: "http_requests",
-			clientmodel.JobLabel:        "api-server",
-			"instance":                  "1",
-			"group":                     "canary",
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "http_requests",
+				clientmodel.JobLabel:        "api-server",
+				"instance":                  "1",
+				"group":                     "canary",
+			},
 		},
 		Values: getTestValueStream(0, 400, 40, testStartTime),
 	},
 	{
-		Metric: clientmodel.Metric{
-			clientmodel.MetricNameLabel: "http_requests",
-			clientmodel.JobLabel:        "app-server",
-			"instance":                  "0",
-			"group":                     "production",
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "http_requests",
+				clientmodel.JobLabel:        "app-server",
+				"instance":                  "0",
+				"group":                     "production",
+			},
 		},
 		Values: getTestValueStream(0, 500, 50, testStartTime),
 	},
 	{
-		Metric: clientmodel.Metric{
-			clientmodel.MetricNameLabel: "http_requests",
-			clientmodel.JobLabel:        "app-server",
-			"instance":                  "1",
-			"group":                     "production",
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "http_requests",
+				clientmodel.JobLabel:        "app-server",
+				"instance":                  "1",
+				"group":                     "production",
+			},
 		},
 		Values: getTestValueStream(0, 600, 60, testStartTime),
 	},
 	{
-		Metric: clientmodel.Metric{
-			clientmodel.MetricNameLabel: "http_requests",
-			clientmodel.JobLabel:        "app-server",
-			"instance":                  "0",
-			"group":                     "canary",
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "http_requests",
+				clientmodel.JobLabel:        "app-server",
+				"instance":                  "0",
+				"group":                     "canary",
+			},
 		},
 		Values: getTestValueStream(0, 700, 70, testStartTime),
 	},
 	{
-		Metric: clientmodel.Metric{
-			clientmodel.MetricNameLabel: "http_requests",
-			clientmodel.JobLabel:        "app-server",
-			"instance":                  "1",
-			"group":                     "canary",
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "http_requests",
+				clientmodel.JobLabel:        "app-server",
+				"instance":                  "1",
+				"group":                     "canary",
+			},
 		},
 		Values: getTestValueStream(0, 800, 80, testStartTime),
 	},
 	// Single-letter metric and label names.
 	{
-		Metric: clientmodel.Metric{
-			clientmodel.MetricNameLabel: "x",
-			"y": "testvalue",
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "x",
+				"y": "testvalue",
+			},
 		},
 		Values: getTestValueStream(0, 100, 10, testStartTime),
 	},
 	// Counter reset in the middle of range.
 	{
-		Metric: clientmodel.Metric{
-			clientmodel.MetricNameLabel: "testcounter_reset_middle",
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "testcounter_reset_middle",
+			},
 		},
 		Values: append(getTestValueStream(0, 40, 10, testStartTime), getTestValueStream(0, 50, 10, testStartTime.Add(testSampleInterval*5))...),
 	},
 	// Counter reset at the end of range.
 	{
-		Metric: clientmodel.Metric{
-			clientmodel.MetricNameLabel: "testcounter_reset_end",
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "testcounter_reset_end",
+			},
 		},
 		Values: append(getTestValueStream(0, 90, 10, testStartTime), getTestValueStream(0, 0, 10, testStartTime.Add(testSampleInterval*10))...),
 	},
diff --git a/rules/manager/manager.go b/rules/manager/manager.go
index 4e1d6957c..0c3324a65 100644
--- a/rules/manager/manager.go
+++ b/rules/manager/manager.go
@@ -222,7 +222,13 @@ func (m *ruleManager) runIteration(results chan<- *extraction.Result) {
 			duration := time.Since(start)
 
 			samples := make(clientmodel.Samples, len(vector))
-			copy(samples, vector)
+			for i, s := range vector {
+				samples[i] = &clientmodel.Sample{
+					Metric:    s.Metric.Metric,
+					Value:     s.Value,
+					Timestamp: s.Timestamp,
+				}
+			}
 			m.results <- &extraction.Result{
 				Samples: samples,
 				Err:     err,
diff --git a/rules/recording.go b/rules/recording.go
index 59e869f7e..3f39ecdb9 100644
--- a/rules/recording.go
+++ b/rules/recording.go
@@ -50,12 +50,12 @@ func (rule RecordingRule) Eval(timestamp clientmodel.Timestamp, storage local.St
 
