add pool for expanded posting slice

pkg/store/bucket.go

@@ -35,6 +35,7 @@ import (
 	"github.com/prometheus/prometheus/tsdb/chunks"
 	"github.com/prometheus/prometheus/tsdb/encoding"
 	"github.com/prometheus/prometheus/tsdb/index"
+	"github.com/prometheus/prometheus/util/zeropool"
 	"github.com/weaveworks/common/httpgrpc"
 	"golang.org/x/sync/errgroup"
 	"google.golang.org/grpc"
@@ -125,8 +126,22 @@ const (
 var (
 	errBlockSyncConcurrencyNotValid = errors.New("the block sync concurrency must be equal or greater than 1.")
 	hashPool                        = sync.Pool{New: func() interface{} { return xxhash.New() }}
+	postingsPool                    zeropool.Pool[[]storage.SeriesRef]
 )
 
+func getPostingsSlice() []storage.SeriesRef {
+	if p := postingsPool.Get(); p != nil {
+		return p
+	}
+
+	// Pre-allocate slice with initial capacity.
+	return make([]storage.SeriesRef, 0, 1024)
+}
+
+func putPostingsSlice(p []storage.SeriesRef) {
+	postingsPool.Put(p[:0])
+}
+
 type bucketStoreMetrics struct {
 	blocksLoaded          prometheus.Gauge
 	blockLoads            prometheus.Counter
@@ -2549,6 +2564,10 @@ type bucketIndexReader struct {
 
 	indexVersion int
 	logger       log.Logger
+
+	// Posting slice to return to the postings pool on close.
+	// A single bucketIndexReader should have at most 1 postings slice to return.
+	postings []storage.SeriesRef
 }
 
 func newBucketIndexReader(block *bucketBlock, logger log.Logger) *bucketIndexReader {
@@ -2678,13 +2697,13 @@ func (r *bucketIndexReader) ExpandedPostings(
 
 // ExpandPostingsWithContext returns the postings expanded as a slice and considers context.
 func ExpandPostingsWithContext(ctx context.Context, p index.Postings) ([]storage.SeriesRef, error) {
-	res := make([]storage.SeriesRef, 0, 1024) // Pre-allocate slice with initial capacity
+	res := getPostingsSlice()
 	i := 0
 	for p.Next() {
 		i++
 		if i%checkContextEveryNIterations == 0 {
 			if err := ctx.Err(); err != nil {
-				return nil, err
+				return res, err
 			}
 		}
 		res = append(res, p.At())
@@ -2978,6 +2997,7 @@ func (r *bucketIndexReader) fetchExpandedPostingsFromCache(ctx context.Context,
 	}
 
 	ps, err := ExpandPostingsWithContext(ctx, p)
+	r.postings = ps
 	if err != nil {
 		level.Error(r.logger).Log("msg", "failed to expand cached expanded postings, refetch postings", "id", r.block.meta.ULID.String(), "err", err)
 		return false, nil, nil
@@ -3414,6 +3434,8 @@ func (r *bucketIndexReader) LoadSeriesForTime(ref storage.SeriesRef, lset *[]sym
 // Close released the underlying resources of the reader.
 func (r *bucketIndexReader) Close() error {
 	r.block.pendingReaders.Done()
+
+	putPostingsSlice(r.postings)
 	return nil
 }
 

pkg/store/bucket_test.go

@@ -2979,7 +2979,7 @@ func TestExpandPostingsWithContextCancel(t *testing.T) {
 	res, err := ExpandPostingsWithContext(ctx, p)
 	testutil.NotOk(t, err)
 	testutil.Equals(t, context.Canceled, err)
-	testutil.Equals(t, []storage.SeriesRef(nil), res)
+	testutil.Equals(t, true, cap(res) == 1024)
 }
 
 func samePostingGroup(a, b *postingGroup) bool {

pkg/store/lazy_postings.go

@@ -310,6 +310,7 @@ func fetchAndExpandPostingGroups(ctx context.Context, r *bucketIndexReader, post
 		return nil, nil, err
 	}
 	ps, err := ExpandPostingsWithContext(ctx, result)
+	r.postings = ps
 	if err != nil {
 		return nil, nil, errors.Wrap(err, "expand")
 	}