app/vmselect: add interpolate function for filling gaps with linearly interpolated values

See https://stackoverflow.com/q/62565021/274937 for details

app/vmselect/promql/exec_test.go

@@ -4113,6 +4113,44 @@ func TestExecSuccess(t *testing.T) {
 		resultExpected := []netstorage.Result{r1}
 		f(q, resultExpected)
 	})
+	t.Run(`interpolate()`, func(t *testing.T) {
+		t.Parallel()
+		q := `interpolate(label_set(time() < 1300 default time() > 1700, "__name__", "foobar", "x", "y"))`
+		r1 := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{1000, 1200, 1400, 1600, 1800, 2000},
+			Timestamps: timestampsExpected,
+		}
+		r1.MetricName.MetricGroup = []byte("foobar")
+		r1.MetricName.Tags = []storage.Tag{{
+			Key:   []byte("x"),
+			Value: []byte("y"),
+		}}
+		resultExpected := []netstorage.Result{r1}
+		f(q, resultExpected)
+	})
+	t.Run(`interpolate(tail)`, func(t *testing.T) {
+		t.Parallel()
+		q := `interpolate(time() < 1300)`
+		r1 := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{1000, 1200, 1200, 1200, 1200, 1200},
+			Timestamps: timestampsExpected,
+		}
+		resultExpected := []netstorage.Result{r1}
+		f(q, resultExpected)
+	})
+	t.Run(`interpolate(head)`, func(t *testing.T) {
+		t.Parallel()
+		q := `interpolate(time() > 1500)`
+		r1 := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{1600, 1600, 1600, 1600, 1800, 2000},
+			Timestamps: timestampsExpected,
+		}
+		resultExpected := []netstorage.Result{r1}
+		f(q, resultExpected)
+	})
 	t.Run(`distinct_over_time([500s])`, func(t *testing.T) {
 		t.Parallel()
 		q := `distinct_over_time((time() < 1700)[500s])`
@@ -5609,6 +5647,7 @@ func TestExecError(t *testing.T) {
 	f(`median("foo", "bar")`)
 	f(`keep_last_value()`)
 	f(`keep_next_value()`)
+	f(`interpolate()`)
 	f(`distinct_over_time()`)
 	f(`distinct()`)
 	f(`alias()`)

app/vmselect/promql/transform.go

@@ -72,6 +72,7 @@ var transformFuncs = map[string]transformFunc{
 	"":                   transformUnion, // empty func is a synonim to union
 	"keep_last_value":    transformKeepLastValue,
 	"keep_next_value":    transformKeepNextValue,
+	"interpolate":        transformInterpolate,
 	"start":              newTransformFuncZeroArgs(transformStart),
 	"end":                newTransformFuncZeroArgs(transformEnd),
 	"step":               newTransformFuncZeroArgs(transformStep),
@@ -764,6 +765,52 @@ func transformKeepNextValue(tfa *transformFuncArg) ([]*timeseries, error) {
 	return rvs, nil
 }
 
+func transformInterpolate(tfa *transformFuncArg) ([]*timeseries, error) {
+	args := tfa.args
+	if err := expectTransformArgsNum(args, 1); err != nil {
+		return nil, err
+	}
+	rvs := args[0]
+	for _, ts := range rvs {
+		values := ts.Values
+		if len(values) == 0 {
+			continue
+		}
+		prevValue := nan
+		var nextValue float64
+		for i := 0; i < len(values); i++ {
+			if !math.IsNaN(values[i]) {
+				continue
+			}
+			if i > 0 {
+				prevValue = values[i-1]
+			}
+			j := i + 1
+			for j < len(values) {
+				if !math.IsNaN(values[j]) {
+					break
+				}
+				j++
+			}
+			if j >= len(values) {
+				nextValue = prevValue
+			} else {
+				nextValue = values[j]
+			}
+			if math.IsNaN(prevValue) {
+				prevValue = nextValue
+			}
+			delta := (nextValue - prevValue) / float64(j-i+1)
+			for i < j {
+				prevValue += delta
+				values[i] = prevValue
+				i++
+			}
+		}
+	}
+	return rvs, nil
+}
+
 func newTransformFuncRunning(rf func(a, b float64, idx int) float64) transformFunc {
 	return func(tfa *transformFuncArg) ([]*timeseries, error) {
 		args := tfa.args

