index: reduce empty postings trees (#509)

Improved Merge when all is empty.
Signed-off-by: NBartek Plotka <bwplotka@gmail.com>

index/index_test.go

@@ -212,7 +212,7 @@ func TestIndexRW_Postings(t *testing.T) {
 	testutil.Ok(t, iw.AddSeries(3, series[2]))
 	testutil.Ok(t, iw.AddSeries(4, series[3]))
 
-	err = iw.WritePostings("a", "1", newListPostings([]uint64{1, 2, 3, 4}))
+	err = iw.WritePostings("a", "1", newListPostings(1, 2, 3, 4))
 	testutil.Ok(t, err)
 
 	testutil.Ok(t, iw.Close())
@@ -323,9 +323,9 @@ func TestPersistence_index_e2e(t *testing.T) {
 	for i := range all {
 		all[i] = uint64(i)
 	}
-	err = iw.WritePostings("", "", newListPostings(all))
+	err = iw.WritePostings("", "", newListPostings(all...))
 	testutil.Ok(t, err)
-	mi.WritePostings("", "", newListPostings(all))
+	mi.WritePostings("", "", newListPostings(all...))
 
 	for n, e := range postings.m {
 		for v := range e {

index/postings.go

@@ -92,7 +92,7 @@ func (p *MemPostings) Get(name, value string) Postings {
 	if lp == nil {
 		return EmptyPostings()
 	}
-	return newListPostings(lp)
+	return newListPostings(lp...)
 }
 
 // All returns a postings list over all documents ever added.
@@ -202,7 +202,7 @@ func (p *MemPostings) Iter(f func(labels.Label, Postings) error) error {
 
 	for n, e := range p.m {
 		for v, p := range e {
-			if err := f(labels.Label{Name: n, Value: v}, newListPostings(p)); err != nil {
+			if err := f(labels.Label{Name: n, Value: v}, newListPostings(p...)); err != nil {
 				return err
 			}
 		}
@@ -283,6 +283,8 @@ func (e errPostings) Err() error       { return e.err }
 var emptyPostings = errPostings{}
 
 // EmptyPostings returns a postings list that's always empty.
+// NOTE: Returning EmptyPostings sentinel when index.Postings struct has no postings is recommended.
+// It triggers optimized flow in other functions like Intersect, Without etc.
 func EmptyPostings() Postings {
 	return emptyPostings
 }
@@ -296,13 +298,20 @@ func ErrPostings(err error) Postings {
 // input postings.
 func Intersect(its ...Postings) Postings {
 	if len(its) == 0 {
-		return emptyPostings
+		return EmptyPostings()
 	}
 	if len(its) == 1 {
 		return its[0]
 	}
+
 	l := len(its) / 2
-	return newIntersectPostings(Intersect(its[:l]...), Intersect(its[l:]...))
+	a := Intersect(its[:l]...)
+	b := Intersect(its[l:]...)
+
+	if a == EmptyPostings() || b == EmptyPostings() {
+		return EmptyPostings()
+	}
+	return newIntersectPostings(a, b)
 }
 
 type intersectPostings struct {
@@ -366,7 +375,12 @@ func Merge(its ...Postings) Postings {
 	if len(its) == 1 {
 		return its[0]
 	}
-	return newMergedPostings(its)
+
+	p, ok := newMergedPostings(its)
+	if !ok {
+		return EmptyPostings()
+	}
+	return p
 }
 
 type postingsHeap []Postings
@@ -395,18 +409,24 @@ type mergedPostings struct {
 	err        error
 }
 
-func newMergedPostings(p []Postings) *mergedPostings {
+func newMergedPostings(p []Postings) (m *mergedPostings, nonEmpty bool) {
 	ph := make(postingsHeap, 0, len(p))
+
 	for _, it := range p {
+		// NOTE: mergedPostings struct requires the user to issue an initial Next.
 		if it.Next() {
 			ph = append(ph, it)
 		} else {
 			if it.Err() != nil {
-				return &mergedPostings{err: it.Err()}
+				return &mergedPostings{err: it.Err()}, true
 			}
 		}
 	}
-	return &mergedPostings{h: ph}
+
+	if len(ph) == 0 {
+		return nil, false
+	}
+	return &mergedPostings{h: ph}, true
 }
 
 func (it *mergedPostings) Next() bool {
@@ -495,8 +515,15 @@ func (it mergedPostings) Err() error {
 }
 
