Allow derivative() function to be used with ORDER BY desc

The derivative function had an arbitrary limitation that would cause it
to set the value to zero if the previous value was after the next value.
This caused all `ORDER BY desc` queries with `derivative()` to always
return zero values.

Fixes #4675.

CHANGELOG.md

@@ -13,6 +13,7 @@
 - [#3166](https://github.com/influxdata/influxdb/issues/3166): Sort the series keys inside of a tag set so output is deterministic.
 - [#1856](https://github.com/influxdata/influxdb/issues/1856): Add `elapsed` function that returns the time delta between subsequent points.
 - [#5502](https://github.com/influxdata/influxdb/issues/5502): Add checksum verification to TSM inspect tool
+- [#4675](https://github.com/influxdata/influxdb/issues/4675): Allow derivative() function to be used with ORDER BY desc.
 
 ### Bugfixes
 

influxql/functions.go

@@ -98,11 +98,7 @@ func (r *FloatDerivativeReducer) Emit() []FloatPoint {
 		if !r.ascending {
 			elapsed = -elapsed
 		}
-
-		value := 0.0
-		if elapsed > 0 {
-			value = diff / (float64(elapsed) / float64(r.interval.Duration))
-		}
+		value := diff / (float64(elapsed) / float64(r.interval.Duration))
 
 		// Drop negative values for non-negative derivatives.
 		if r.isNonNegative && diff < 0 {
@@ -149,11 +145,7 @@ func (r *IntegerDerivativeReducer) Emit() []FloatPoint {
 		if !r.ascending {
 			elapsed = -elapsed
 		}
-
-		value := 0.0
-		if elapsed > 0 {
-			value = diff / (float64(elapsed) / float64(r.interval.Duration))
-		}
+		value := diff / (float64(elapsed) / float64(r.interval.Duration))
 
 		// Drop negative values for non-negative derivatives.
 		if r.isNonNegative && diff < 0 {

influxql/select_test.go

@@ -1895,6 +1895,54 @@ func TestSelect_Derivative_Integer(t *testing.T) {
 	}
 }
 
+func TestSelect_Derivative_Desc_Float(t *testing.T) {
+	var ic IteratorCreator
+	ic.CreateIteratorFn = func(opt influxql.IteratorOptions) (influxql.Iterator, error) {
+		return &FloatIterator{Points: []influxql.FloatPoint{
+			{Name: "cpu", Time: 12 * Second, Value: 3},
+			{Name: "cpu", Time: 8 * Second, Value: 19},
+			{Name: "cpu", Time: 4 * Second, Value: 10},
+			{Name: "cpu", Time: 0 * Second, Value: 20},
+		}}, nil
+	}
+
+	// Execute selection.
+	itrs, err := influxql.Select(MustParseSelectStatement(`SELECT derivative(value, 1s) FROM cpu WHERE time >= '1970-01-01T00:00:00Z' AND time < '1970-01-01T00:00:16Z' ORDER BY desc`), &ic, nil)
+	if err != nil {
+		t.Fatal(err)
+	} else if a := Iterators(itrs).ReadAll(); !deep.Equal(a, [][]influxql.Point{
+		{&influxql.FloatPoint{Name: "cpu", Time: 8 * Second, Value: 4}},
+		{&influxql.FloatPoint{Name: "cpu", Time: 4 * Second, Value: -2.25}},
+		{&influxql.FloatPoint{Name: "cpu", Time: 0 * Second, Value: 2.5}},
+	}) {
+		t.Fatalf("unexpected points: %s", spew.Sdump(a))
+	}
+}
+
+func TestSelect_Derivative_Desc_Integer(t *testing.T) {
+	var ic IteratorCreator
+	ic.CreateIteratorFn = func(opt influxql.IteratorOptions) (influxql.Iterator, error) {
+		return &IntegerIterator{Points: []influxql.IntegerPoint{
+			{Name: "cpu", Time: 12 * Second, Value: 3},
+			{Name: "cpu", Time: 8 * Second, Value: 19},
+			{Name: "cpu", Time: 4 * Second, Value: 10},
+			{Name: "cpu", Time: 0 * Second, Value: 20},
+		}}, nil
+	}
+
+	// Execute selection.
+	itrs, err := influxql.Select(MustParseSelectStatement(`SELECT derivative(value, 1s) FROM cpu WHERE time >= '1970-01-01T00:00:00Z' AND time < '1970-01-01T00:00:16Z' ORDER BY desc`), &ic, nil)
+	if err != nil {
+		t.Fatal(err)
+	} else if a := Iterators(itrs).ReadAll(); !deep.Equal(a, [][]influxql.Point{
+		{&influxql.FloatPoint{Name: "cpu", Time: 8 * Second, Value: 4}},
+		{&influxql.FloatPoint{Name: "cpu", Time: 4 * Second, Value: -2.25}},
+		{&influxql.FloatPoint{Name: "cpu", Time: 0 * Second, Value: 2.5}},
+	}) {
+		t.Fatalf("unexpected points: %s", spew.Sdump(a))
+	}
+}
+
 func TestSelect_Difference_Float(t *testing.T) {
 	var ic IteratorCreator
 	ic.CreateIteratorFn = func(opt influxql.IteratorOptions) (influxql.Iterator, error) {