app/vmselect/promql: add drop_empty_series() function for dropping empty series before performing additional calculations

This can be useful in the following queries:

   drop_empty_series(temperature <= 30) default 40

This query drops temperature series with all the values bigger than 30 on the selected time range,
while replacing gaps in the remaining series with 40.

The query without drop_empty_series:

  (temperature <= 30) default 40

would leave all the temperature series with all the values bigger than 30 on the selected time range,
and replace all their values with 40. This is not what could be epxected in some cases
like here - https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5071

app/vmselect/promql/exec_test.go

@@ -2013,6 +2013,46 @@ func TestExecSuccess(t *testing.T) {
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})
+	t.Run(`drop_empty_series()`, func(t *testing.T) {
+		t.Parallel()
+		q := `sort(drop_empty_series(
+			(
+				alias(time(), "foo"),
+				alias(500 + time(), "bar"),
+			) > 2000
+		) default 123)`
+		r := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{123, 123, 123, 2100, 2300, 2500},
+			Timestamps: timestampsExpected,
+		}
+		r.MetricName.MetricGroup = []byte("bar")
+		resultExpected := []netstorage.Result{r}
+		f(q, resultExpected)
+	})
+	t.Run(`no drop_empty_series()`, func(t *testing.T) {
+		t.Parallel()
+		q := `sort((
+			(
+				alias(time(), "foo"),
+				alias(500 + time(), "bar"),
+			) > 2000
+		) default 123)`
+		r1 := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{123, 123, 123, 123, 123, 123},
+			Timestamps: timestampsExpected,
+		}
+		r1.MetricName.MetricGroup = []byte("foo")
+		r2 := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{123, 123, 123, 2100, 2300, 2500},
+			Timestamps: timestampsExpected,
+		}
+		r2.MetricName.MetricGroup = []byte("bar")
+		resultExpected := []netstorage.Result{r1, r2}
+		f(q, resultExpected)
+	})
 	t.Run(`drop_common_labels(single_series)`, func(t *testing.T) {
 		t.Parallel()
 		q := `drop_common_labels(label_set(time(), "foo", "bar", "__name__", "xxx", "q", "we"))`
@@ -8960,6 +9000,8 @@ func TestExecError(t *testing.T) {
 	f(`delta_prometheus()`)
 	f(`rollup_candlestick()`)
 	f(`rollup()`)
+	f(`drop_empty_series()`)
+	f(`drop_common_labels()`)
 
 	// Invalid argument type
 	f(`median_over_time({}, 2)`)

app/vmselect/promql/transform.go

@@ -45,6 +45,7 @@ var transformFuncs = map[string]transformFunc{
 	"days_in_month":              newTransformFuncDateTime(transformDaysInMonth),
 	"deg":                        newTransformFuncOneArg(transformDeg),
 	"drop_common_labels":         transformDropCommonLabels,
+	"drop_empty_series":          transformDropEmptySeries,
 	"end":                        newTransformFuncZeroArgs(transformEnd),
 	"exp":                        newTransformFuncOneArg(transformExp),
 	"floor":                      newTransformFuncOneArg(transformFloor),
@@ -1840,6 +1841,15 @@ func transformDropCommonLabels(tfa *transformFuncArg) ([]*timeseries, error) {
 	return rvs, nil
 }
 
+func transformDropEmptySeries(tfa *transformFuncArg) ([]*timeseries, error) {
+	args := tfa.args
+	if len(args) != 1 {
+		return nil, fmt.Errorf("unexpected number of args; got %d; want 1", len(args))
+	}
+	rvs := removeEmptySeries(args[0])
+	return rvs, nil
+}
+
 func transformLabelCopy(tfa *transformFuncArg) ([]*timeseries, error) {
 	return transformLabelCopyExt(tfa, false)
 }

docs/CHANGELOG.md

@@ -32,6 +32,7 @@ The sandbox cluster installation is running under the constant load generated by
 
 * SECURITY: upgrade Go builder from Go1.21.1 to Go1.21.3. See [the list of issues addressed in Go1.21.2](https://github.com/golang/go/issues?q=milestone%3AGo1.21.2+label%3ACherryPickApproved) and [the list of issues addressed in Go1.21.3](https://github.com/golang/go/issues?q=milestone%3AGo1.21.3+label%3ACherryPickApproved).
 