 // Without returns a new postings list that contains all elements from the full list that
-// are not in the drop list
+// are not in the drop list.
 func Without(full, drop Postings) Postings {
+	if full == EmptyPostings() {
+		return EmptyPostings()
+	}
+
+	if drop == EmptyPostings() {
+		return full
+	}
 	return newRemovedPostings(full, drop)
 }
 
@@ -580,10 +607,10 @@ type listPostings struct {
 }
 
 func NewListPostings(list []uint64) Postings {
-	return newListPostings(list)
+	return newListPostings(list...)
 }
 
-func newListPostings(list []uint64) *listPostings {
+func newListPostings(list ...uint64) *listPostings {
 	return &listPostings{list: list}
 }
 

index/postings_test.go

@@ -62,38 +62,120 @@ func TestMemPostings_ensureOrder(t *testing.T) {
 }
 
 func TestIntersect(t *testing.T) {
+	a := newListPostings(1, 2, 3)
+	b := newListPostings(2, 3, 4)
+
 	var cases = []struct {
-		a, b []uint64
-		res  []uint64
+		in []Postings
+
+		res Postings
 	}{
 		{
-			a:   []uint64{1, 2, 3, 4, 5},
-			b:   []uint64{6, 7, 8, 9, 10},
-			res: nil,
+			in:  []Postings{},
+			res: EmptyPostings(),
 		},
 		{
-			a:   []uint64{1, 2, 3, 4, 5},
-			b:   []uint64{4, 5, 6, 7, 8},
-			res: []uint64{4, 5},
+			in:  []Postings{a, b, EmptyPostings()},
+			res: EmptyPostings(),
 		},
 		{
-			a:   []uint64{1, 2, 3, 4, 9, 10},
-			b:   []uint64{1, 4, 5, 6, 7, 8, 10, 11},
-			res: []uint64{1, 4, 10},
-		}, {
-			a:   []uint64{1},
-			b:   []uint64{0, 1},
-			res: []uint64{1},
+			in:  []Postings{b, a, EmptyPostings()},
+			res: EmptyPostings(),
+		},
+		{
+			in:  []Postings{EmptyPostings(), b, a},
+			res: EmptyPostings(),
+		},
+		{
+			in:  []Postings{EmptyPostings(), a, b},
+			res: EmptyPostings(),
+		},
+		{
+			in:  []Postings{a, EmptyPostings(), b},
+			res: EmptyPostings(),
+		},
+		{
+			in:  []Postings{b, EmptyPostings(), a},
+			res: EmptyPostings(),
+		},
+		{
+			in:  []Postings{b, EmptyPostings(), a, a, b, a, a, a},
+			res: EmptyPostings(),
+		},
+		{
+			in: []Postings{
+				newListPostings(1, 2, 3, 4, 5),
+				newListPostings(6, 7, 8, 9, 10),
+			},
+			res: newListPostings(),
+		},
+		{
+			in: []Postings{
+				newListPostings(1, 2, 3, 4, 5),
+				newListPostings(4, 5, 6, 7, 8),
+			},
+			res: newListPostings(4, 5),
+		},
+		{
+			in: []Postings{
+				newListPostings(1, 2, 3, 4, 9, 10),
+				newListPostings(1, 4, 5, 6, 7, 8, 10, 11),
+			},
+			res: newListPostings(1, 4, 10),
+		},
+		{
+			in: []Postings{
+				newListPostings(1),
+				newListPostings(0, 1),
+			},
+			res: newListPostings(1),
+		},
+		{
+			in: []Postings{
+				newListPostings(1),
+			},
+			res: newListPostings(1),
+		},
+		{
+			in: []Postings{
+				newListPostings(1),
+				newListPostings(),
+			},
+			res: newListPostings(),
+		},
+		{
+			in: []Postings{
+				newListPostings(),
+				newListPostings(),
+			},
+			res: newListPostings(),
 		},
 	}
 
 	for _, c := range cases {
-		a := newListPostings(c.a)
-		b := newListPostings(c.b)
+		t.Run("", func(t *testing.T) {
+			if c.res == nil {
+				t.Fatal("intersect result expectancy cannot be nil")
+			}
 
