diff --git a/compact.go b/compact.go
index a3bc7d17a..17a4da041 100644
--- a/compact.go
+++ b/compact.go
@@ -457,7 +457,7 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 
 		indexr := b.Index()
 
-		all, err := indexr.Postings("", "")
+		all, err := indexr.Postings(allPostingsKey.Name, allPostingsKey.Value)
 		if err != nil {
 			return err
 		}
diff --git a/head.go b/head.go
index a5ce94e45..3ead78fc4 100644
--- a/head.go
+++ b/head.go
@@ -207,7 +207,7 @@ func (h *Head) ReadWAL() error {
 			}
 			ms := h.series.getByID(s.Ref)
 			if ms == nil {
-				h.logger.Log("msg", "unknown series reference in WAL", "ref", s.Ref)
+				h.logger.Log("msg", "unknown series reference in WAL", "ref", s.Ref, "ts", s.T, "mint", mint)
 				continue
 			}
 			_, chunkCreated := ms.append(s.T, s.V)
@@ -267,7 +267,7 @@ func (h *Head) Truncate(mint int64) error {
 
 	start = time.Now()
 
-	p, err := h.indexRange(mint, math.MaxInt64).Postings("", "")
+	p, err := h.indexRange(mint, math.MaxInt64).Postings(allPostingsKey.Name, allPostingsKey.Value)
 	if err != nil {
 		return err
 	}
@@ -1038,8 +1038,6 @@ func (s *stripeSeries) getOrSet(hash uint64, series *memSeries) (*memSeries, boo
 		return prev, false
 	}
 	s.hashes[i].set(hash, series)
-
-	s.hashes[i][hash] = append(s.hashes[i][hash], series)
 	s.locks[i].Unlock()
 
 	i = series.ref & stripeMask
diff --git a/postings.go b/postings.go
index 97a29ab19..0e51b221b 100644
--- a/postings.go
+++ b/postings.go
@@ -45,7 +45,7 @@ func (p *memPostings) get(name, value string) Postings {
 	return newListPostings(l)
 }
 
-var allLabel = labels.Label{}
+var allPostingsKey = labels.Label{}
 
 // add adds a document to the index. The caller has to ensure that no
 // term argument appears twice.
@@ -53,13 +53,36 @@ func (p *memPostings) add(id uint64, lset labels.Labels) {
 	p.mtx.Lock()
 
 	for _, l := range lset {
-		p.m[l] = append(p.m[l], id)
+		p.addFor(id, l)
 	}
-	p.m[allLabel] = append(p.m[allLabel], id)
+	p.addFor(id, allPostingsKey)
 
 	p.mtx.Unlock()
 }
 
+func (p *memPostings) addFor(id uint64, l labels.Label) {
+	list := append(p.m[l], id)
+	p.m[l] = list
+
+	// There is no guarantee that no higher ID was inserted before as they may
+	// be generated independently before adding them to postings.
+	// We repair order violations on insert. The invariant is that the first n-1
+	// items in the list are already sorted.
+	for i := len(list) - 1; i >= 1; i-- {
+		if list[i] >= list[i-1] {
+			break
+		}
+		list[i], list[i-1] = list[i-1], list[i]
+	}
+}
+
+func expandPostings(p Postings) (res []uint64, err error) {
+	for p.Next() {
+		res = append(res, p.At())
+	}
+	return res, p.Err()
+}
+
 // Postings provides iterative access over a postings list.
 type Postings interface {
 	// Next advances the iterator and returns true if another value was found.
diff --git a/postings_test.go b/postings_test.go
index 5d726ca3a..48cd2b608 100644
--- a/postings_test.go
+++ b/postings_test.go
@@ -21,6 +21,15 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestMemPostings_addFor(t *testing.T) {
+	p := newMemPostings()
+	p.m[allPostingsKey] = []uint64{1, 2, 3, 4, 6, 7, 8}
+
+	p.addFor(5, allPostingsKey)
+
+	require.Equal(t, []uint64{1, 2, 3, 4, 5, 6, 7, 8}, p.m[allPostingsKey])
+}
+
 type mockPostings struct {
 	next  func() bool
 	seek  func(uint64) bool
@@ -33,13 +42,6 @@ func (m *mockPostings) Seek(v uint64) bool { return m.seek(v) }
 func (m *mockPostings) Value() uint64      { return m.value() }
 func (m *mockPostings) Err() error         { return m.err() }
 
-func expandPostings(p Postings) (res []uint64, err error) {
-	for p.Next() {
-		res = append(res, p.At())
-	}
-	return res, p.Err()
-}
-
 func TestIntersect(t *testing.T) {
 	var cases = []struct {
 		a, b []uint64