docs/MetricsQL.md

@@ -80,6 +80,7 @@ This functionality can be tried at [an editable Grafana dashboard](http://play-g
   Use `limitk(1, q)` if you need retaining all the labels from `q`.
 - `keep_last_value(q)` - fills missing data (gaps) in `q` with the previous non-empty value.
 - `keep_next_value(q)` - fills missing data (gaps) in `q` with the next non-empty value.
+- `interpolate(q)` - fills missing data (gaps) in `q` with linearly interpolated values.
 - `distinct_over_time(m[d])` - returns distinct number of values for `m` data points over `d` duration.
 - `distinct(q)` - returns a time series with the number of unique values for each timestamp in `q`.
 - `sum2_over_time(m[d])` - returns sum of squares for all the `m` values over `d` duration.

go.mod

@@ -9,7 +9,7 @@ require (
 	// like https://github.com/valyala/fasthttp/commit/996610f021ff45fdc98c2ce7884d5fa4e7f9199b
 	github.com/VictoriaMetrics/fasthttp v1.0.1
 	github.com/VictoriaMetrics/metrics v1.11.3
-	github.com/VictoriaMetrics/metricsql v0.2.3
+	github.com/VictoriaMetrics/metricsql v0.2.4
 	github.com/aws/aws-sdk-go v1.32.13
 	github.com/cespare/xxhash/v2 v2.1.1
 	github.com/golang/snappy v0.0.1

go.sum

@@ -53,8 +53,8 @@ github.com/VictoriaMetrics/metrics v1.11.2 h1:t/ceLP6SvagUqypCKU7cI7+tQn54+TIV/t
 github.com/VictoriaMetrics/metrics v1.11.2/go.mod h1:LU2j9qq7xqZYXz8tF3/RQnB2z2MbZms5TDiIg9/NHiQ=
 github.com/VictoriaMetrics/metrics v1.11.3 h1:eSfXc0CrquKa1VTNUvhP+dhNjLUZHQGTFfp19mYCQWE=
 github.com/VictoriaMetrics/metrics v1.11.3/go.mod h1:LU2j9qq7xqZYXz8tF3/RQnB2z2MbZms5TDiIg9/NHiQ=
-github.com/VictoriaMetrics/metricsql v0.2.3 h1:xGscDmLoeIV7+8qX/mdHnOY0vu4m+wHIVGMoy/nBovY=
-github.com/VictoriaMetrics/metricsql v0.2.3/go.mod h1:UIjd9S0W1UnTWlJdM0wLS+2pfuPqjwqKoK8yTos+WyE=
+github.com/VictoriaMetrics/metricsql v0.2.4 h1:240YwT8B4KITVFE7EOrf1rVvsY+5fYsAzyb+bI6/q50=
+github.com/VictoriaMetrics/metricsql v0.2.4/go.mod h1:UIjd9S0W1UnTWlJdM0wLS+2pfuPqjwqKoK8yTos+WyE=
 github.com/allegro/bigcache v1.2.1-0.20190218064605-e24eb225f156 h1:eMwmnE/GDgah4HI848JfFxHt+iPb26b4zyfspmqY0/8=
 github.com/allegro/bigcache v1.2.1-0.20190218064605-e24eb225f156/go.mod h1:Cb/ax3seSYIx7SuZdm2G2xzfwmv3TPSk2ucNfQESPXM=
 github.com/aws/aws-sdk-go v1.32.13 h1:zzyXF7SUxJcJa3hTcYCl1/Ey+kh2N8TjK5tWnL0wieo=

vendor/github.com/VictoriaMetrics/metricsql/transform.go

@@ -51,6 +51,7 @@ var transformFuncs = map[string]bool{
 	"":                   true, // empty func is a synonim to union
 	"keep_last_value":    true,
 	"keep_next_value":    true,
+	"interpolate":        true,
 	"start":              true,
 	"end":                true,
 	"step":               true,

vendor/modules.txt

@@ -16,7 +16,7 @@ github.com/VictoriaMetrics/fasthttp/fasthttputil
 github.com/VictoriaMetrics/fasthttp/stackless
 # github.com/VictoriaMetrics/metrics v1.11.3
 github.com/VictoriaMetrics/metrics
-# github.com/VictoriaMetrics/metricsql v0.2.3
+# github.com/VictoriaMetrics/metricsql v0.2.4
 github.com/VictoriaMetrics/metricsql
 github.com/VictoriaMetrics/metricsql/binaryop
 # github.com/aws/aws-sdk-go v1.32.13