-		res, err := ExpandPostings(Intersect(a, b))
-		testutil.Ok(t, err)
-		testutil.Equals(t, c.res, res)
+			expected, err := ExpandPostings(c.res)
+			testutil.Ok(t, err)
+
+			i := Intersect(c.in...)
+
+			if c.res == EmptyPostings() {
+				testutil.Equals(t, EmptyPostings(), i)
+				return
+			}
+
+			if i == EmptyPostings() {
+				t.Fatal("intersect unexpected result: EmptyPostings sentinel")
+			}
+
+			res, err := ExpandPostings(i)
+			testutil.Ok(t, err)
+			testutil.Equals(t, expected, res)
+		})
 	}
 }
 
@@ -128,7 +210,7 @@ func TestMultiIntersect(t *testing.T) {
 	for _, c := range cases {
 		ps := make([]Postings, 0, len(c.p))
 		for _, postings := range c.p {
-			ps = append(ps, newListPostings(postings))
+			ps = append(ps, newListPostings(postings...))
 		}
 
 		res, err := ExpandPostings(Intersect(ps...))
@@ -157,10 +239,10 @@ func BenchmarkIntersect(t *testing.B) {
 		d = append(d, uint64(i))
 	}
 
-	i1 := newListPostings(a)
-	i2 := newListPostings(b)
-	i3 := newListPostings(c)
-	i4 := newListPostings(d)
+	i1 := newListPostings(a...)
+	i2 := newListPostings(b...)
+	i3 := newListPostings(c...)
+	i4 := newListPostings(d...)
 
 	t.ResetTimer()
 
@@ -172,60 +254,117 @@ func BenchmarkIntersect(t *testing.B) {
 }
 
 func TestMultiMerge(t *testing.T) {
-	var cases = []struct {
-		a, b, c []uint64
-		res     []uint64
-	}{
-		{
-			a:   []uint64{1, 2, 3, 4, 5, 6, 1000, 1001},
-			b:   []uint64{2, 4, 5, 6, 7, 8, 999, 1001},
-			c:   []uint64{1, 2, 5, 6, 7, 8, 1001, 1200},
-			res: []uint64{1, 2, 3, 4, 5, 6, 7, 8, 999, 1000, 1001, 1200},
-		},
-	}
+	i1 := newListPostings(1, 2, 3, 4, 5, 6, 1000, 1001)
+	i2 := newListPostings(2, 4, 5, 6, 7, 8, 999, 1001)
+	i3 := newListPostings(1, 2, 5, 6, 7, 8, 1001, 1200)
 
-	for _, c := range cases {
-		i1 := newListPostings(c.a)
-		i2 := newListPostings(c.b)
-		i3 := newListPostings(c.c)
-
-		res, err := ExpandPostings(Merge(i1, i2, i3))
-		testutil.Ok(t, err)
-		testutil.Equals(t, c.res, res)
-	}
+	res, err := ExpandPostings(Merge(i1, i2, i3))
+	testutil.Ok(t, err)
+	testutil.Equals(t, []uint64{1, 2, 3, 4, 5, 6, 7, 8, 999, 1000, 1001, 1200}, res)
 }
 