+* FEATURE: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): add [drop_empty_series()](https://docs.victoriametrics.com/MetricsQL.html#drop_empty_series) function, which can be used for filtering out empty series before performing additional calculations as shown in [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5071).
 * FEATURE: [vmalert](https://docs.victoriametrics.com/vmalert.html): add `eval_alignment` attribute for [Groups](https://docs.victoriametrics.com/vmalert.html#groups), it will align group query requests timestamp with interval like `datasource.queryTimeAlignment` did.
   This also means that `datasource.queryTimeAlignment` command-line flag becomes deprecated now and will have no effect if configured. If `datasource.queryTimeAlignment` was set to `false` before, then `eval_alignment` has to be set to `false` explicitly under group.
   See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5049).

docs/MetricsQL.md

@@ -1055,6 +1055,17 @@ Metric names are stripped from the resulting series. Add [keep_metric_names](#ke
 
 This function is supported by PromQL. See also [rad](#rad).
 
+#### drop_empty_series
+
+`drop_empty_series(q) is a [transform function](#transform-functions), which drops empty series from `q`.
+
+This function can be used when `default` operator should be applied only to non-empty series. For example,
+`drop_empty_series(temperature < 30) default 42` returns series, which have at least a single sample smaller than 30 on the selected time range,
+while filling gaps in the returned series with 42.
+
+On the other hand `(temperature < 30) default 40` returns all the `temperature` series, even if they have no samples smaller than 30,
+by replacing all the values bigger or equal to 30 with 40.
+
 #### end
 
 `end()` is a [transform function](#transform-functions), which returns the unix timestamp in seconds for the last point.

go.mod

@@ -12,7 +12,7 @@ require (
 	// like https://github.com/valyala/fasthttp/commit/996610f021ff45fdc98c2ce7884d5fa4e7f9199b
 	github.com/VictoriaMetrics/fasthttp v1.2.0
 	github.com/VictoriaMetrics/metrics v1.24.0
-	github.com/VictoriaMetrics/metricsql v0.66.1
+	github.com/VictoriaMetrics/metricsql v0.67.0
 	github.com/aws/aws-sdk-go-v2 v1.21.2
 	github.com/aws/aws-sdk-go-v2/config v1.18.45
 	github.com/aws/aws-sdk-go-v2/feature/s3/manager v1.11.90

go.sum

@@ -70,8 +70,8 @@ github.com/VictoriaMetrics/fasthttp v1.2.0 h1:nd9Wng4DlNtaI27WlYh5mGXCJOmee/2c2b
 github.com/VictoriaMetrics/fasthttp v1.2.0/go.mod h1:zv5YSmasAoSyv8sBVexfArzFDIGGTN4TfCKAtAw7IfE=
 github.com/VictoriaMetrics/metrics v1.24.0 h1:ILavebReOjYctAGY5QU2F9X0MYvkcrG3aEn2RKa1Zkw=
 github.com/VictoriaMetrics/metrics v1.24.0/go.mod h1:eFT25kvsTidQFHb6U0oa0rTrDRdz4xTYjpL8+UPohys=
-github.com/VictoriaMetrics/metricsql v0.66.1 h1:H+HfOHOznVNml0O3QkXarFxrlY+enrcWRUZ1xX73Kig=
-github.com/VictoriaMetrics/metricsql v0.66.1/go.mod h1:k4UaP/+CjuZslIjd+kCigNG9TQmUqh5v0TP/nMEy90I=
+github.com/VictoriaMetrics/metricsql v0.67.0 h1:IYbKA6rhd8UWmW1LbaKiEk1q5ixFQagu9jL6z3Mt03o=
+github.com/VictoriaMetrics/metricsql v0.67.0/go.mod h1:k4UaP/+CjuZslIjd+kCigNG9TQmUqh5v0TP/nMEy90I=
 github.com/VividCortex/ewma v1.2.0 h1:f58SaIzcDXrSy3kWaHNvuJgJ3Nmz59Zji6XoJR/q1ow=
 github.com/VividCortex/ewma v1.2.0/go.mod h1:nz4BbCtbLyFDeC9SUHbtcT5644juEuWfUAUnGx7j5l4=
 github.com/alecthomas/template v0.0.0-20160405071501-a0175ee3bccc/go.mod h1:LOuyumcjzFXgccqObfd/Ljyb9UuFJ6TxHnclSeseNhc=

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

@@ -29,6 +29,7 @@ var transformFuncs = map[string]bool{
 	"days_in_month":              true,
 	"deg":                        true,
 	"drop_common_labels":         true,
+	"drop_empty_series":          true,
 	"end":                        true,
 	"exp":                        true,
 	"floor":                      true,

vendor/modules.txt

@@ -99,7 +99,7 @@ github.com/VictoriaMetrics/fasthttp/stackless
 # github.com/VictoriaMetrics/metrics v1.24.0
 ## explicit; go 1.20
 github.com/VictoriaMetrics/metrics
-# github.com/VictoriaMetrics/metricsql v0.66.1
+# github.com/VictoriaMetrics/metricsql v0.67.0
 ## explicit; go 1.13
 github.com/VictoriaMetrics/metricsql
 github.com/VictoriaMetrics/metricsql/binaryop