app/vmselect/promql: return back the behaviour for deriv() function when the lookbehind window doesnt contain enough points

It is expected that the `deriv(m[d])` returns non-empty value if the lookbehind window `d`
contains less than 2 samples in the same way as `rate()` does.

This is a follow-up after 3e084be06b .

app/vmselect/promql/exec_test.go

@@ -6239,6 +6239,39 @@ func TestExecSuccess(t *testing.T) {
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})
+	t.Run(`deriv(1)`, func(t *testing.T) {
+		t.Parallel()
+		q := `deriv(1)`
+		r := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{0, 0, 0, 0, 0, 0},
+			Timestamps: timestampsExpected,
+		}
+		resultExpected := []netstorage.Result{r}
+		f(q, resultExpected)
+	})
+	t.Run(`deriv(time())`, func(t *testing.T) {
+		t.Parallel()
+		q := `deriv(2*time())`
+		r := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{2, 2, 2, 2, 2, 2},
+			Timestamps: timestampsExpected,
+		}
+		resultExpected := []netstorage.Result{r}
+		f(q, resultExpected)
+	})
+	t.Run(`deriv(-time())`, func(t *testing.T) {
+		t.Parallel()
+		q := `deriv(-time())`
+		r := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{-1, -1, -1, -1, -1, -1},
+			Timestamps: timestampsExpected,
+		}
+		resultExpected := []netstorage.Result{r}
+		f(q, resultExpected)
+	})
 	t.Run(`delta(time())`, func(t *testing.T) {
 		t.Parallel()
 		q := `delta(time())`

app/vmselect/promql/rollup.go

@@ -151,7 +151,6 @@ var rollupFuncsCannotAdjustWindow = map[string]bool{
 	"holt_winters":        true,
 	"idelta":              true,
 	"increase":            true,
-	"deriv":               true,
 	"predict_linear":      true,
 	"resets":              true,
 	"avg_over_time":       true,
@@ -865,9 +864,13 @@ func linearRegression(rfa *rollupFuncArg) (float64, float64) {
 	// before calling rollup funcs.
 	values := rfa.values
 	timestamps := rfa.timestamps
-	if len(values) < 2 {
+	n := float64(len(values))
+	if n == 0 {
 		return nan, nan
 	}
+	if n == 1 {
+		return values[0], 0
+	}
 
 	// See https://en.wikipedia.org/wiki/Simple_linear_regression#Numerical_example
 	interceptTime := rfa.currTimestamp
@@ -882,7 +885,6 @@ func linearRegression(rfa *rollupFuncArg) (float64, float64) {
 		tvSum += dt * v
 		ttSum += dt * dt
 	}
-	n := float64(len(values))
 	k := (tvSum - tSum*vSum/n) / (ttSum - tSum*tSum/n)
 	v := vSum/n - k*tSum/n
 	return v, k

app/vmselect/promql/rollup_test.go

@@ -979,7 +979,7 @@ func TestRollupFuncsNoWindow(t *testing.T) {
 		}
 		rc.Timestamps = getTimestamps(rc.Start, rc.End, rc.Step)
 		values := rc.Do(nil, testValues, testTimestamps)
-		valuesExpected := []float64{nan, -2879.310344827588, 127.87627310448904, -496.5831435079728, nan}
+		valuesExpected := []float64{nan, -2879.310344827588, 127.87627310448904, -496.5831435079728, 0}
 		timestampsExpected := []int64{0, 40, 80, 120, 160}
 		testRowsEqual(t, values, rc.Timestamps, valuesExpected, timestampsExpected)
 	})