app/vmselect: do not return empty time series in /api/v1/query result

app/vmselect/prometheus/prometheus.go

@@ -426,7 +426,7 @@ func QueryHandler(w http.ResponseWriter, r *http.Request) error {
 		Step:     step,
 		Deadline: deadline,
 	}
-	result, err := promql.Exec(&ec, query)
+	result, err := promql.Exec(&ec, query, true)
 	if err != nil {
 		return fmt.Errorf("cannot execute %q: %s", query, err)
 	}
@@ -484,7 +484,7 @@ func QueryRangeHandler(w http.ResponseWriter, r *http.Request) error {
 		Deadline: deadline,
 		MayCache: mayCache,
 	}
-	result, err := promql.Exec(&ec, query)
+	result, err := promql.Exec(&ec, query, false)
 	if err != nil {
 		return fmt.Errorf("cannot execute %q: %s", query, err)
 	}

app/vmselect/promql/exec.go

@@ -27,8 +27,8 @@ func ExpandWithExprs(q string) (string, error) {
 	return string(buf), nil
 }
 
-// Exec executes q for the given ec until the deadline.
+// Exec executes q for the given ec.
-func Exec(ec *EvalConfig, q string) ([]netstorage.Result, error) {
+func Exec(ec *EvalConfig, q string, isFirstPointOnly bool) ([]netstorage.Result, error) {
 	if *logSlowQueryDuration > 0 {
 		startTime := time.Now()
 		defer func() {
@@ -65,6 +65,14 @@ func Exec(ec *EvalConfig, q string) ([]netstorage.Result, error) {
 	}
 	ec.End -= ec.Step
 
+	if isFirstPointOnly {
+		// Remove all the points except the first one from every time series.
+		for _, ts := range rv {
+			ts.Values = ts.Values[:1]
+			ts.Timestamps = ts.Timestamps[:1]
+		}
+	}
+
 	maySort := maySortResults(e, rv)
 	result, err := timeseriesToResult(rv, maySort)
 	if err != nil {

app/vmselect/promql/exec_test.go

@@ -62,7 +62,7 @@ func TestExecSuccess(t *testing.T) {
 			Deadline: netstorage.NewDeadline(time.Minute),
 		}
 		for i := 0; i < 5; i++ {
-			result, err := Exec(ec, q)
+			result, err := Exec(ec, q, false)
 			if err != nil {
 				t.Fatalf(`unexpected error when executing %q: %s`, q, err)
 			}
@@ -3665,7 +3665,14 @@ func TestExecError(t *testing.T) {
 			Deadline: netstorage.NewDeadline(time.Minute),
 		}
 		for i := 0; i < 4; i++ {
-			rv, err := Exec(ec, q)
+			rv, err := Exec(ec, q, false)
+			if err == nil {
+				t.Fatalf(`expecting non-nil error on %q`, q)
+			}
+			if rv != nil {
+				t.Fatalf(`expecting nil rv`)
+			}
+			rv, err = Exec(ec, q, true)
 			if err == nil {
 				t.Fatalf(`expecting non-nil error on %q`, q)
 			}