app/vmselect/promql: add lag(q[d]) function, which returns the lag between the current timestamp and the timstamp for the last data point in q

app/vmselect/promql/exec_test.go

@@ -3720,6 +3720,17 @@ func TestExecSuccess(t *testing.T) {
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})
+	t.Run(`lag()`, func(t *testing.T) {
+		t.Parallel()
+		q := `lag(time()[60s:17s])`
+		r := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{14, 10, 6, 2, 15, 11},
+			Timestamps: timestampsExpected,
+		}
+		resultExpected := []netstorage.Result{r}
+		f(q, resultExpected)
+	})
 	t.Run(`()`, func(t *testing.T) {
 		t.Parallel()
 		q := `()`
@@ -4189,6 +4200,8 @@ func TestExecError(t *testing.T) {
 	f(`alias()`)
 	f(`alias(1)`)
 	f(`alias(1, "foo", "bar")`)
+	f(`lifetime()`)
+	f(`lag()`)
 
 	// Invalid argument type
 	f(`median_over_time({}, 2)`)

app/vmselect/promql/rollup.go

@@ -48,6 +48,7 @@ var rollupFuncs = map[string]newRollupFunc{
 	"integrate":           newRollupFuncOneArg(rollupIntegrate),
 	"ideriv":              newRollupFuncOneArg(rollupIderiv),
 	"lifetime":            newRollupFuncOneArg(rollupLifetime),
+	"lag":                 newRollupFuncOneArg(rollupLag),
 	"scrape_interval":     newRollupFuncOneArg(rollupScrapeInterval),
 	"rollup":              newRollupFuncOneArg(rollupFake),
 	"rollup_rate":         newRollupFuncOneArg(rollupFake), // + rollupFuncsRemoveCounterResets
@@ -113,6 +114,7 @@ type rollupFuncArg struct {
 	values        []float64
 	timestamps    []int64
 
+	currTimestamp int64
 	idx           int
 	step          int64
 }
@@ -122,6 +124,7 @@ func (rfa *rollupFuncArg) reset() {
 	rfa.prevTimestamp = 0
 	rfa.values = nil
 	rfa.timestamps = nil
+	rfa.currTimestamp = 0
 	rfa.idx = 0
 	rfa.step = 0
 }
@@ -226,6 +229,7 @@ func (rc *rollupConfig) Do(dstValues []float64, values []float64, timestamps []i
 
 		rfa.values = values[i:j]
 		rfa.timestamps = timestamps[i:j]
+		rfa.currTimestamp = tEnd
 		value := rc.Func(rfa)
 		rfa.idx++
 		dstValues = append(dstValues, value)
@@ -799,6 +803,18 @@ func rollupLifetime(rfa *rollupFuncArg) float64 {
 	return float64(timestamps[len(timestamps)-1]-rfa.prevTimestamp) * 1e-3
 }
 
+func rollupLag(rfa *rollupFuncArg) float64 {
+	// Calculate the duration between the current timestamp and the last data point.
+	timestamps := rfa.timestamps
+	if len(timestamps) == 0 {
+		if math.IsNaN(rfa.prevValue) {
+			return nan
+		}
+		return float64(rfa.currTimestamp-rfa.prevTimestamp) * 1e-3
+	}
+	return float64(rfa.currTimestamp-timestamps[len(timestamps)-1]) * 1e-3
+}
+
 func rollupScrapeInterval(rfa *rollupFuncArg) float64 {
 	// Calculate the average interval between data points.
 	timestamps := rfa.timestamps

app/vmselect/promql/rollup_test.go

@@ -632,6 +632,20 @@ func TestRollupFuncsNoWindow(t *testing.T) {
 		timestampsExpected := []int64{10, 50, 90, 130}
 		testRowsEqual(t, values, rc.Timestamps, valuesExpected, timestampsExpected)
 	})
+	t.Run("lag", func(t *testing.T) {
+		rc := rollupConfig{
+			Func:   rollupLag,
+			Start:  0,
+			End:    160,
+			Step:   40,
+			Window: 0,
+		}
+		rc.Timestamps = getTimestamps(rc.Start, rc.End, rc.Step)
+		values := rc.Do(nil, testValues, testTimestamps)
+		valuesExpected := []float64{nan, 0.004, 0, 0, 0.03}
+		timestampsExpected := []int64{0, 40, 80, 120, 160}
+		testRowsEqual(t, values, rc.Timestamps, valuesExpected, timestampsExpected)
+	})
 	t.Run("lifetime_1", func(t *testing.T) {
 		rc := rollupConfig{
 			Func:   rollupLifetime,