Revert ListPostings change (#12955)

Reverts change from https://github.com/prometheus/prometheus/pull/12906

The benchmarks show that it's slower when intersecting, which is a
common usage for ListPostings (when intersecting matchers from Head)

(old is before #12906, new is #12906):

                           │     old     │                 new                 │
                           │   sec/op    │   sec/op     vs base                │
Intersect/LongPostings1-16   20.54µ ± 1%   21.11µ ± 1%   +2.76% (p=0.000 n=20)
Intersect/LongPostings2-16   51.03m ± 1%   52.40m ± 2%   +2.69% (p=0.000 n=20)
Intersect/ManyPostings-16    194.2m ± 3%   332.1m ± 1%  +71.00% (p=0.000 n=20)
geomean                      5.882m        7.161m       +21.74%

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

tsdb/index/postings.go

@@ -748,7 +748,7 @@ func (rp *removedPostings) Err() error {
 // ListPostings implements the Postings interface over a plain list.
 type ListPostings struct {
 	list []storage.SeriesRef
-	pos  int
+	cur  storage.SeriesRef
 }
 
 func NewListPostings(list []storage.SeriesRef) Postings {
@@ -760,34 +760,39 @@ func newListPostings(list ...storage.SeriesRef) *ListPostings {
 }
 
 func (it *ListPostings) At() storage.SeriesRef {
-	return it.list[it.pos-1]
+	return it.cur
 }
 
 func (it *ListPostings) Next() bool {
-	if it.pos < len(it.list) {
-		it.pos++
+	if len(it.list) > 0 {
+		it.cur = it.list[0]
+		it.list = it.list[1:]
 		return true
 	}
+	it.cur = 0
 	return false
 }
 
 func (it *ListPostings) Seek(x storage.SeriesRef) bool {
-	if it.pos == 0 {
-		it.pos++
-	}
-	if it.pos > len(it.list) {
-		return false
-	}
 	// If the current value satisfies, then return.
-	if it.list[it.pos-1] >= x {
+	if it.cur >= x {
 		return true
 	}
+	if len(it.list) == 0 {
+		return false
+	}
 
 	// Do binary search between current position and end.
-	it.pos = sort.Search(len(it.list[it.pos-1:]), func(i int) bool {
-		return it.list[i+it.pos-1] >= x
-	}) + it.pos
-	return it.pos-1 < len(it.list)
+	i := sort.Search(len(it.list), func(i int) bool {
+		return it.list[i] >= x
+	})
+	if i < len(it.list) {
+		it.cur = it.list[i]
+		it.list = it.list[i+1:]
+		return true
+	}
+	it.list = nil
+	return false
 }
 
 func (it *ListPostings) Err() error {

tsdb/index/postings_test.go

@@ -1242,6 +1242,8 @@ func TestListPostings(t *testing.T) {
 	})
 }
 
+// BenchmarkListPostings benchmarks ListPostings by iterating Next/At sequentially.
+// See also BenchmarkIntersect as it performs more `At` calls than `Next` calls when intersecting.
 func BenchmarkListPostings(b *testing.B) {
 	const maxCount = 1e6
 	input := make([]storage.SeriesRef, maxCount)