Improvement on postings intersection (#616)

* improvement on postings intersection

Signed-off-by: naivewong <867245430@qq.com>

index/postings.go

@@ -303,68 +303,68 @@ func Intersect(its ...Postings) Postings {
 	if len(its) == 1 {
 		return its[0]
 	}
-
-	l := len(its) / 2
-	a := Intersect(its[:l]...)
-	b := Intersect(its[l:]...)
-
-	if a == EmptyPostings() || b == EmptyPostings() {
+	for _, p := range its {
+		if p == EmptyPostings() {
 			return EmptyPostings()
 		}
-	return newIntersectPostings(a, b)
+	}
+
+	return newIntersectPostings(its...)
 }
 
 type intersectPostings struct {
-	a, b Postings
+	arr []Postings
 	cur uint64
 }
 
-func newIntersectPostings(a, b Postings) *intersectPostings {
-	return &intersectPostings{a: a, b: b}
+func newIntersectPostings(its ...Postings) *intersectPostings {
+	return &intersectPostings{arr: its}
 }
 
 func (it *intersectPostings) At() uint64 {
 	return it.cur
 }
 
-func (it *intersectPostings) doNext(id uint64) bool {
+func (it *intersectPostings) doNext() bool {
+Loop:
 	for {
-		if !it.b.Seek(id) {
+		for _, p := range it.arr {
+			if !p.Seek(it.cur) {
 				return false
 			}
-		if vb := it.b.At(); vb != id {
-			if !it.a.Seek(vb) {
-				return false
-			}
-			id = it.a.At()
-			if vb != id {
-				continue
+			if p.At() > it.cur {
+				it.cur = p.At()
+				continue Loop
 			}
 		}
-		it.cur = id
 		return true
 	}
 }
 
 func (it *intersectPostings) Next() bool {
-	if !it.a.Next() {
+	for _, p := range it.arr {
+		if !p.Next() {
 			return false
 		}
-	return it.doNext(it.a.At())
+		if p.At() > it.cur {
+			it.cur = p.At()
+		}
+	}
+	return it.doNext()
 }
 
 func (it *intersectPostings) Seek(id uint64) bool {
-	if !it.a.Seek(id) {
-		return false
-	}
-	return it.doNext(it.a.At())
+	it.cur = id
+	return it.doNext()
 }
 
 func (it *intersectPostings) Err() error {
-	if it.a.Err() != nil {
-		return it.a.Err()
+	for _, p := range it.arr {
+		if p.Err() != nil {
+			return p.Err()
+		}
 	}
-	return it.b.Err()
+	return nil
 }
 
 // Merge returns a new iterator over the union of the input iterators.

index/postings_test.go

@@ -221,6 +221,7 @@ func TestMultiIntersect(t *testing.T) {
 }
 
 func BenchmarkIntersect(t *testing.B) {
+	t.Run("LongPostings1", func(bench *testing.B) {
 		var a, b, c, d []uint64
 
 		for i := 0; i < 10000000; i += 2 {
@@ -244,13 +245,66 @@ func BenchmarkIntersect(t *testing.B) {
 		i3 := newListPostings(c...)
 		i4 := newListPostings(d...)
 
-	t.ResetTimer()
+		bench.ResetTimer()
+		bench.ReportAllocs()
+		for i := 0; i < bench.N; i++ {
+			if _, err := ExpandPostings(Intersect(i1, i2, i3, i4)); err != nil {
+				bench.Fatal(err)
+			}
+		}
+	})
+
+	t.Run("LongPostings2", func(bench *testing.B) {
+		var a, b, c, d []uint64
+
+		for i := 0; i < 12500000; i++ {
+			a = append(a, uint64(i))
+		}
+		for i := 7500000; i < 12500000; i++ {
+			b = append(b, uint64(i))
+		}
+		for i := 9000000; i < 20000000; i++ {
+			c = append(c, uint64(i))
+		}
+		for i := 10000000; i < 12000000; i++ {
+			d = append(d, uint64(i))
+		}
 
-	for i := 0; i < t.N; i++ {
+		i1 := newListPostings(a...)
+		i2 := newListPostings(b...)
+		i3 := newListPostings(c...)
+		i4 := newListPostings(d...)
+
+		bench.ResetTimer()
+		bench.ReportAllocs()
+		for i := 0; i < bench.N; i++ {
 			if _, err := ExpandPostings(Intersect(i1, i2, i3, i4)); err != nil {
-			t.Fatal(err)
+				bench.Fatal(err)
+			}
+		}
+	})
+
+	// Many matchers(k >> n).
+	t.Run("ManyPostings", func(bench *testing.B) {
+		var its []Postings
+
+		// 100000 matchers(k=100000).
+		for i := 0; i < 100000; i++ {
+			var temp []uint64
+			for j := 1; j < 100; j++ {
+				temp = append(temp, uint64(j))
 			}
+			its = append(its, newListPostings(temp...))
 		}
+
+		bench.ResetTimer()
+		bench.ReportAllocs()
+		for i := 0; i < bench.N; i++ {
+			if _, err := ExpandPostings(Intersect(its...)); err != nil {
+				bench.Fatal(err)
+			}
+		}
+	})
 }
 
 func TestMultiMerge(t *testing.T) {