diff --git a/tsdb/block_test.go b/tsdb/block_test.go
index 3c2990cf3..c292cdcf5 100644
--- a/tsdb/block_test.go
+++ b/tsdb/block_test.go
@@ -323,7 +323,7 @@ func TestBlockSize(t *testing.T) {
 		require.NoError(t, err)
 		require.Equal(t, expAfterDelete, actAfterDelete, "after a delete reported block size doesn't match actual disk size")
 
-		c, err := NewLeveledCompactor(context.Background(), nil, log.NewNopLogger(), []int64{0}, nil)
+		c, err := NewLeveledCompactor(context.Background(), nil, log.NewNopLogger(), []int64{0}, nil, nil)
 		require.NoError(t, err)
 		blockDirAfterCompact, err := c.Compact(tmpdir, []string{blockInit.Dir()}, nil)
 		require.NoError(t, err)
@@ -426,7 +426,7 @@ func createBlock(tb testing.TB, dir string, series []storage.Series) string {
 }
 
 func createBlockFromHead(tb testing.TB, dir string, head *Head) string {
-	compactor, err := NewLeveledCompactor(context.Background(), nil, log.NewNopLogger(), []int64{1000000}, nil)
+	compactor, err := NewLeveledCompactor(context.Background(), nil, log.NewNopLogger(), []int64{1000000}, nil, nil)
 	require.NoError(tb, err)
 
 	require.NoError(tb, os.MkdirAll(dir, 0777))
diff --git a/tsdb/blockwriter.go b/tsdb/blockwriter.go
index 2d19c7705..95d6f2854 100644
--- a/tsdb/blockwriter.go
+++ b/tsdb/blockwriter.go
@@ -100,7 +100,7 @@ func (w *BlockWriter) Flush(ctx context.Context) (ulid.ULID, error) {
 		nil,
 		w.logger,
 		[]int64{w.blockSize},
-		chunkenc.NewPool())
+		chunkenc.NewPool(), nil)
 	if err != nil {
 		return ulid.ULID{}, errors.Wrap(err, "create leveled compactor")
 	}
diff --git a/tsdb/compact.go b/tsdb/compact.go
index 9befdc552..ca7baa191 100644
--- a/tsdb/compact.go
+++ b/tsdb/compact.go
@@ -82,6 +82,7 @@ type LeveledCompactor struct {
 	chunkPool                chunkenc.Pool
 	ctx                      context.Context
 	maxBlockChunkSegmentSize int64
+	mergeFunc                storage.VerticalChunkSeriesMergeFunc
 }
 
 type compactorMetrics struct {
@@ -145,11 +146,11 @@ func newCompactorMetrics(r prometheus.Registerer) *compactorMetrics {
 }
 
 // NewLeveledCompactor returns a LeveledCompactor.
-func NewLeveledCompactor(ctx context.Context, r prometheus.Registerer, l log.Logger, ranges []int64, pool chunkenc.Pool) (*LeveledCompactor, error) {
-	return NewLeveledCompactorWithChunkSize(ctx, r, l, ranges, pool, chunks.DefaultChunkSegmentSize)
+func NewLeveledCompactor(ctx context.Context, r prometheus.Registerer, l log.Logger, ranges []int64, pool chunkenc.Pool, mergeFunc storage.VerticalChunkSeriesMergeFunc) (*LeveledCompactor, error) {
+	return NewLeveledCompactorWithChunkSize(ctx, r, l, ranges, pool, chunks.DefaultChunkSegmentSize, mergeFunc)
 }
 
-func NewLeveledCompactorWithChunkSize(ctx context.Context, r prometheus.Registerer, l log.Logger, ranges []int64, pool chunkenc.Pool, maxBlockChunkSegmentSize int64) (*LeveledCompactor, error) {
+func NewLeveledCompactorWithChunkSize(ctx context.Context, r prometheus.Registerer, l log.Logger, ranges []int64, pool chunkenc.Pool, maxBlockChunkSegmentSize int64, mergeFunc storage.VerticalChunkSeriesMergeFunc) (*LeveledCompactor, error) {
 	if len(ranges) == 0 {
 		return nil, errors.Errorf("at least one range must be provided")
 	}
@@ -159,6 +160,9 @@ func NewLeveledCompactorWithChunkSize(ctx context.Context, r prometheus.Register
 	if l == nil {
 		l = log.NewNopLogger()
 	}
+	if mergeFunc == nil {
+		mergeFunc = storage.NewCompactingChunkSeriesMerger(storage.ChainedSeriesMerge)
+	}
 	return &LeveledCompactor{
 		ranges:                   ranges,
 		chunkPool:                pool,
@@ -166,6 +170,7 @@ func NewLeveledCompactorWithChunkSize(ctx context.Context, r prometheus.Register
 		metrics:                  newCompactorMetrics(r),
 		ctx:                      ctx,
 		maxBlockChunkSegmentSize: maxBlockChunkSegmentSize,
+		mergeFunc:                mergeFunc,
 	}, nil
 }
 
