Optimise topk where k==1 (#9365)

* Add benchmark for query_range with topk

Modify sample data so values within a metric differ

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

* Optimise topk where k==1

In this case we don't need a heap to keep track of values; just a single
slot is fine.

Simplify the initialization of the heap: since all cases start off as a
single-item heap we can just assign the value directly.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

* Allow at least one slot in results for topk, quantile

k isn't set for quantile, but we need space to start collecting values

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

promql/bench_test.go

@@ -71,7 +71,7 @@ func BenchmarkRangeQuery(b *testing.B) {
 		a := storage.Appender(context.Background())
 		ts := int64(s * 10000) // 10s interval.
 		for i, metric := range metrics {
-			ref, _ := a.Append(refs[i], metric, ts, float64(s))
+			ref, _ := a.Append(refs[i], metric, ts, float64(s)+float64(i)/float64(len(metrics)))
 			refs[i] = ref
 		}
 		if err := a.Commit(); err != nil {
@@ -159,6 +159,9 @@ func BenchmarkRangeQuery(b *testing.B) {
 		{
 			expr: "count_values('value', h_X)",
 		},
+		{
+			expr: "topk(1, a_X)",
+		},
 		// Combinations.
 		{
 			expr: "rate(a_X[1m]) + rate(b_X[1m])",

promql/engine.go

@@ -2210,22 +2210,24 @@ func (ev *evaluator) aggregation(op parser.ItemType, grouping []string, without
 			resultSize := k
 			if k > inputVecLen {
 				resultSize = inputVecLen
+			} else if k == 0 {
+				resultSize = 1
 			}
 			switch op {
 			case parser.STDVAR, parser.STDDEV:
 				result[groupingKey].value = 0
 			case parser.TOPK, parser.QUANTILE:
-				result[groupingKey].heap = make(vectorByValueHeap, 0, resultSize)
+				result[groupingKey].heap = make(vectorByValueHeap, 1, resultSize)
-				heap.Push(&result[groupingKey].heap, &Sample{
+				result[groupingKey].heap[0] = Sample{
 					Point:  Point{V: s.V},
 					Metric: s.Metric,
-				})
+				}
 			case parser.BOTTOMK:
-				result[groupingKey].reverseHeap = make(vectorByReverseValueHeap, 0, resultSize)
+				result[groupingKey].reverseHeap = make(vectorByReverseValueHeap, 1, resultSize)
-				heap.Push(&result[groupingKey].reverseHeap, &Sample{
+				result[groupingKey].reverseHeap[0] = Sample{
 					Point:  Point{V: s.V},
 					Metric: s.Metric,
-				})
+				}
 			case parser.GROUP:
 				result[groupingKey].value = 1
 			}
@@ -2283,6 +2285,13 @@ func (ev *evaluator) aggregation(op parser.ItemType, grouping []string, without
 		case parser.TOPK:
 			if int64(len(group.heap)) < k || group.heap[0].V < s.V || math.IsNaN(group.heap[0].V) {
 				if int64(len(group.heap)) == k {
+					if k == 1 { // For k==1 we can replace in-situ.
+						group.heap[0] = Sample{
+							Point:  Point{V: s.V},
+							Metric: s.Metric,
+						}
+						break
+					}
 					heap.Pop(&group.heap)
 				}
 				heap.Push(&group.heap, &Sample{
@@ -2294,6 +2303,13 @@ func (ev *evaluator) aggregation(op parser.ItemType, grouping []string, without
 		case parser.BOTTOMK:
 			if int64(len(group.reverseHeap)) < k || group.reverseHeap[0].V > s.V || math.IsNaN(group.reverseHeap[0].V) {
 				if int64(len(group.reverseHeap)) == k {
+					if k == 1 { // For k==1 we can replace in-situ.
+						group.reverseHeap[0] = Sample{
+							Point:  Point{V: s.V},
+							Metric: s.Metric,
+						}
+						break
+					}
 					heap.Pop(&group.reverseHeap)
 				}
 				heap.Push(&group.reverseHeap, &Sample{
@@ -2327,7 +2343,9 @@ func (ev *evaluator) aggregation(op parser.ItemType, grouping []string, without
 
 		case parser.TOPK:
 			// The heap keeps the lowest value on top, so reverse it.
+			if len(aggr.heap) > 1 {
 				sort.Sort(sort.Reverse(aggr.heap))
+			}
 			for _, v := range aggr.heap {
 				enh.Out = append(enh.Out, Sample{
 					Metric: v.Metric,
@@ -2338,7 +2356,9 @@ func (ev *evaluator) aggregation(op parser.ItemType, grouping []string, without
 
 		case parser.BOTTOMK:
 			// The heap keeps the highest value on top, so reverse it.
+			if len(aggr.reverseHeap) > 1 {
 				sort.Sort(sort.Reverse(aggr.reverseHeap))
+			}
 			for _, v := range aggr.reverseHeap {
 				enh.Out = append(enh.Out, Sample{
 					Metric: v.Metric,