 func TestMergedPostings(t *testing.T) {
 	var cases = []struct {
-		a, b []uint64
-		res  []uint64
+		in []Postings
+
+		res Postings
 	}{
 		{
-			a:   []uint64{1, 2, 3, 4, 5},
-			b:   []uint64{6, 7, 8, 9, 10},
-			res: []uint64{1, 2, 3, 4, 5, 6, 7, 8, 9, 10},
+			in:  []Postings{},
+			res: EmptyPostings(),
 		},
 		{
-			a:   []uint64{1, 2, 3, 4, 5},
-			b:   []uint64{4, 5, 6, 7, 8},
-			res: []uint64{1, 2, 3, 4, 5, 6, 7, 8},
+			in: []Postings{
+				newListPostings(),
+				newListPostings(),
+			},
+			res: EmptyPostings(),
 		},
 		{
-			a:   []uint64{1, 2, 3, 4, 9, 10},
-			b:   []uint64{1, 4, 5, 6, 7, 8, 10, 11},
-			res: []uint64{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11},
+			in: []Postings{
+				newListPostings(),
+			},
+			res: newListPostings(),
+		},
+		{
+			in: []Postings{
+				EmptyPostings(),
+				EmptyPostings(),
+				EmptyPostings(),
+				EmptyPostings(),
+			},
+			res: EmptyPostings(),
+		},
+		{
+			in: []Postings{
+				newListPostings(1, 2, 3, 4, 5),
+				newListPostings(6, 7, 8, 9, 10),
+			},
+			res: newListPostings(1, 2, 3, 4, 5, 6, 7, 8, 9, 10),
+		},
+		{
+			in: []Postings{
+				newListPostings(1, 2, 3, 4, 5),
+				newListPostings(4, 5, 6, 7, 8),
+			},
+			res: newListPostings(1, 2, 3, 4, 5, 6, 7, 8),
+		},
+		{
+			in: []Postings{
+				newListPostings(1, 2, 3, 4, 9, 10),
+				newListPostings(1, 4, 5, 6, 7, 8, 10, 11),
+			},
+			res: newListPostings(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11),
+		},
+		{
+			in: []Postings{
+				newListPostings(1, 2, 3, 4, 9, 10),
+				EmptyPostings(),
+				newListPostings(1, 4, 5, 6, 7, 8, 10, 11),
+			},
+			res: newListPostings(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11),
+		},
+		{
+			in: []Postings{
+				newListPostings(1, 2),
+				newListPostings(),
+			},
+			res: newListPostings(1, 2),
+		},
+		{
+			in: []Postings{
+				newListPostings(1, 2),
+				EmptyPostings(),
+			},
+			res: newListPostings(1, 2),
 		},
 	}
 
 	for _, c := range cases {
-		a := newListPostings(c.a)
-		b := newListPostings(c.b)
+		t.Run("", func(t *testing.T) {
+			if c.res == nil {
+				t.Fatal("merge result expectancy cannot be nil")
+			}
 
-		res, err := ExpandPostings(Merge(a, b))
-		testutil.Ok(t, err)
-		testutil.Equals(t, c.res, res)
-	}
+			expected, err := ExpandPostings(c.res)
+			testutil.Ok(t, err)
+
+			m := Merge(c.in...)
 
+			if c.res == EmptyPostings() {
+				testutil.Equals(t, EmptyPostings(), m)
+				return
+			}
+
+			if m == EmptyPostings() {
+				t.Fatal("merge unexpected result: EmptyPostings sentinel")
+			}
+
+			res, err := ExpandPostings(m)
+			testutil.Ok(t, err)
+			testutil.Equals(t, expected, res)
+		})
+	}
 }
 
 func TestMergedPostingsSeek(t *testing.T) {
@@ -271,8 +410,8 @@ func TestMergedPostingsSeek(t *testing.T) {
 	}
 
 	for _, c := range cases {
-		a := newListPostings(c.a)
-		b := newListPostings(c.b)
+		a := newListPostings(c.a...)
+		b := newListPostings(c.b...)
 
 		p := Merge(a, b)
 
@@ -333,8 +472,8 @@ func TestRemovedPostings(t *testing.T) {
 	}
 
 	for _, c := range cases {
-		a := newListPostings(c.a)
-		b := newListPostings(c.b)
+		a := newListPostings(c.a...)
+		b := newListPostings(c.b...)
 
 		res, err := ExpandPostings(newRemovedPostings(a, b))
 		testutil.Ok(t, err)
@@ -353,8 +492,8 @@ func TestRemovedNextStackoverflow(t *testing.T) {
 		remove = append(remove, i)
 	}
 