 	// Override the metric name and labels.
 	for _, sample := range vector {
-		sample.Metric[clientmodel.MetricNameLabel] = clientmodel.LabelValue(rule.name)
+		sample.Metric.Set(clientmodel.MetricNameLabel, clientmodel.LabelValue(rule.name))
 		for label, value := range rule.labels {
 			if value == "" {
-				delete(sample.Metric, label)
+				sample.Metric.Delete(label)
 			} else {
-				sample.Metric[label] = value
+				sample.Metric.Set(label, value)
 			}
 		}
 	}
diff --git a/rules/rules_test.go b/rules/rules_test.go
index 094965255..028da4006 100644
--- a/rules/rules_test.go
+++ b/rules/rules_test.go
@@ -25,6 +25,7 @@ import (
 	"github.com/prometheus/prometheus/rules/ast"
 	"github.com/prometheus/prometheus/stats"
 	"github.com/prometheus/prometheus/storage/local"
+	"github.com/prometheus/prometheus/storage/metric"
 	"github.com/prometheus/prometheus/utility/test"
 )
 
@@ -60,7 +61,7 @@ func newTestStorage(t testing.TB) (storage local.Storage, closer test.Closer) {
 
 func TestExpressions(t *testing.T) {
 	// Labels in expected output need to be alphabetically sorted.
-	var expressionTests = []struct {
+	expressionTests := []struct {
 		expr           string
 		output         []string
 		shouldFail     bool
@@ -705,6 +706,188 @@ func TestExpressions(t *testing.T) {
 	}
 }
 
+func TestRangedEvaluationRegressions(t *testing.T) {
+	scenarios := []struct {
+		in   ast.Matrix
+		out  ast.Matrix
+		expr string
+	}{
+		{
+			// Testing COWMetric behavior in drop_common_labels.
+			in: ast.Matrix{
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{
+							clientmodel.MetricNameLabel: "testmetric",
+							"testlabel":                 "1",
+						},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime,
+							Value:     1,
+						},
+						{
+							Timestamp: testStartTime.Add(time.Hour),
+							Value:     1,
+						},
+					},
+				},
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{
+							clientmodel.MetricNameLabel: "testmetric",
+							"testlabel":                 "2",
+						},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime.Add(time.Hour),
+							Value:     2,
+						},
+					},
+				},
+			},
+			out: ast.Matrix{
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{
+							clientmodel.MetricNameLabel: "testmetric",
+						},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime,
+							Value:     1,
+						},
+					},
+				},
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{
+							clientmodel.MetricNameLabel: "testmetric",
+							"testlabel":                 "1",
+						},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime.Add(time.Hour),
+							Value:     1,
+						},
+					},
+				},
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{
+							clientmodel.MetricNameLabel: "testmetric",
+							"testlabel":                 "2",
+						},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime.Add(time.Hour),
+							Value:     2,
+						},
+					},
+				},
+			},
+			expr: "drop_common_labels(testmetric)",
+		},
+		{
+			// Testing COWMetric behavior in vector aggregation.
+			in: ast.Matrix{
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{
+							clientmodel.MetricNameLabel: "testmetric",
+							"testlabel":                 "1",
+						},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime,
+							Value:     1,
+						},
+						{
+							Timestamp: testStartTime.Add(time.Hour),
+							Value:     1,
+						},
+					},
+				},
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{
+							clientmodel.MetricNameLabel: "testmetric",
+							"testlabel":                 "2",
+						},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime,
+							Value:     2,
+						},
+					},
+				},
+			},
+			out: ast.Matrix{
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime,
+							Value:     3,
+						},
+					},
+				},
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{
+							"testlabel": "1",
+						},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime.Add(time.Hour),
+							Value:     1,
+						},
+					},
+				},
+			},
+			expr: "sum(testmetric) keeping_extra",
+		},
+	}
+
+	for i, s := range scenarios {
+		storage, closer := local.NewTestStorage(t)
+		storeMatrix(storage, s.in)
+
+		expr, err := LoadExprFromString(s.expr)
+		if err != nil {
+			t.Fatalf("%d. Error parsing expression: %v", i, err)
+		}
+
+		got, err := ast.EvalVectorRange(
+			expr.(ast.VectorNode),
+			testStartTime,
+			testStartTime.Add(time.Hour),
+			time.Hour,
+			storage,
+			stats.NewTimerGroup(),
+		)
+		if err != nil {
+			t.Fatalf("%d. Error evaluating expression: %v", i, err)
+		}
+
+		if got.String() != s.out.String() {
+			t.Fatalf("%d. Expression: %s\n\ngot:\n=====\n%v\n====\n\nwant:\n=====\n%v\n=====\n", i, s.expr, got.String(), s.out.String())
+		}
+
+		closer.Close()
+	}
+}
+
 var ruleTests = []struct {
 	inputFile         string
 	shouldFail        bool
diff --git a/storage/local/interface.go b/storage/local/interface.go
index 4b8b8e39e..733da1a76 100644
--- a/storage/local/interface.go
+++ b/storage/local/interface.go
@@ -14,9 +14,9 @@
 package local
 
