diff --git a/head.go b/head.go
index 1b8af29f0..5eafb4f95 100644
--- a/head.go
+++ b/head.go
@@ -403,9 +403,6 @@ func (h *HeadBlock) fullness() float64 {
 }
 
 func (h *HeadBlock) updateMapping() {
-	h.mapper.mtx.Lock()
-	defer h.mapper.mtx.Unlock()
-
 	h.mtx.RLock()
 
 	if h.mapper.sortable != nil && h.mapper.Len() == len(h.descs) {
@@ -435,15 +432,12 @@ func (h *HeadBlock) remapPostings(p Postings) Postings {
 		return errPostings{err: err}
 	}
 
-	h.mapper.mtx.RLock()
-	defer h.mapper.mtx.RUnlock()
+	h.mapper.mtx.Lock()
+	defer h.mapper.mtx.Unlock()
 
+	h.updateMapping()
 	h.mapper.Sort(list)
 
-	slice.Sort(list, func(i, j int) bool {
-		return h.mapper.fw[list[i]] < h.mapper.fw[list[j]]
-	})
-
 	return newListPostings(list)
 }
 
diff --git a/querier.go b/querier.go
index 73158b6ec..bbbc7abf3 100644
--- a/querier.go
+++ b/querier.go
@@ -64,11 +64,8 @@ func (s *DB) Querier(mint, maxt int64) Querier {
 
 		// TODO(fabxc): find nicer solution.
 		if hb, ok := b.(*HeadBlock); ok {
-			hb.updateMapping()
 			q.postingsMapper = hb.remapPostings
-			q.series = &headSeriesReader{
-				h: hb,
-			}
+			q.series = &headSeriesReader{h: hb}
 		}
 
 		sq.blocks = append(sq.blocks, q)