diff --git a/promql/engine.go b/promql/engine.go
index 35dc52942..140c0a0e4 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -1067,8 +1067,6 @@ func (ev *evaluator) Eval(expr parser.Expr) (v parser.Value, ws annotations.Anno
 
 // EvalSeriesHelper stores extra information about a series.
 type EvalSeriesHelper struct {
-	// The grouping key used by aggregation.
-	groupingKey uint64
 	// Used to map left-hand to right-hand in binary operations.
 	signature string
 }
@@ -1316,13 +1314,25 @@ func (ev *evaluator) rangeEvalAgg(aggExpr *parser.AggregateExpr, sortedGrouping
 	seriess := make(map[uint64]Series, biggestLen) // Output series by series hash.
 	tempNumSamples := ev.currentSamples
 
-	// Initialise series helpers with the grouping key.
+	// Create a mapping from input series to output groups.
 	buf := make([]byte, 0, 1024)
-
-	seriesHelper := make([]EvalSeriesHelper, len(inputMatrix))
+	groupToResultIndex := make(map[uint64]int)
+	seriesToResult := make([]int, len(inputMatrix))
+	orderedResult := make([]*groupedAggregation, 0, 16)
 
 	for si, series := range inputMatrix {
-		seriesHelper[si].groupingKey, buf = generateGroupingKey(series.Metric, sortedGrouping, aggExpr.Without, buf)
+		var groupingKey uint64
+		groupingKey, buf = generateGroupingKey(series.Metric, sortedGrouping, aggExpr.Without, buf)
+		index, ok := groupToResultIndex[groupingKey]
+		// Add a new group if it doesn't exist.
+		if !ok {
+			m := generateGroupingLabels(enh, series.Metric, aggExpr.Without, sortedGrouping)
+			newAgg := &groupedAggregation{labels: m}
+			index = len(orderedResult)
+			groupToResultIndex[groupingKey] = index
+			orderedResult = append(orderedResult, newAgg)
+		}
+		seriesToResult[si] = index
 	}
 
 	for ts := ev.startTimestamp; ts <= ev.endTimestamp; ts += ev.interval {
@@ -1334,7 +1344,7 @@ func (ev *evaluator) rangeEvalAgg(aggExpr *parser.AggregateExpr, sortedGrouping
 
 		// Make the function call.
 		enh.Ts = ts
-		result, ws := ev.aggregation(aggExpr, sortedGrouping, param, inputMatrix, seriesHelper, enh, seriess)
+		result, ws := ev.aggregation(aggExpr, param, inputMatrix, seriesToResult, orderedResult, enh, seriess)
 
 		warnings.Merge(ws)
 
@@ -2698,12 +2708,10 @@ type groupedAggregation struct {
 
 // aggregation evaluates an aggregation operation on a Vector. The provided grouping labels
 // must be sorted.
-func (ev *evaluator) aggregation(e *parser.AggregateExpr, grouping []string, q float64, inputMatrix Matrix, seriesHelper []EvalSeriesHelper, enh *EvalNodeHelper, seriess map[uint64]Series) (Matrix, annotations.Annotations) {
+func (ev *evaluator) aggregation(e *parser.AggregateExpr, q float64, inputMatrix Matrix, seriesToResult []int, orderedResult []*groupedAggregation, enh *EvalNodeHelper, seriess map[uint64]Series) (Matrix, annotations.Annotations) {
 	op := e.Op
-	without := e.Without
 	var annos annotations.Annotations
-	result := map[uint64]*groupedAggregation{}
-	orderedResult := []*groupedAggregation{}
+	seen := make([]bool, len(orderedResult)) // Which output groups were seen in the input at this timestamp.
 	k := 1
 	if op == parser.TOPK || op == parser.BOTTOMK {
 		if !convertibleToInt64(q) {
@@ -2743,53 +2751,47 @@ func (ev *evaluator) aggregation(e *parser.AggregateExpr, grouping []string, q f
 			ev.error(ErrTooManySamples(env))
 		}
 
-		metric := s.Metric
-		groupingKey := seriesHelper[si].groupingKey
-
-		group, ok := result[groupingKey]
-		// Add a new group if it doesn't exist.
-		if !ok {
-			m := generateGroupingLabels(enh, metric, without, grouping)
-			newAgg := &groupedAggregation{
-				labels:     m,
+		group := orderedResult[seriesToResult[si]]
+		// Initialize this group if it's the first time we've seen it.
+		if !seen[seriesToResult[si]] {
+			*group = groupedAggregation{
+				labels:     group.labels,
 				floatValue: s.F,
 				floatMean:  s.F,
 				groupCount: 1,
 			}
 			switch {
 			case s.H == nil:
-				newAgg.hasFloat = true
+				group.hasFloat = true
 			case op == parser.SUM:
-				newAgg.histogramValue = s.H.Copy()
-				newAgg.hasHistogram = true
+				group.histogramValue = s.H.Copy()
+				group.hasHistogram = true
 			case op == parser.AVG:
-				newAgg.histogramMean = s.H.Copy()
-				newAgg.hasHistogram = true
+				group.histogramMean = s.H.Copy()
+				group.hasHistogram = true
 			case op == parser.STDVAR || op == parser.STDDEV:
-				newAgg.groupCount = 0
+				group.groupCount = 0
 			}
 
 			switch op {
 			case parser.STDVAR, parser.STDDEV:
-				newAgg.floatValue = 0
+				group.floatValue = 0
 			case parser.TOPK, parser.QUANTILE:
-				newAgg.heap = make(vectorByValueHeap, 1, k)
-				newAgg.heap[0] = Sample{
+				group.heap = make(vectorByValueHeap, 1, k)
+				group.heap[0] = Sample{
 					F:      s.F,
 					Metric: s.Metric,
 				}
 			case parser.BOTTOMK:
-				newAgg.reverseHeap = make(vectorByReverseValueHeap, 1, k)
-				newAgg.reverseHeap[0] = Sample{
+				group.reverseHeap = make(vectorByReverseValueHeap, 1, k)
+				group.reverseHeap[0] = Sample{
 					F:      s.F,
 					Metric: s.Metric,
 				}
 			case parser.GROUP:
-				newAgg.floatValue = 1
+				group.floatValue = 1
 			}
-
-			result[groupingKey] = newAgg
-			orderedResult = append(orderedResult, newAgg)
+			seen[seriesToResult[si]] = true
 			continue
 		}
 
@@ -2950,7 +2952,10 @@ func (ev *evaluator) aggregation(e *parser.AggregateExpr, grouping []string, q f
 			seriess[hash] = ss
 		}
 	}
-	for _, aggr := range orderedResult {
+	for ri, aggr := range orderedResult {
+		if !seen[ri] {
+			continue
+		}
 		switch op {
 		case parser.AVG:
 			if aggr.hasFloat && aggr.hasHistogram {