 import (
-	"time"
 	clientmodel "github.com/prometheus/client_golang/model"
 	"github.com/prometheus/client_golang/prometheus"
+	"time"
 
 	"github.com/prometheus/prometheus/storage/metric"
 )
@@ -38,7 +38,7 @@ type Storage interface {
 	// Get all of the label values that are associated with a given label name.
 	GetLabelValuesForLabelName(clientmodel.LabelName) clientmodel.LabelValues
 	// Get the metric associated with the provided fingerprint.
-	GetMetricForFingerprint(clientmodel.Fingerprint) clientmodel.Metric
+	GetMetricForFingerprint(clientmodel.Fingerprint) clientmodel.COWMetric
 	// Construct an iterator for a given fingerprint.
 	NewIterator(clientmodel.Fingerprint) SeriesIterator
 	// Run the various maintenance loops in goroutines. Returns when the
diff --git a/storage/local/storage.go b/storage/local/storage.go
index 7544e0149..0618ba57d 100644
--- a/storage/local/storage.go
+++ b/storage/local/storage.go
@@ -323,25 +323,25 @@ func (s *memorySeriesStorage) GetLabelValuesForLabelName(labelName clientmodel.L
 }
 
 // GetMetricForFingerprint implements Storage.
-func (s *memorySeriesStorage) GetMetricForFingerprint(fp clientmodel.Fingerprint) clientmodel.Metric {
+func (s *memorySeriesStorage) GetMetricForFingerprint(fp clientmodel.Fingerprint) clientmodel.COWMetric {
 	s.fpLocker.Lock(fp)
 	defer s.fpLocker.Unlock(fp)
 
 	series, ok := s.fpToSeries.get(fp)
 	if ok {
-		// Copy required here because caller might mutate the returned
-		// metric.
-		m := make(clientmodel.Metric, len(series.metric))
-		for ln, lv := range series.metric {
-			m[ln] = lv
+		// Wrap the returned metric in a copy-on-write (COW) metric here because
+		// the caller might mutate it.
+		return clientmodel.COWMetric{
+			Metric: series.metric,
 		}
-		return m
 	}
 	metric, err := s.persistence.getArchivedMetric(fp)
 	if err != nil {
 		glog.Errorf("Error retrieving archived metric for fingerprint %v: %v", fp, err)
 	}
-	return metric
+	return clientmodel.COWMetric{
+		Metric: metric,
+	}
 }
 
 // AppendSamples implements Storage.
diff --git a/storage/metric/sample.go b/storage/metric/sample.go
index 2291fd743..532f0b900 100644
--- a/storage/metric/sample.go
+++ b/storage/metric/sample.go
@@ -47,12 +47,6 @@ func (s *SamplePair) String() string {
 // Values is a slice of SamplePairs.
 type Values []SamplePair
 
-// SampleSet is Values with a Metric attached.
-type SampleSet struct {
-	Metric clientmodel.Metric
-	Values Values
-}
-
 // Interval describes the inclusive interval between two Timestamps.
 type Interval struct {
 	OldestInclusive clientmodel.Timestamp
diff --git a/templates/templates.go b/templates/templates.go
index 03e94e4e4..9b0928b31 100644
--- a/templates/templates.go
+++ b/templates/templates.go
@@ -76,7 +76,7 @@ func query(q string, timestamp clientmodel.Timestamp, storage local.Storage) (qu
 			Value:  float64(v.Value),
 			Labels: make(map[string]string),
 		}
-		for label, value := range v.Metric {
+		for label, value := range v.Metric.Metric {
 			s.Labels[string(label)] = string(value)
 		}
 		result[n] = &s