-	flp := newListPostings(full)
-	rlp := newListPostings(remove)
+	flp := newListPostings(full...)
+	rlp := newListPostings(remove...)
 	rp := newRemovedPostings(flp, rlp)
 	gotElem := false
 	for rp.Next() {
@@ -432,8 +571,8 @@ func TestRemovedPostingsSeek(t *testing.T) {
 	}
 
 	for _, c := range cases {
-		a := newListPostings(c.a)
-		b := newListPostings(c.b)
+		a := newListPostings(c.a...)
+		b := newListPostings(c.b...)
 
 		p := newRemovedPostings(a, b)
 
@@ -526,13 +665,13 @@ func TestBigEndian(t *testing.T) {
 }
 
 func TestIntersectWithMerge(t *testing.T) {
-	// One of the reproduceable cases for:
+	// One of the reproducible cases for:
 	// https://github.com/prometheus/prometheus/issues/2616
-	a := newListPostings([]uint64{21, 22, 23, 24, 25, 30})
+	a := newListPostings(21, 22, 23, 24, 25, 30)
 
 	b := Merge(
-		newListPostings([]uint64{10, 20, 30}),
-		newListPostings([]uint64{15, 26, 30}),
+		newListPostings(10, 20, 30),
+		newListPostings(15, 26, 30),
 	)
 
 	p := Intersect(a, b)
@@ -541,3 +680,81 @@ func TestIntersectWithMerge(t *testing.T) {
 	testutil.Ok(t, err)
 	testutil.Equals(t, []uint64{30}, res)
 }
+
+func TestWithoutPostings(t *testing.T) {
+	var cases = []struct {
+		base Postings
+		drop Postings
+
+		res Postings
+	}{
+		{
+			base: EmptyPostings(),
+			drop: EmptyPostings(),
+
+			res: EmptyPostings(),
+		},
+		{
+			base: EmptyPostings(),
+			drop: newListPostings(1, 2),
+
+			res: EmptyPostings(),
+		},
+		{
+			base: newListPostings(1, 2),
+			drop: EmptyPostings(),
+
+			res: newListPostings(1, 2),
+		},
+		{
+			base: newListPostings(),
+			drop: newListPostings(),
+
+			res: newListPostings(),
+		},
+		{
+			base: newListPostings(1, 2, 3),
+			drop: newListPostings(),
+
+			res: newListPostings(1, 2, 3),
+		},
+		{
+			base: newListPostings(1, 2, 3),
+			drop: newListPostings(4, 5, 6),
+
+			res: newListPostings(1, 2, 3),
+		},
+		{
+			base: newListPostings(1, 2, 3),
+			drop: newListPostings(3, 4, 5),
+
+			res: newListPostings(1, 2),
+		},
+	}
+
+	for _, c := range cases {
+		t.Run("", func(t *testing.T) {
+			if c.res == nil {
+				t.Fatal("without result expectancy cannot be nil")
+			}
+
+			expected, err := ExpandPostings(c.res)
+			testutil.Ok(t, err)
+
+			w := Without(c.base, c.drop)
+
+			if c.res == EmptyPostings() {
+				testutil.Equals(t, EmptyPostings(), w)
+				return
+			}
+
+			if w == EmptyPostings() {
+				t.Fatal("without unexpected result: EmptyPostings sentinel")
+			}
+
+			res, err := ExpandPostings(w)
+			testutil.Ok(t, err)
+			testutil.Equals(t, expected, res)
+		})
+	}
+}

querier.go

@@ -264,6 +264,7 @@ func (q *blockQuerier) Close() error {
 // PostingsForMatchers assembles a single postings iterator against the index reader
 // based on the given matchers. It returns a list of label names that must be manually
 // checked to not exist in series the postings list points to.
+// It returns EmptyPostings() if it can be determined beforehand that no results will be found.
 func PostingsForMatchers(ix IndexReader, ms ...labels.Matcher) (index.Postings, error) {
 	var its []index.Postings