app/vmselect/promql: add hoeffding_bound_upper(phi, m[d]) and hoeffding_bound_lower(phi, m[d]) functions

These functions can be used for calculating Hoeffding bounds
for `m` over `d` time range and for the given `phi` in the range `[0..1]`.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/283

app/vmselect/promql/eval.go

@@ -605,6 +605,14 @@ func evalRollupFuncWithMetricExpr(ec *EvalConfig, name string, rf rollupFunc,
 		rollupResultCacheMiss.Inc()
 	}
 
+	// Obtain rollup configs before fetching data from db,
+	// so type errors can be caught earlier.
+	sharedTimestamps := getTimestamps(start, ec.End, ec.Step)
+	preFunc, rcs, err := getRollupConfigs(name, rf, expr, start, ec.End, ec.Step, window, ec.LookbackDelta, sharedTimestamps)
+	if err != nil {
+		return nil, err
+	}
+
 	// Fetch the remaining part of the result.
 	tfs := toTagFilters(me.LabelFilters)
 	sq := &storage.SearchQuery{
@@ -629,12 +637,6 @@ func evalRollupFuncWithMetricExpr(ec *EvalConfig, name string, rf rollupFunc,
 		tss = mergeTimeseries(tssCached, tss, start, ec)
 		return tss, nil
 	}
-	sharedTimestamps := getTimestamps(start, ec.End, ec.Step)
-	preFunc, rcs, err := getRollupConfigs(name, rf, expr, start, ec.End, ec.Step, window, ec.LookbackDelta, sharedTimestamps)
-	if err != nil {
-		rss.Cancel()
-		return nil, err
-	}
 
 	// Verify timeseries fit available memory after the rollup.
 	// Take into account points from tssCached.

app/vmselect/promql/exec_test.go

@@ -4480,6 +4480,29 @@ func TestExecSuccess(t *testing.T) {
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})
+	t.Run(`hoeffding_bound_lower()`, func(t *testing.T) {
+		t.Parallel()
+		q := `hoeffding_bound_lower(0.9, rand(0)[:10s])`
+		r := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{0.2516770508510652, 0.2830570387745462, 0.27716232108436645, 0.3679356319931767, 0.3168460474120903, 0.23156726248243734},
+			Timestamps: timestampsExpected,
+		}
+		resultExpected := []netstorage.Result{r}
+		f(q, resultExpected)
+	})
+	t.Run(`hoeffding_bound_upper()`, func(t *testing.T) {
+		t.Parallel()
+		q := `hoeffding_bound_upper(0.9, alias(rand(0), "foobar")[:10s])`
+		r := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{0.6510581320042821, 0.7261021731890429, 0.7245290097397009, 0.8113950442584258, 0.7736122275568004, 0.6658564048254882},
+			Timestamps: timestampsExpected,
+		}
+		r.MetricName.MetricGroup = []byte("foobar")
+		resultExpected := []netstorage.Result{r}
+		f(q, resultExpected)
+	})
 	t.Run(`aggr_over_time(single-func)`, func(t *testing.T) {
 		t.Parallel()
 		q := `aggr_over_time("increase", rand(0)[:10s])`
@@ -5267,6 +5290,12 @@ func TestExecError(t *testing.T) {
 	f(`aggr_over_time()`)
 	f(`aggr_over_time(foo)`)
 	f(`aggr_over_time("foo", bar, 1)`)
+	f(`hoeffding_bound_lower()`)
+	f(`hoeffding_bound_lower(1)`)
+	f(`hoeffding_bound_lower(0.99, foo, 1)`)
+	f(`hoeffding_bound_upper()`)
+	f(`hoeffding_bound_upper(1)`)
+	f(`hoeffding_bound_upper(0.99, foo, 1)`)
 
 	// Invalid argument type
 	f(`median_over_time({}, 2)`)

app/vmselect/promql/rollup.go

@@ -65,6 +65,8 @@ var rollupFuncs = map[string]newRollupFunc{
 	"rollup_increase":       newRollupFuncOneArg(rollupFake), // + rollupFuncsRemoveCounterResets
 	"rollup_candlestick":    newRollupFuncOneArg(rollupFake),
 	"aggr_over_time":        newRollupFuncTwoArgs(rollupFake),
+	"hoeffding_bound_upper": newRollupHoeffdingBoundUpper,
+	"hoeffding_bound_lower": newRollupHoeffdingBoundLower,
 }
 
 // rollupAggrFuncs are functions that can be passed to `aggr_over_time()`
@@ -143,6 +145,8 @@ var rollupFuncsKeepMetricGroup = map[string]bool{
 	"quantile_over_time":    true,
 	"rollup":                true,
 	"geomean_over_time":     true,
+	"hoeffding_bound_lower": true,
+	"hoeffding_bound_upper": true,
 }
 
 func getRollupAggrFuncNames(expr metricsql.Expr) ([]string, error) {
@@ -200,7 +204,8 @@ func getRollupArgIdx(funcName string) int {
 		logger.Panicf("BUG: getRollupArgIdx is called for non-rollup func %q", funcName)
 	}
 	switch funcName {
-	case "quantile_over_time", "aggr_over_time":
+	case "quantile_over_time", "aggr_over_time",
+		"hoeffding_bound_lower", "hoeffding_bound_upper":
 		return 1
 	default:
 		return 0
@@ -828,6 +833,66 @@ func newRollupShareFilter(args []interface{}, countFilter func(values []float64,
 	return rf, nil
 }
 
