app/vmselect/promql: return at most one time series from absent_over_time() in the same way as Prometheus does

See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2130

app/vmselect/promql/eval.go

@@ -650,6 +650,9 @@ func evalRollupFuncWithoutAt(ec *EvalConfig, funcName string, rf rollupFunc, exp
 	if err != nil {
 		return nil, err
 	}
+	if funcName == "absent_over_time" {
+		rvs = aggregateAbsentOverTime(ec, re.Expr, rvs)
+	}
 	ec.updateIsPartialResponse(ecNew.IsPartialResponse)
 	if offset != 0 && len(rvs) > 0 {
 		// Make a copy of timestamps, since they may be used in other values.
@@ -665,6 +668,27 @@ func evalRollupFuncWithoutAt(ec *EvalConfig, funcName string, rf rollupFunc, exp
 	return rvs, nil
 }
 
+// aggregateAbsentOverTime collapses tss to a single time series with 1 and nan values.
+//
+// Values for returned series are set to nan if at least a single tss series contains nan at that point.
+// This means that tss contains a series with non-empty results at that point.
+// This follows Prometheus logic - see https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2130
+func aggregateAbsentOverTime(ec *EvalConfig, expr metricsql.Expr, tss []*timeseries) []*timeseries {
+	rvs := getAbsentTimeseries(ec, expr)
+	if len(tss) == 0 {
+		return rvs
+	}
+	for i := range tss[0].Values {
+		for _, ts := range tss {
+			if math.IsNaN(ts.Values[i]) {
+				rvs[0].Values[i] = nan
+				break
+			}
+		}
+	}
+	return rvs
+}
+
 func evalRollupFuncWithSubquery(ec *EvalConfig, funcName string, rf rollupFunc, expr metricsql.Expr, re *metricsql.RollupExpr) ([]*timeseries, error) {
 	// TODO: determine whether to use rollupResultCacheV here.
 	step := re.Step.Duration(ec.Step)
@@ -688,10 +712,6 @@ func evalRollupFuncWithSubquery(ec *EvalConfig, funcName string, rf rollupFunc,
 	}
 	ec.updateIsPartialResponse(ecSQ.IsPartialResponse)
 	if len(tssSQ) == 0 {
-		if funcName == "absent_over_time" {
-			tss := evalNumber(ec, 1)
-			return tss, nil
-		}
 		return nil, nil
 	}
 	sharedTimestamps := getTimestamps(ec.Start, ec.End, ec.Step)
@@ -842,14 +862,7 @@ func evalRollupFuncWithMetricExpr(ec *EvalConfig, funcName string, rf rollupFunc
 	rssLen := rss.Len()
 	if rssLen == 0 {
 		rss.Cancel()
-		var tss []*timeseries
-		if funcName == "absent_over_time" {
-			tss = getAbsentTimeseries(ec, me)
-		}
-		// Add missing points until ec.End.
-		// Do not cache the result, since missing points
-		// may be backfilled in the future.
-		tss = mergeTimeseries(tssCached, tss, start, ec)
+		tss := mergeTimeseries(tssCached, nil, start, ec)
 		return tss, nil
 	}
 

app/vmselect/promql/exec_test.go

@@ -877,19 +877,37 @@ func TestExecSuccess(t *testing.T) {
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})
-	t.Run(`absent_over_time(scalar(multi-timeseries))`, func(t *testing.T) {
+	t.Run(`absent_over_time(non-nan)`, func(t *testing.T) {
 		t.Parallel()
 		q := `
-		absent_over_time(label_set(scalar(1 or label_set(2, "xx", "foo")), "yy", "foo"))`
+		absent_over_time(time())`
+		resultExpected := []netstorage.Result{}
+		f(q, resultExpected)
+	})
+	t.Run(`absent_over_time(nan)`, func(t *testing.T) {
+		t.Parallel()
+		q := `
+		absent_over_time((time() < 1500)[300s:])`
 		r := netstorage.Result{
 			MetricName: metricNameExpected,
-			Values:     []float64{1, 1, 1, 1, 1, 1},
+			Values:     []float64{nan, nan, nan, nan, 1, 1},
+			Timestamps: timestampsExpected,
+		}
+		resultExpected := []netstorage.Result{r}
+		f(q, resultExpected)
+	})
+	t.Run(`absent_over_time(multi-ts)`, func(t *testing.T) {
+		t.Parallel()
+		q := `
+		absent_over_time((
+			alias((time() < 1400)[200s:], "one"),
+			alias((time() > 1600)[200s:], "two"),
+		))`
+		r := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{nan, nan, nan, 1, nan, nan},
 			Timestamps: timestampsExpected,
 		}
-		r.MetricName.Tags = []storage.Tag{{
-			Key:   []byte("yy"),
-			Value: []byte("foo"),
-		}}
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})

docs/CHANGELOG.md

@@ -34,6 +34,7 @@ The following tip changes can be tested by building VictoriaMetrics components f
 * FEATURE: add `__meta_kubernetes_endpointslice_label*` and `__meta_kubernetes_endpointslice_annotation*` labels for `role: endpointslice` targets in [kubernetes_sd_config](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#kubernetes_sd_config) to be consistent with other `role` values. See [this issue](https://github.com/prometheus/prometheus/issues/10284).
 * FEATURE: vmagent: support Prometheus-like durations in `-promscrape.config`. See [this comment](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/817#issuecomment-1033384766).
 
+* BUGFIX: calculate [absent_over_time()](https://docs.victoriametrics.com/MetricsQL.html#absent_over_time) in the same way as Prometheus does. Previously it could return multiple time series instead of at most one time series like Prometheus does. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2130).
 * BUGFIX: return proper results from `highestMax()` function at [Graphite render API](https://docs.victoriametrics.com/#graphite-render-api-usage). Previously it was incorrectly returning timeseries with min peaks instead of max peaks.
 * BUGFIX: properly limit indexdb cache sizes. Previously they could exceed values set via `-memory.allowedPercent` and/or `-memory.allowedBytes` when `indexdb` contained many data parts. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2007).
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): fix a bug, which could break time range picker when editing `From` or `To` input fields. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2080).