app/vmselect/promql: fill gaps on right side with values from left side of or operator in the same way as Prometheus does

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/552

app/vmselect/promql/binary_op.go

@@ -310,9 +310,22 @@ func binaryOpOr(bfa *binaryOpFuncArg) ([]*timeseries, error) {
 	for _, tss := range mLeft {
 		rvs = append(rvs, tss...)
 	}
-	for k, tss := range mRight {
+	for k, tssRight := range mRight {
-		if mLeft[k] == nil {
+		tssLeft := mLeft[k]
-			rvs = append(rvs, tss...)
+		if tssLeft == nil {
+			rvs = append(rvs, tssRight...)
+			continue
+		}
+		// Fill gaps in tssLeft with values from tssRight as Prometheus does.
+		// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/552
+		valuesRight := tssRight[0].Values
+		for _, tsLeft := range tssLeft {
+			valuesLeft := tsLeft.Values
+			for i, v := range valuesLeft {
+				if math.IsNaN(v) {
+					valuesLeft[i] = valuesRight[i]
+				}
+			}
 		}
 	}
 	return rvs, nil

app/vmselect/promql/exec_test.go

@@ -1849,6 +1849,17 @@ func TestExecSuccess(t *testing.T) {
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})
+	t.Run(`scalar or scalar`, func(t *testing.T) {
+		t.Parallel()
+		q := `time() > 1400 or 123`
+		r := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{123, 123, 123, 1600, 1800, 2000},
+			Timestamps: timestampsExpected,
+		}
+		resultExpected := []netstorage.Result{r}
+		f(q, resultExpected)
+	})
 	t.Run(`timseries-with-tags unless 2`, func(t *testing.T) {
 		t.Parallel()
 		q := `label_set(time(), "foo", "bar") unless 2`