app/vmselect/promql: assume that time series value doesnt change during gaps when calculating increase() and delta()

This should remove unexpected spikes at the end of gaps.
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/894

app/vmselect/promql/rollup.go

@@ -326,14 +326,29 @@ func getRollupFunc(funcName string) newRollupFunc {
 }
 
 type rollupFuncArg struct {
-	prevValue     float64
-	prevTimestamp int64
-	values        []float64
-	timestamps    []int64
+	// The value preceeding values if it fits staleness interval.
+	prevValue float64
 
+	// The timestamp for prevValue.
+	prevTimestamp int64
+
+	// Values that fit window ending at currTimestamp.
+	values []float64
+
+	// Timestamps for values.
+	timestamps []int64
+
+	// Actual value preceeding values without restrictions on staleness interval.
+	realPrevValue float64
+
+	// Current timestamp for rollup evaluation.
 	currTimestamp int64
-	idx           int
-	window        int64
+
+	// Index for the currently evaluated point relative to time range for query evaluation.
+	idx int
+
+	// Time window for rollup calculations.
+	window int64
 
 	tsm *timeseriesMap
 }
@@ -538,6 +553,11 @@ func (rc *rollupConfig) doInternal(dstValues []float64, tsm *timeseriesMap, valu
 			rfa.values = nil
 			rfa.timestamps = nil
 		}
+		if i > 0 {
+			rfa.realPrevValue = values[i-1]
+		} else {
+			rfa.realPrevValue = nan
+		}
 		rfa.currTimestamp = tEnd
 		value := rc.Func(rfa)
 		rfa.idx++
@@ -1244,6 +1264,12 @@ func rollupDelta(rfa *rollupFuncArg) float64 {
 		if len(values) == 0 {
 			return nan
 		}
+		if !math.IsNaN(rfa.realPrevValue) {
+			// Assume that the value didn't change during the current gap.
+			// This should fix high delta() and increase() values at the end of gaps.
+			// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/894
+			return values[len(values)-1] - rfa.realPrevValue
+		}
 		// Assume that the previous non-existing value was 0
 		// only if the first value doesn't exceed too much the delta with the next value.
 		//

app/vmselect/promql/rollup_test.go

@@ -1103,11 +1103,12 @@ func testRowsEqual(t *testing.T, values []float64, timestamps []int64, valuesExp
 }
 
 func TestRollupDelta(t *testing.T) {
-	f := func(prevValue float64, values []float64, resultExpected float64) {
+	f := func(prevValue, realPrevValue float64, values []float64, resultExpected float64) {
 		t.Helper()
 		rfa := &rollupFuncArg{
-			prevValue: prevValue,
-			values:    values,
+			prevValue:     prevValue,
+			values:        values,
+			realPrevValue: realPrevValue,
 		}
 		result := rollupDelta(rfa)
 		if math.IsNaN(result) {
@@ -1120,22 +1121,26 @@ func TestRollupDelta(t *testing.T) {
 			t.Fatalf("unexpected result; got %v; want %v", result, resultExpected)
 		}
 	}
-	f(nan, nil, nan)
+	f(nan, nan, nil, nan)
 
 	// Small initial value
-	f(nan, []float64{1}, 1)
-	f(nan, []float64{10}, 10)
-	f(nan, []float64{100}, 100)
-	f(nan, []float64{1, 2, 3}, 3)
-	f(1, []float64{1, 2, 3}, 2)
-	f(nan, []float64{5, 6, 8}, 8)
-	f(2, []float64{5, 6, 8}, 6)
+	f(nan, nan, []float64{1}, 1)
+	f(nan, nan, []float64{10}, 10)
+	f(nan, nan, []float64{100}, 100)
+	f(nan, nan, []float64{1, 2, 3}, 3)
+	f(1, nan, []float64{1, 2, 3}, 2)
+	f(nan, nan, []float64{5, 6, 8}, 8)
+	f(2, nan, []float64{5, 6, 8}, 6)
 
 	// Too big initial value must be skipped.
-	f(nan, []float64{1000}, 0)
-	f(nan, []float64{1000, 1001, 1002}, 2)
+	f(nan, nan, []float64{1000}, 0)
+	f(nan, nan, []float64{1000, 1001, 1002}, 2)
+
+	// Delta calculations against non-nan realPrevValue
+	f(nan, 900, []float64{1000}, 100)
+	f(nan, 900, []float64{1000, 1001, 1002}, 102)
 
 	// Empty values
-	f(1, nil, 0)
-	f(100, nil, 0)
+	f(1, nan, nil, 0)
+	f(100, nan, nil, 0)
 }

docs/CHANGELOG.md

@@ -15,6 +15,7 @@
 
 * BUGFIX: do not return data points in the end of the selected time range for time series ending in the middle of the selected time range.
   See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/887 and https://github.com/VictoriaMetrics/VictoriaMetrics/issues/845
+* BUGFIX: remove spikes at the end of time series gaps for `increase()` or `delta()` functions. See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/894
 * BUGFIX: vminsert: properly return HTTP 503 status code when all the vmstorage nodes are unavailable. See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/896