@@ -746,8 +751,9 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 
 	set := sets[0]
 	if len(sets) > 1 {
-		// Merge series using compacting chunk series merger.
-		set = storage.NewMergeChunkSeriesSet(sets, storage.NewCompactingChunkSeriesMerger(storage.ChainedSeriesMerge))
+		// Merge series using specified chunk series merger.
+		// The default one is the compacting series merger.
+		set = storage.NewMergeChunkSeriesSet(sets, c.mergeFunc)
 	}
 
 	// Iterate over all sorted chunk series.
diff --git a/tsdb/compact_test.go b/tsdb/compact_test.go
index a08464216..255a4c14d 100644
--- a/tsdb/compact_test.go
+++ b/tsdb/compact_test.go
@@ -158,7 +158,7 @@ func TestNoPanicFor0Tombstones(t *testing.T) {
 		},
 	}
 
-	c, err := NewLeveledCompactor(context.Background(), nil, nil, []int64{50}, nil)
+	c, err := NewLeveledCompactor(context.Background(), nil, nil, []int64{50}, nil, nil)
 	require.NoError(t, err)
 
 	c.plan(metas)
@@ -172,7 +172,7 @@ func TestLeveledCompactor_plan(t *testing.T) {
 		180,
 		540,
 		1620,
-	}, nil)
+	}, nil, nil)
 	require.NoError(t, err)
 
 	cases := map[string]struct {
@@ -381,7 +381,7 @@ func TestRangeWithFailedCompactionWontGetSelected(t *testing.T) {
 		240,
 		720,
 		2160,
-	}, nil)
+	}, nil, nil)
 	require.NoError(t, err)
 
 	cases := []struct {
@@ -431,7 +431,7 @@ func TestCompactionFailWillCleanUpTempDir(t *testing.T) {
 		240,
 		720,
 		2160,
-	}, nil)
+	}, nil, nil)
 	require.NoError(t, err)
 
 	tmpdir, err := ioutil.TempDir("", "test")
@@ -940,7 +940,7 @@ func TestCompaction_populateBlock(t *testing.T) {
 				blocks = append(blocks, &mockBReader{ir: ir, cr: cr, mint: mint, maxt: maxt})
 			}
 
-			c, err := NewLeveledCompactor(context.Background(), nil, nil, []int64{0}, nil)
+			c, err := NewLeveledCompactor(context.Background(), nil, nil, []int64{0}, nil, nil)
 			require.NoError(t, err)
 
 			meta := &BlockMeta{
@@ -1065,7 +1065,7 @@ func BenchmarkCompaction(b *testing.B) {
 				blockDirs = append(blockDirs, block.Dir())
 			}
 
-			c, err := NewLeveledCompactor(context.Background(), nil, log.NewNopLogger(), []int64{0}, nil)
+			c, err := NewLeveledCompactor(context.Background(), nil, log.NewNopLogger(), []int64{0}, nil, nil)
 			require.NoError(b, err)
 
 			b.ResetTimer()
diff --git a/tsdb/db.go b/tsdb/db.go
index d6036f5e5..2bb56a85c 100644
--- a/tsdb/db.go
+++ b/tsdb/db.go
@@ -370,6 +370,7 @@ func (db *DBReadOnly) FlushWAL(dir string) (returnErr error) {
 		db.logger,
 		ExponentialBlockRanges(DefaultOptions().MinBlockDuration, 3, 5),
 		chunkenc.NewPool(),
+		nil,
 	)
 	if err != nil {
 		return errors.Wrap(err, "create leveled compactor")
@@ -648,7 +649,7 @@ func open(dir string, l log.Logger, r prometheus.Registerer, opts *Options, rngs
 
 	var err error
 	ctx, cancel := context.WithCancel(context.Background())
-	db.compactor, err = NewLeveledCompactorWithChunkSize(ctx, r, l, rngs, db.chunkPool, opts.MaxBlockChunkSegmentSize)
+	db.compactor, err = NewLeveledCompactorWithChunkSize(ctx, r, l, rngs, db.chunkPool, opts.MaxBlockChunkSegmentSize, nil)
 	if err != nil {
 		cancel()
 		return nil, errors.Wrap(err, "create leveled compactor")