app/vmselect/promql: return a single time series at max from absent() function like Prometheus does

CHANGELOG.md

@@ -53,6 +53,7 @@
 
 * BUGFIX: properly handle stale time series after K8S deployment. Previously such time series could be double-counted.
   See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/748
+* BUGFIX: return a single time series at max from `absent()` function like Prometheus does.
 * BUGFIX: vmalert: accept days, weeks and years in `for: ` part of config like Prometheus does. See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/817
 * BUGFIX: fix `mode_over_time(m[d])` calculations. Previously the function could return incorrect results.
 

app/vmselect/promql/exec_test.go

@@ -592,10 +592,6 @@ func TestExecSuccess(t *testing.T) {
 			Values:     []float64{1, 1, 1, 1, 1, 1},
 			Timestamps: timestampsExpected,
 		}
-		r.MetricName.Tags = []storage.Tag{{
-			Key:   []byte("yy"),
-			Value: []byte("foo"),
-		}}
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})

app/vmselect/promql/transform.go

@@ -167,23 +167,24 @@ func transformAbsent(tfa *transformFuncArg) ([]*timeseries, error) {
 	if err := expectTransformArgsNum(args, 1); err != nil {
 		return nil, err
 	}
-	arg := args[0]
-	if len(arg) == 0 {
-		rvs := getAbsentTimeseries(tfa.ec, tfa.fe.Args[0])
+	tss := args[0]
+	rvs := getAbsentTimeseries(tfa.ec, tfa.fe.Args[0])
+	if len(tss) == 0 {
 		return rvs, nil
 	}
-	for _, ts := range arg {
-		ts.MetricName.ResetMetricGroup()
-		for i, v := range ts.Values {
-			if !math.IsNaN(v) {
-				v = nan
-			} else {
-				v = 1
+	for i := range tss[0].Values {
+		isAbsent := true
+		for _, ts := range tss {
+			if !math.IsNaN(ts.Values[i]) {
+				isAbsent = false
+				break
 			}
-			ts.Values[i] = v
+		}
+		if !isAbsent {
+			rvs[0].Values[i] = nan
 		}
 	}
-	return arg, nil
+	return rvs, nil
 }
 
 func getAbsentTimeseries(ec *EvalConfig, arg metricsql.Expr) []*timeseries {