Add interface for heap to support Reverse for `order by desc`

CHANGELOG.md

@@ -57,6 +57,7 @@ There are breaking changes in this release:
 - [#4685](https://github.com/influxdb/influxdb/pull/4685): Automatically promote node to raft peer if drop server results in removing a raft peer.
 
 ### Bugfixes
+- [#4235](https://github.com/influxdb/influxdb/issues/4235): "ORDER BY DESC" doesn't properly order
 - [#4789](https://github.com/influxdb/influxdb/pull/4789): Decode WHERE fields during aggregates. Fix [issue #4701](https://github.com/influxdb/influxdb/issues/4701).
 - [#4778](https://github.com/influxdb/influxdb/pull/4778): If there are no points to count, count is 0.
 - [#4715](https://github.com/influxdb/influxdb/pull/4715): Fix panic during Raft-close. Fix [issue #4707](https://github.com/influxdb/influxdb/issues/4707). Thanks @oiooj

cmd/influxd/run/server_test.go

@@ -5118,6 +5118,11 @@ func TestServer_Query_OrderByTime(t *testing.T) {
 		fmt.Sprintf(`cpu,host=server1 value=1 %d`, mustParseTime(time.RFC3339Nano, "2000-01-01T00:00:01Z").UnixNano()),
 		fmt.Sprintf(`cpu,host=server1 value=2 %d`, mustParseTime(time.RFC3339Nano, "2000-01-01T00:00:02Z").UnixNano()),
 		fmt.Sprintf(`cpu,host=server1 value=3 %d`, mustParseTime(time.RFC3339Nano, "2000-01-01T00:00:03Z").UnixNano()),
+
+		fmt.Sprintf(`power,presence=true value=1 %d`, mustParseTime(time.RFC3339Nano, "2000-01-01T00:00:01Z").UnixNano()),
+		fmt.Sprintf(`power,presence=true value=2 %d`, mustParseTime(time.RFC3339Nano, "2000-01-01T00:00:02Z").UnixNano()),
+		fmt.Sprintf(`power,presence=true value=3 %d`, mustParseTime(time.RFC3339Nano, "2000-01-01T00:00:03Z").UnixNano()),
+		fmt.Sprintf(`power,presence=false value=4 %d`, mustParseTime(time.RFC3339Nano, "2000-01-01T00:00:04Z").UnixNano()),
 	}
 
 	test := NewTest("db0", "rp0")
@@ -5130,6 +5135,13 @@ func TestServer_Query_OrderByTime(t *testing.T) {
 			command: `select value from "cpu" ORDER BY time DESC`,
 			exp:     `{"results":[{"series":[{"name":"cpu","columns":["time","value"],"values":[["2000-01-01T00:00:03Z",3],["2000-01-01T00:00:02Z",2],["2000-01-01T00:00:01Z",1]]}]}]}`,
 		},
+
+		&Query{
+			name:    "order desc with tags",
+			params:  url.Values{"db": []string{"db0"}},
+			command: `select value from "power" ORDER BY time DESC`,
+			exp:     `{"results":[{"series":[{"name":"power","columns":["time","value"],"values":[["2000-01-01T00:00:04Z",4],["2000-01-01T00:00:03Z",3],["2000-01-01T00:00:02Z",2],["2000-01-01T00:00:01Z",1]]}]}]}`,
+		},
 	}...)
 
 	for i, query := range test.queries {

tsdb/cursor.go

@@ -153,22 +153,24 @@ type TagSetCursor struct {
 	tags          map[string]string // Tag key-value pairs
 	cursors       []*TagsCursor     // Underlying tags cursors.
 	currentTags   map[string]string // the current tags for the underlying series cursor in play
+	ascending     bool
 
 	SelectFields []string // fields to be selected
 
 	// Min-heap of cursors ordered by timestamp.
-	heap *pointHeap
+	heap heap.Interface
 
 	// Memoize the cursor's tagset-based key.
 	memokey string
 }
 
 // NewTagSetCursor returns a instance of TagSetCursor.
-func NewTagSetCursor(m string, t map[string]string, c []*TagsCursor) *TagSetCursor {
+func NewTagSetCursor(m string, t map[string]string, c []*TagsCursor, ascending bool) *TagSetCursor {
 	return &TagSetCursor{
 		measurement: m,
 		tags:        t,
 		cursors:     c,
+		ascending:   ascending,
 		heap:        newPointHeap(),
 	}
 }
@@ -185,7 +187,11 @@ func (tsc *TagSetCursor) key() string {
 }
 
 func (tsc *TagSetCursor) Init(seek int64) {
-	tsc.heap = newPointHeap()
+	if tsc.ascending {
+		tsc.heap = newPointHeap()
+	} else {
+		tsc.heap = newPointHeapReverse()
+	}
 
 	// Prime the buffers.
 	for i := 0; i < len(tsc.cursors); i++ {
@@ -297,13 +303,27 @@ type pointHeapItem struct {
 }
 
 type pointHeap []*pointHeapItem
+type pointHeapReverse struct {
+	pointHeap
+}
 
-func newPointHeap() *pointHeap {
+func newPointHeap() heap.Interface {
 	q := make(pointHeap, 0)
 	heap.Init(&q)
 	return &q
 }
 
+func newPointHeapReverse() heap.Interface {
+	q := make(pointHeap, 0)
+	heap.Init(&q)
+
+	return &pointHeapReverse{q}
+}
+
+func (pq *pointHeapReverse) Less(i, j int) bool {
+	return pq.pointHeap[i].timestamp > pq.pointHeap[j].timestamp
+}
+
 func (pq pointHeap) Len() int { return len(pq) }
 
 func (pq pointHeap) Less(i, j int) bool {

tsdb/raw.go

@@ -873,7 +873,7 @@ func (m *RawMapper) openMeasurement(mm *Measurement) error {
 			cursors = append(cursors, cm)
 		}
 
-		tsc := NewTagSetCursor(mm.Name, t.Tags, cursors)
+		tsc := NewTagSetCursor(mm.Name, t.Tags, cursors, ascending)
 		tsc.SelectFields = m.selectFields
 		if ascending {
 			tsc.Init(m.qmin)