[REFACTOR] PromQL: simplify rangeEvalTimestampFunctionOverVectorSelector (#14021)

The function `rangeEvalTimestampFunctionOverVectorSelector` appeared to be checking histogram size, however the value it used was always 0 due to subtle variable shadowing.
However we don't need to pass sample values to the `timestamp` function, since the latter only cares about timestamps. This also affects peak sample count in statistics, since we are no longer copying histogram samples.

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

promql/engine.go

@@ -2024,27 +2024,23 @@ func (ev *evaluator) rangeEvalTimestampFunctionOverVectorSelector(vs *parser.Vec
 		vec := make(Vector, 0, len(vs.Series))
 		for i, s := range vs.Series {
 			it := seriesIterators[i]
-			t, f, h, ok := ev.vectorSelectorSingle(it, vs, enh.Ts)
-			if ok {
+			t, _, _, ok := ev.vectorSelectorSingle(it, vs, enh.Ts)
+			if !ok {
+				continue
+			}
+
+			// Note that we ignore the sample values because call only cares about the timestamp.
 			vec = append(vec, Sample{
 				Metric: s.Labels(),
 				T:      t,
-					F:      f,
-					H:      h,
 			})
-				histSize := 0
-				if h != nil {
-					histSize := h.Size() / 16 // 16 bytes per sample.
-					ev.currentSamples += histSize
-				}
-				ev.currentSamples++
 
-				ev.samplesStats.IncrementSamplesAtTimestamp(enh.Ts, int64(1+histSize))
+			ev.currentSamples++
+			ev.samplesStats.IncrementSamplesAtTimestamp(enh.Ts, 1)
 			if ev.currentSamples > ev.maxSamples {
 				ev.error(ErrTooManySamples(env))
 			}
 		}
-		}
 		ev.samplesStats.UpdatePeak(ev.currentSamples)
 		vec, annos := call([]parser.Value{vec}, e.Args, enh)
 		return vec, ws.Merge(annos)

promql/engine_test.go

@@ -818,7 +818,7 @@ load 10s
 		{
 			Query:        "timestamp(metricWith1HistogramEvery10Seconds)",
 			Start:        time.Unix(21, 0),
-			PeakSamples:  13, // histogram size 12 + 1 extra because of timestamp
+			PeakSamples:  2,
 			TotalSamples: 1, // 1 float sample (because of timestamp) / 10 seconds
 			TotalSamplesPerStep: stats.TotalSamplesPerStep{
 				21000: 1,
@@ -1116,7 +1116,7 @@ load 10s
 			Start:        time.Unix(201, 0),
 			End:          time.Unix(220, 0),
 			Interval:     5 * time.Second,
-			PeakSamples:  16,
+			PeakSamples:  5,
 			TotalSamples: 4, // 1 sample per query * 4 steps
 			TotalSamplesPerStep: stats.TotalSamplesPerStep{
 				201000: 1,