diff --git a/tsdb/head_bench_test.go b/tsdb/head_bench_test.go
index aa2cf221486..dc682602b12 100644
--- a/tsdb/head_bench_test.go
+++ b/tsdb/head_bench_test.go
@@ -43,7 +43,7 @@ func BenchmarkHeadStripeSeriesCreate(b *testing.B) {
 	defer h.Close()
 
 	for i := 0; i < b.N; i++ {
-		h.getOrCreate(uint64(i), labels.FromStrings(labels.MetricName, "test", "a", strconv.Itoa(i), "b", strconv.Itoa(i%10), "c", strconv.Itoa(i%100), "d", strconv.Itoa(i/2), "e", strconv.Itoa(i/4)))
+		h.getOrCreate(uint64(i), labels.FromStrings("a", strconv.Itoa(i)))
 	}
 }
 
@@ -61,8 +61,8 @@ func BenchmarkHeadStripeSeriesCreateParallel(b *testing.B) {
 
 	b.RunParallel(func(pb *testing.PB) {
 		for pb.Next() {
-			i := int(count.Inc())
-			h.getOrCreate(uint64(i), labels.FromStrings(labels.MetricName, "test", "a", strconv.Itoa(i), "b", strconv.Itoa(i%10), "c", strconv.Itoa(i%100), "d", strconv.Itoa(i/2), "e", strconv.Itoa(i/4)))
+			i := count.Inc()
+			h.getOrCreate(uint64(i), labels.FromStrings("a", strconv.Itoa(int(i))))
 		}
 	})
 }
@@ -82,7 +82,7 @@ func BenchmarkHeadStripeSeriesCreate_PreCreationFailure(b *testing.B) {
 	defer h.Close()
 
 	for i := 0; i < b.N; i++ {
-		h.getOrCreate(uint64(i), labels.FromStrings(labels.MetricName, "test", "a", strconv.Itoa(i), "b", strconv.Itoa(i%10), "c", strconv.Itoa(i%100), "d", strconv.Itoa(i/2), "e", strconv.Itoa(i/4)))
+		h.getOrCreate(uint64(i), labels.FromStrings("a", strconv.Itoa(i)))
 	}
 }
 
diff --git a/tsdb/index/postings.go b/tsdb/index/postings.go
index 58f3473dae0..5ed41f76988 100644
--- a/tsdb/index/postings.go
+++ b/tsdb/index/postings.go
@@ -345,14 +345,13 @@ func (p *MemPostings) Add(id storage.SeriesRef, lset labels.Labels) {
 	p.mtx.Unlock()
 }
 
-func appendWithExponentialGrowth[T any](a []T, v T) (_ []T, copied bool) {
+func appendWithExponentialGrowth[T any](a []T, v T) []T {
 	if cap(a) < len(a)+1 {
 		newList := make([]T, len(a), len(a)*2+1)
 		copy(newList, a)
 		a = newList
-		copied = true
 	}
-	return append(a, v), copied
+	return append(a, v)
 }
 
 func (p *MemPostings) addFor(id storage.SeriesRef, l labels.Label) {
@@ -361,26 +360,16 @@ func (p *MemPostings) addFor(id storage.SeriesRef, l labels.Label) {
 		nm = map[string][]storage.SeriesRef{}
 		p.m[l.Name] = nm
 	}
-	list, copied := appendWithExponentialGrowth(nm[l.Value], id)
+	list := appendWithExponentialGrowth(nm[l.Value], id)
 	nm[l.Value] = list
 
-	// Return if it shouldn't be ordered, if it only has one element or if it's already ordered.
-	// The invariant is that the first n-1 items in the list are already sorted.
-	if !p.ordered || len(list) == 1 || list[len(list)-1] >= list[len(list)-2] {
+	if !p.ordered {
 		return
 	}
-
-	if !copied {
-		// We have appended to the existing slice,
-		// and readers may already have a copy of this postings slice,
-		// so we need to copy it before sorting.
-		old := list
-		list = make([]storage.SeriesRef, len(old), cap(old))
-		copy(list, old)
-		nm[l.Value] = list
-	}
-
-	// Repair order violations.
+	// 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
diff --git a/tsdb/index/postings_test.go b/tsdb/index/postings_test.go
index 7d0b717bf0e..96c9ed124b4 100644
--- a/tsdb/index/postings_test.go
+++ b/tsdb/index/postings_test.go
@@ -1475,58 +1475,3 @@ func TestMemPostings_PostingsForLabelMatchingHonorsContextCancel(t *testing.T) {
 	require.Error(t, p.Err())
 	require.Equal(t, failAfter+1, ctx.Count()) // Plus one for the Err() call that puts the error in the result.
 }
-
-func TestMemPostings_Unordered_Add_Get(t *testing.T) {
-	mp := NewMemPostings()
-	for ref := storage.SeriesRef(1); ref < 8; ref += 2 {
-		// First, add next series.
-		next := ref + 1
-		mp.Add(next, labels.FromStrings(labels.MetricName, "test", "series", strconv.Itoa(int(next))))
-		nextPostings := mp.Get(labels.MetricName, "test")
-
-		// Now add current ref.
-		mp.Add(ref, labels.FromStrings(labels.MetricName, "test", "series", strconv.Itoa(int(ref))))
-
-		// Next postings should still reference the next series.
-		nextExpanded, err := ExpandPostings(nextPostings)
-		require.NoError(t, err)
-		require.Len(t, nextExpanded, int(ref))
-		require.Equal(t, next, nextExpanded[len(nextExpanded)-1])
-	}
-}
-
-func TestMemPostings_Concurrent_Add_Get(t *testing.T) {
-	refs := make(chan storage.SeriesRef)
-	wg := sync.WaitGroup{}
-	wg.Add(1)
-	t.Cleanup(wg.Wait)
-	t.Cleanup(func() { close(refs) })
-
-	mp := NewMemPostings()
-	go func() {
-		defer wg.Done()
-		for ref := range refs {
-			mp.Add(ref, labels.FromStrings(labels.MetricName, "test", "series", strconv.Itoa(int(ref))))
-			p := mp.Get(labels.MetricName, "test")
-
-			_, err := ExpandPostings(p)
-			if err != nil {
-				t.Errorf("unexpected error: %s", err)
-			}
-		}
-	}()
-
-	for ref := storage.SeriesRef(1); ref < 8; ref += 2 {
-		// Add next ref in another goroutine so they would race.
-		refs <- ref + 1
-		// Add current ref here
-		mp.Add(ref, labels.FromStrings(labels.MetricName, "test", "series", strconv.Itoa(int(ref))))
-
-		// We don't read the value of the postings here,
-		// this is tested in TestMemPostings_Unordered_Add_Get where it's easier to achieve the determinism.
-		// This test just checks that there's no data race.
-		p := mp.Get(labels.MetricName, "test")
-		_, err := ExpandPostings(p)
-		require.NoError(t, err)
-	}
-}