app/vmselect/promql: properly calculate quantiles_over_time() over a single raw sample

app/vmselect/promql/exec_test.go

@@ -5110,7 +5110,40 @@ func TestExecSuccess(t *testing.T) {
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})
-	t.Run(`quantiles_over_time`, func(t *testing.T) {
+	t.Run(`quantiles_over_time(single_sample)`, func(t *testing.T) {
+		t.Parallel()
+		q := `sort_by_label(
+			quantiles_over_time("phi", 0.5, 0.9,
+				time()[100s:100s]
+			),
+			"phi",
+		)`
+		r1 := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{1000, 1200, 1400, 1600, 1800, 2000},
+			Timestamps: timestampsExpected,
+		}
+		r1.MetricName.Tags = []storage.Tag{
+			{
+				Key:   []byte("phi"),
+				Value: []byte("0.5"),
+			},
+		}
+		r2 := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{1000, 1200, 1400, 1600, 1800, 2000},
+			Timestamps: timestampsExpected,
+		}
+		r2.MetricName.Tags = []storage.Tag{
+			{
+				Key:   []byte("phi"),
+				Value: []byte("0.9"),
+			},
+		}
+		resultExpected := []netstorage.Result{r1, r2}
+		f(q, resultExpected)
+	})
+	t.Run(`quantiles_over_time(multiple_samples)`, func(t *testing.T) {
 		t.Parallel()
 		q := `sort_by_label(
 			quantiles_over_time("phi", 0.5, 0.9,

app/vmselect/promql/rollup.go

@@ -1112,11 +1112,7 @@ func newRollupQuantiles(args []interface{}) (rollupFunc, error) {
 		// before calling rollup funcs.
 		values := rfa.values
 		if len(values) == 0 {
-			return rfa.prevValue
-		}
-		if len(values) == 1 {
-			// Fast path - only a single value.
-			return values[0]
+			return nan
 		}
 		qs := getFloat64s()
 		qs.A = quantiles(qs.A[:0], phis, values)

docs/CHANGELOG.md

@@ -25,6 +25,7 @@ The following tip changes can be tested by building VictoriaMetrics components f
 * BUGFIX: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): properly calculate `rate_over_sum(m[d])` as `sum_over_time(m[d])/d`. Previously the `sum_over_time(m[d])` could be improperly divided by smaller than `d` time range. See [rate_over_sum() docs](https://docs.victoriametrics.com/MetricsQL.html#rate_over_sum) and [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3045).
 * BUGFIX: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): properly calculate `increase(m[d])` over slow-changing counters with values smaller than 100. Previously [increase](https://docs.victoriametrics.com/MetricsQL.html#increase) could return unexpectedly big results in this case. See [the related issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/962) and [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/3163).
 * BUGFIX: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): ignore empty series when applying [limit_offset](https://docs.victoriametrics.com/MetricsQL.html#limit_offset). It should improve queries with additional filters by value in expressions like `limit_offset(1,1, foo > 1)`.
+* BUGFIX: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): properly calculate [quantiles_over_time](https://docs.victoriametrics.com/MetricsQL.html#quantiles_over_time) when the lookbehind window contains only a single sample. Previously an empty result was incorrectly returned in this case.
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): fix `RangeError: Maximum call stack size exceeded` error when the query returns too many data points at `Table` view. See [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/3092/files).
 * BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert.html): re-evaluate annotations per each alert evaluation. Previously, annotations were evaluated only on alert's value ch
 ange. This could result in stale annotations in some cases described in [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/3119).