app/vmselect/promql: fix comparison to nan

The comparison to nan has been broken in d335cc886c

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/150

app/vmselect/promql/binary_op.go

@@ -84,18 +84,22 @@ func newBinaryOpFunc(bf func(left, right float64, isBool bool) float64) binaryOp
 	return func(bfa *binaryOpFuncArg) ([]*timeseries, error) {
 		left := bfa.left
 		right := bfa.right
-		switch bfa.be.Op {
-		case "ifnot":
+		op := bfa.be.Op
+		switch true {
+		case op == "ifnot":
 			left = removeEmptySeries(left)
 			// Do not remove empty series on the right side,
 			// so the left-side series could be matched against them.
-		case "default":
+		case op == "default":
 			// Do not remove empty series on the left and the right side,
-			// since this may result in missing result:
+			// since this may lead to missing result:
 			// - if empty time series are removed on the left side,
 			// then they won't be substituted by time series from the right side.
 			// - if empty time series are removed on the right side,
 			// then this may result in missing time series from the left side.
+		case metricsql.IsBinaryOpCmp(op):
+			// Do not remove empty series for comparison operations,
+			// since this may lead to missing result.
 		default:
 			left = removeEmptySeries(left)
 			right = removeEmptySeries(right)

app/vmselect/promql/exec_test.go

@@ -2625,6 +2625,23 @@ func TestExecSuccess(t *testing.T) {
 		resultExpected := []netstorage.Result{}
 		f(q, resultExpected)
 	})
+	t.Run(`compare_to_nan_right`, func(t *testing.T) {
+		t.Parallel()
+		q := `1 != nan`
+		r := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values: []float64{1, 1, 1, 1, 1, 1},
+			Timestamps: timestampsExpected,
+		}
+		resultExpected := []netstorage.Result{r}
+		f(q, resultExpected)
+	})
+	t.Run(`compare_to_nan_left`, func(t *testing.T) {
+		t.Parallel()
+		q := `nan != 1`
+		resultExpected := []netstorage.Result{}
+		f(q, resultExpected)
+	})
 	t.Run(`-1 < 2`, func(t *testing.T) {
 		t.Parallel()
 		q := `-1 < 2`