+func newRollupHoeffdingBoundLower(args []interface{}) (rollupFunc, error) {
+	if err := expectRollupArgsNum(args, 2); err != nil {
+		return nil, err
+	}
+	phis, err := getScalar(args[0], 0)
+	if err != nil {
+		return nil, err
+	}
+	rf := func(rfa *rollupFuncArg) float64 {
+		bound, avg := rollupHoeffdingBoundInternal(rfa, phis)
+		return avg - bound
+	}
+	return rf, nil
+}
+
+func newRollupHoeffdingBoundUpper(args []interface{}) (rollupFunc, error) {
+	if err := expectRollupArgsNum(args, 2); err != nil {
+		return nil, err
+	}
+	phis, err := getScalar(args[0], 0)
+	if err != nil {
+		return nil, err
+	}
+	rf := func(rfa *rollupFuncArg) float64 {
+		bound, avg := rollupHoeffdingBoundInternal(rfa, phis)
+		return avg + bound
+	}
+	return rf, nil
+}
+
+func rollupHoeffdingBoundInternal(rfa *rollupFuncArg, phis []float64) (float64, float64) {
+	// There is no need in handling NaNs here, since they must be cleaned up
+	// before calling rollup funcs.
+	values := rfa.values
+	if len(values) == 0 {
+		return nan, nan
+	}
+	if len(values) == 1 {
+		return 0, values[0]
+	}
+	vMax := rollupMax(rfa)
+	vMin := rollupMin(rfa)
+	vAvg := rollupAvg(rfa)
+	vRange := vMax - vMin
+	if vRange <= 0 {
+		return 0, vAvg
+	}
+	phi := phis[rfa.idx]
+	if phi >= 1 {
+		return inf, vAvg
+	}
+	if phi <= 0 {
+		return 0, vAvg
+	}
+	// See https://en.wikipedia.org/wiki/Hoeffding%27s_inequality
+	// and https://www.youtube.com/watch?v=6UwcqiNsZ8U&feature=youtu.be&t=1237
+	bound := vRange * math.Sqrt(math.Log(1/(1-phi))/(2*float64(len(values))))
+	return bound, vAvg
+}
+
 func newRollupQuantile(args []interface{}) (rollupFunc, error) {
 	if err := expectRollupArgsNum(args, 2); err != nil {
 		return nil, err

app/vmselect/promql/rollup_test.go

@@ -310,6 +310,48 @@ func TestRollupHoltWinters(t *testing.T) {
 	f(0.9, 0.9, 33.99637566941818)
 }
 
+func TestRollupHoeffdingBoundLower(t *testing.T) {
+	f := func(phi, vExpected float64) {
+		t.Helper()
+		phis := []*timeseries{{
+			Values:     []float64{phi},
+			Timestamps: []int64{123},
+		}}
+		var me metricsql.MetricExpr
+		args := []interface{}{phis, &metricsql.RollupExpr{Expr: &me}}
+		testRollupFunc(t, "hoeffding_bound_lower", args, &me, vExpected)
+	}
+
+	f(0.5, 28.21949401521037)
+	f(-1, 47.083333333333336)
+	f(0, 47.083333333333336)
+	f(1, -inf)
+	f(2, -inf)
+	f(0.1, 39.72878000047643)
+	f(0.9, 12.701803086472331)
+}
+
+func TestRollupHoeffdingBoundUpper(t *testing.T) {
+	f := func(phi, vExpected float64) {
+		t.Helper()
+		phis := []*timeseries{{
+			Values:     []float64{phi},
+			Timestamps: []int64{123},
+		}}
+		var me metricsql.MetricExpr
+		args := []interface{}{phis, &metricsql.RollupExpr{Expr: &me}}
+		testRollupFunc(t, "hoeffding_bound_upper", args, &me, vExpected)
+	}
+
+	f(0.5, 65.9471726514563)
+	f(-1, 47.083333333333336)
+	f(0, 47.083333333333336)
+	f(1, inf)
+	f(2, inf)
+	f(0.1, 54.43788666619024)
+	f(0.9, 81.46486358019433)
+}
+
 func TestRollupNewRollupFuncSuccess(t *testing.T) {
 	f := func(funcName string, vExpected float64) {
 		t.Helper()

docs/ExtendedPromQL.md

@@ -103,3 +103,5 @@ This functionality can be tried at [an editable Grafana dashboard](http://play-g
 - `aggr_over_time(("aggr_func1", "aggr_func2", ...), m[d])` - simultaneously calculates all the listed `aggr_func*` for `m` over `d` time range.
   `aggr_func*` can contain any functions that accept range vector. For instance, `aggr_over_time(("min_over_time", "max_over_time", "rate"), m[d])`
   would calculate `min_over_time`, `max_over_time` and `rate` for `m[d]`.
+- `hoeffding_bound_upper(phi, m[d])` and `hoeffding_bound_lower(phi, m[d])` - return upper and lower [Hoeffding bounds](https://en.wikipedia.org/wiki/Hoeffding%27s_inequality)
+  for the given `phi` in the range `[0..1]`.

lib/metricsql/rollup.go

@@ -54,6 +54,8 @@ var rollupFuncs = map[string]bool{
 	"rollup_increase":       true,
 	"rollup_candlestick":    true,
 	"aggr_over_time":        true,
+	"hoeffding_bound_upper": true,
+	"hoeffding_bound_lower": true,
 }
 
 // IsRollupFunc returns whether funcName is known rollup function.