diff --git a/promql/engine.go b/promql/engine.go
index 91c5e9e249..821a0e3698 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -1293,29 +1293,27 @@ func (ev *evaluator) rangeEval(prepSeries func(labels.Labels, *EvalSeriesHelper)
 
 func (ev *evaluator) rangeEvalAgg(aggExpr *parser.AggregateExpr, sortedGrouping []string) (Matrix, annotations.Annotations) {
 	numSteps := int((ev.endTimestamp-ev.startTimestamp)/ev.interval) + 1
-	matrixes := make([]Matrix, 2)
-	origMatrixes := make([]Matrix, 2)
 	originalNumSamples := ev.currentSamples
-
 	var warnings annotations.Annotations
-	for i, e := range []parser.Expr{aggExpr.Param, aggExpr.Expr} {
-		// Functions will take string arguments from the expressions, not the values.
-		if e != nil && e.Type() != parser.ValueTypeString {
-			// ev.currentSamples will be updated to the correct value within the ev.eval call.
-			val, ws := ev.eval(e)
-			warnings.Merge(ws)
-			matrixes[i] = val.(Matrix)
 
-			// Keep a copy of the original point slices so that they
-			// can be returned to the pool.
-			origMatrixes[i] = make(Matrix, len(matrixes[i]))
-			copy(origMatrixes[i], matrixes[i])
-		}
+	// param is the number k for topk/bottomk.
+	var param float64
+	if aggExpr.Param != nil {
+		val, ws := ev.eval(aggExpr.Param)
+		warnings.Merge(ws)
+		param = val.(Matrix)[0].Floats[0].F
 	}
+	// Now fetch the data to be aggregated.
+	// ev.currentSamples will be updated to the correct value within the ev.eval call.
+	val, ws := ev.eval(aggExpr.Expr)
+	warnings.Merge(ws)
+	inputMatrix := val.(Matrix)
 
-	vectors := make([]Vector, 2)    // Input vectors for the function.
-	args := make([]parser.Value, 2) // Argument to function.
-	biggestLen := len(matrixes[1])
+	// Keep a copy of the original point slice so that it can be returned to the pool.
+	origMatrix := inputMatrix
+
+	var vector Vector // Input vectors for the function.
+	biggestLen := len(inputMatrix)
 	enh := &EvalNodeHelper{Out: make(Vector, 0, biggestLen)}
 	type seriesAndTimestamp struct {
 		Series
@@ -1324,16 +1322,14 @@ func (ev *evaluator) rangeEvalAgg(aggExpr *parser.AggregateExpr, sortedGrouping
 	seriess := make(map[uint64]seriesAndTimestamp, biggestLen) // Output series by series hash.
 	tempNumSamples := ev.currentSamples
 
-	seriesHelpers := make([][]EvalSeriesHelper, 2)
-	bufHelpers := make([][]EvalSeriesHelper, 2)
-	// Prepare a function to initialise series helpers with the grouping key.
+	// Initialise series helpers with the grouping key.
 	buf := make([]byte, 0, 1024)
 
-	seriesHelpers[1] = make([]EvalSeriesHelper, len(matrixes[1]))
-	bufHelpers[1] = make([]EvalSeriesHelper, len(matrixes[1]))
+	seriesHelper := make([]EvalSeriesHelper, len(inputMatrix))
+	bufHelper := make([]EvalSeriesHelper, len(inputMatrix))
 
-	for si, series := range matrixes[1] {
-		seriesHelpers[1][si].groupingKey, buf = generateGroupingKey(series.Metric, sortedGrouping, aggExpr.Without, buf)
+	for si, series := range inputMatrix {
+		seriesHelper[si].groupingKey, buf = generateGroupingKey(series.Metric, sortedGrouping, aggExpr.Without, buf)
 	}
 
 	for ts := ev.startTimestamp; ts <= ev.endTimestamp; ts += ev.interval {
@@ -1343,42 +1339,35 @@ func (ev *evaluator) rangeEvalAgg(aggExpr *parser.AggregateExpr, sortedGrouping
 		// Reset number of samples in memory after each timestamp.
 		ev.currentSamples = tempNumSamples
 		// Gather input vectors for this timestamp.
-		for i := range []parser.Expr{aggExpr.Param, aggExpr.Expr} {
-			vectors[i] = vectors[i][:0]
-			bufHelpers[i] = bufHelpers[i][:0]
+		{
+			vector = vector[:0]
+			bufHelper = bufHelper[:0]
 
-			for si, series := range matrixes[i] {
+			for si, series := range inputMatrix {
 				switch {
 				case len(series.Floats) > 0 && series.Floats[0].T == ts:
-					vectors[i] = append(vectors[i], Sample{Metric: series.Metric, F: series.Floats[0].F, T: ts})
+					vector = append(vector, Sample{Metric: series.Metric, F: series.Floats[0].F, T: ts})
 					// Move input vectors forward so we don't have to re-scan the same
 					// past points at the next step.
-					matrixes[i][si].Floats = series.Floats[1:]
+					inputMatrix[si].Floats = series.Floats[1:]
 				case len(series.Histograms) > 0 && series.Histograms[0].T == ts:
-					vectors[i] = append(vectors[i], Sample{Metric: series.Metric, H: series.Histograms[0].H, T: ts})
-					matrixes[i][si].Histograms = series.Histograms[1:]
+					vector = append(vector, Sample{Metric: series.Metric, H: series.Histograms[0].H, T: ts})
+					inputMatrix[si].Histograms = series.Histograms[1:]
 				default:
 					continue
 				}
-				if seriesHelpers[i] != nil {
-					bufHelpers[i] = append(bufHelpers[i], seriesHelpers[i][si])
-				}
+				bufHelper = append(bufHelper, seriesHelper[si])
 				ev.currentSamples++
 				if ev.currentSamples > ev.maxSamples {
 					ev.error(ErrTooManySamples(env))
 				}
 			}
-			args[i] = vectors[i]
 			ev.samplesStats.UpdatePeak(ev.currentSamples)
 		}
 
 		// Make the function call.
 		enh.Ts = ts
-		var param float64
-		if aggExpr.Param != nil {
-			param = args[0].(Vector)[0].F
-		}
-		result, ws := ev.aggregation(aggExpr, sortedGrouping, param, args[1].(Vector), bufHelpers[1], enh)
+		result, ws := ev.aggregation(aggExpr, sortedGrouping, param, vector, bufHelper, enh)
 
 		enh.Out = result[:0] // Reuse result vector.
 		warnings.Merge(ws)
@@ -1440,12 +1429,10 @@ func (ev *evaluator) rangeEvalAgg(aggExpr *parser.AggregateExpr, sortedGrouping
 		}
 	}
 
-	// Reuse the original point slices.
-	for _, m := range origMatrixes {
-		for _, s := range m {
-			putFPointSlice(s.Floats)
-			putHPointSlice(s.Histograms)
-		}
+	// Reuse the original point slice.
+	for _, s := range origMatrix {
+		putFPointSlice(s.Floats)
+		putHPointSlice(s.Histograms)
 	}
 	// Assemble the output matrix. By the time we get here we know we don't have too many samples.
 	mat := make(Matrix, 0, len(seriess))