app/vmselect/promql: remove spikes from increase() and delta() results on time series with spare irregular data points

Do not take into account spare data point value if the next point will is located too far from the current point.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/894

app/vmselect/promql/rollup.go

@@ -338,9 +338,12 @@ type rollupFuncArg struct {
 	// Timestamps for values.
 	timestamps []int64
 
-	// Actual value preceeding values without restrictions on staleness interval.
+	// Real value preceeding values without restrictions on staleness interval.
 	realPrevValue float64
 
+	// Real value which goes after values.
+	realNextValue float64
+
 	// Current timestamp for rollup evaluation.
 	currTimestamp int64
 
@@ -558,6 +561,11 @@ func (rc *rollupConfig) doInternal(dstValues []float64, tsm *timeseriesMap, valu
 		} else {
 			rfa.realPrevValue = nan
 		}
+		if j < len(values) {
+			rfa.realNextValue = values[j]
+		} else {
+			rfa.realNextValue = nan
+		}
 		rfa.currTimestamp = tEnd
 		value := rc.Func(rfa)
 		rfa.idx++
@@ -1282,6 +1290,8 @@ func rollupDelta(rfa *rollupFuncArg) float64 {
 		d := float64(10)
 		if len(values) > 1 {
 			d = values[1] - values[0]
+		} else if !math.IsNaN(rfa.realNextValue) {
+			d = rfa.realNextValue - values[0]
 		}
 		if math.Abs(values[0]) < 10*(math.Abs(d)+1) {
 			prevValue = 0

app/vmselect/promql/rollup_test.go

@@ -1103,12 +1103,13 @@ func testRowsEqual(t *testing.T, values []float64, timestamps []int64, valuesExp
 }
 
 func TestRollupDelta(t *testing.T) {
-	f := func(prevValue, realPrevValue float64, values []float64, resultExpected float64) {
+	f := func(prevValue, realPrevValue, realNextValue float64, values []float64, resultExpected float64) {
 		t.Helper()
 		rfa := &rollupFuncArg{
 			prevValue:     prevValue,
 			values:        values,
 			realPrevValue: realPrevValue,
+			realNextValue: realNextValue,
 		}
 		result := rollupDelta(rfa)
 		if math.IsNaN(result) {
@@ -1121,26 +1122,36 @@ func TestRollupDelta(t *testing.T) {
 			t.Fatalf("unexpected result; got %v; want %v", result, resultExpected)
 		}
 	}
-	f(nan, nan, nil, nan)
+	f(nan, nan, nan, nil, nan)
 
 	// Small initial value
-	f(nan, nan, []float64{1}, 1)
+	f(nan, nan, nan, []float64{1}, 1)
-	f(nan, nan, []float64{10}, 10)
+	f(nan, nan, nan, []float64{10}, 10)
-	f(nan, nan, []float64{100}, 100)
+	f(nan, nan, nan, []float64{100}, 100)
-	f(nan, nan, []float64{1, 2, 3}, 3)
+	f(nan, nan, nan, []float64{1, 2, 3}, 3)
-	f(1, nan, []float64{1, 2, 3}, 2)
+	f(1, nan, nan, []float64{1, 2, 3}, 2)
-	f(nan, nan, []float64{5, 6, 8}, 8)
+	f(nan, nan, nan, []float64{5, 6, 8}, 8)
-	f(2, nan, []float64{5, 6, 8}, 6)
+	f(2, nan, nan, []float64{5, 6, 8}, 6)
 
 	// Too big initial value must be skipped.
-	f(nan, nan, []float64{1000}, 0)
+	f(nan, nan, nan, []float64{1000}, 0)
-	f(nan, nan, []float64{1000, 1001, 1002}, 2)
+	f(nan, nan, nan, []float64{1000, 1001, 1002}, 2)
 
-	// Delta calculations against non-nan realPrevValue
+	// Non-nan realPrevValue
-	f(nan, 900, []float64{1000}, 100)
+	f(nan, 900, nan, []float64{1000}, 100)
-	f(nan, 900, []float64{1000, 1001, 1002}, 102)
+	f(nan, 1000, nan, []float64{1000}, 0)
+	f(nan, 1100, nan, []float64{1000}, -100)
+	f(nan, 900, nan, []float64{1000, 1001, 1002}, 102)
+
+	// Small delta between realNextValue and values
+	f(nan, nan, 990, []float64{1000}, 0)
+	f(nan, nan, 1005, []float64{1000}, 0)
+
+	// Big delta between relaNextValue and values
+	f(nan, nan, 800, []float64{1000}, 1000)
+	f(nan, nan, 1300, []float64{1000}, 1000)
 
 	// Empty values
-	f(1, nan, nil, 0)
+	f(1, nan, nan, nil, 0)
-	f(100, nan, nil, 0)
+	f(100, nan, nan, nil, 0)
 }