Ref: thanos-io/pull/5588

pkg/store/bucket.go

@@ -1809,6 +1809,10 @@ func (r *bucketIndexReader) ExpandedPostings(ctx context.Context, ms []*labels.M
 		keys = append(keys, allPostingsLabel)
 	}
 
+	// Sorting will improve the performance dramatically if the dataset is relatively large
+	// since entries in the postings offset table was sorted by label name and value,
+	// the sequential reading is much faster.
+	sort.Sort(labels.Labels(keys))
 	fetchedPostings, err := r.fetchPostings(ctx, keys)
 	if err != nil {
 		return nil, errors.Wrap(err, "get postings")
@@ -1890,7 +1894,6 @@ func checkNilPosting(l labels.Label, p index.Postings) index.Postings {
 func toPostingGroup(lvalsFn func(name string) ([]string, error), m *labels.Matcher) (*postingGroup, error) {
 	if m.Type == labels.MatchRegexp {
 		if vals := findSetMatches(m.Value); len(vals) > 0 {
-			sort.Strings(vals)
 			toAdd := make([]labels.Label, 0, len(vals))
 			for _, val := range vals {
 				toAdd = append(toAdd, labels.Label{Name: m.Name, Value: val})