diff --git a/block.go b/block.go
index d24198a68..ef6d71a73 100644
--- a/block.go
+++ b/block.go
@@ -19,12 +19,11 @@ const (
 type Block interface{}
 
 type block interface {
-	stats() *seriesStats
+	stats() *blockStats
 	seriesData() seriesDataIterator
 }
 
 type persistedBlock struct {
-    
 }
 
 type seriesDataIterator interface {
@@ -50,7 +49,7 @@ const (
 
 const (
 	seriesMetaSize  = int(unsafe.Sizeof(seriesMeta{}))
-	seriesStatsSize = int(unsafe.Sizeof(seriesStats{}))
+	seriesStatsSize = int(unsafe.Sizeof(blockStats{}))
 )
 
 type seriesMeta struct {
@@ -59,7 +58,7 @@ type seriesMeta struct {
 	_     [7]byte // padding/reserved
 }
 
-type seriesStats struct {
+type blockStats struct {
 	series  uint32
 	samples uint64
 	_       [4]byte // padding/reserved
@@ -69,9 +68,9 @@ func (s *persistedSeries) meta() *seriesMeta {
 	return (*seriesMeta)(unsafe.Pointer(&s.data[0]))
 }
 
-func (s *persistedSeries) stats() *seriesStats {
+func (s *persistedSeries) stats() *blockStats {
 	// The stats start right behind the block meta data.
-	return (*seriesStats)(unsafe.Pointer(&s.data[seriesMetaSize]))
+	return (*blockStats)(unsafe.Pointer(&s.data[seriesMetaSize]))
 }
 
 // seriesAt returns the series stored at offset as a skiplist and the chunks
diff --git a/db.go b/db.go
index 6cd2b4a3c..0b28409d8 100644
--- a/db.go
+++ b/db.go
@@ -77,8 +77,8 @@ func (db *DB) Close() error {
 
 	for i, shard := range db.shards {
 		fmt.Println("shard", i)
-		fmt.Println("	num chunks", len(shard.head.forward))
-		fmt.Println("	num samples", shard.head.samples)
+		fmt.Println("	num chunks", shard.head.stats().series)
+		fmt.Println("	num samples", shard.head.stats().samples)
 
 		wg.Add(1)
 		go func(i int, shard *SeriesShard) {
@@ -185,16 +185,16 @@ func NewSeriesShard(path string) *SeriesShard {
 	return &SeriesShard{
 		path: path,
 		// TODO(fabxc): restore from checkpoint.
-		head: &HeadBlock{
-			ivIndex: newMemIndex(),
-			descs:   map[uint64][]*chunkDesc{},
-			values:  map[string]stringset{},
-			forward: map[uint32]*chunkDesc{},
-		},
+		head: NewHeadBlock(),
 		// TODO(fabxc): provide access to persisted blocks.
 	}
 }
 
+// blockFor returns the block of shard series that contains the given timestamp.
+func (s *SeriesShard) blockFor(ts int64) block {
+	return nil
+}
+
 // chunkDesc wraps a plain data chunk and provides cached meta data about it.
 type chunkDesc struct {
 	lset  Labels
diff --git a/head.go b/head.go
index 5b283abb3..7b191fb62 100644
--- a/head.go
+++ b/head.go
@@ -2,8 +2,6 @@ package tsdb
 
 import (
 	"math"
-	"sort"
-	"strings"
 	"sync"
 
 	"github.com/fabxc/tsdb/chunks"
@@ -11,15 +9,20 @@ import (
 
 // HeadBlock handles reads and writes of time series data within a time window.
 type HeadBlock struct {
-	mtx     sync.RWMutex
-	descs   map[uint64][]*chunkDesc // labels hash to possible chunks descs
-	forward map[uint32]*chunkDesc   // chunk ID to chunk desc
-	values  map[string]stringset    // label names to possible values
-	ivIndex *memIndex               // inverted index for label pairs
+	mtx   sync.RWMutex
+	descs map[uint64][]*chunkDesc // labels hash to possible chunks descs
+	index *memIndex
 
 	samples uint64 // total samples in the block.
 }
 
+func NewHeadBlock() *HeadBlock {
+	return &HeadBlock{
+		descs: make(map[uint64][]*chunkDesc, 2048),
+		index: newMemIndex(),
+	}
+}
+
 // get retrieves the chunk with the hash and label set and creates
 // a new one if it doesn't exist yet.
 func (h *HeadBlock) get(hash uint64, lset Labels) *chunkDesc {
@@ -34,39 +37,12 @@ func (h *HeadBlock) get(hash uint64, lset Labels) *chunkDesc {
 		lset:  lset,
 		chunk: chunks.NewXORChunk(int(math.MaxInt64)),
 	}
-	h.index(cd)
+	h.index.add(cd)
 
 	h.descs[hash] = append(cds, cd)
 	return cd
 }
 
-func (h *HeadBlock) index(chkd *chunkDesc) {
-	// Add each label pair as a term to the inverted index.
-	terms := make([]string, 0, len(chkd.lset))
-	b := make([]byte, 0, 64)
-
-	for _, l := range chkd.lset {
-		b = append(b, l.Name...)
-		b = append(b, sep)
-		b = append(b, l.Value...)
-
-		terms = append(terms, string(b))
-		b = b[:0]
-
-		// Add to label name to values index.
-		valset, ok := h.values[l.Name]
-		if !ok {
-			valset = stringset{}
-			h.values[l.Name] = valset
-		}
-		valset.set(l.Value)
-	}
-	id := h.ivIndex.add(terms...)
-
-	// Store forward index for the returned ID.
-	h.forward[id] = chkd
-}
-
 // append adds the sample to the headblock.
 func (h *HeadBlock) append(hash uint64, lset Labels, ts int64, v float64) error {
 	if err := h.get(hash, lset).append(ts, v); err != nil {
@@ -76,9 +52,9 @@ func (h *HeadBlock) append(hash uint64, lset Labels, ts int64, v float64) error
 	return nil
 }
 
-func (h *HeadBlock) stats() *seriesStats {
-	return &seriesStats{
-		series:  uint32(len(h.forward)),
+func (h *HeadBlock) stats() *blockStats {
+	return &blockStats{
+		series:  uint32(h.index.numSeries()),
 		samples: h.samples,
 	}
 }
@@ -88,11 +64,11 @@ func (h *HeadBlock) seriesData() seriesDataIterator {
 	defer h.mtx.RUnlock()
 
 	it := &chunkDescsIterator{
-		descs: make([]*chunkDesc, 0, len(h.forward)),
+		descs: make([]*chunkDesc, 0, len(h.index.forward)),
 		i:     -1,
 	}
 
-	for _, cd := range h.forward {
+	for _, cd := range h.index.forward {
 		it.descs = append(it.descs, cd)
 	}
 	return it
@@ -115,27 +91,3 @@ func (it *chunkDescsIterator) values() (skiplist, []chunks.Chunk) {
 func (it *chunkDescsIterator) err() error {
 	return nil
 }
-
-type stringset map[string]struct{}
-
-func (ss stringset) set(s string) {
-	ss[s] = struct{}{}
-}
-
-func (ss stringset) has(s string) bool {
-	_, ok := ss[s]
-	return ok
-}
-
-func (ss stringset) String() string {
-	return strings.Join(ss.slice(), ",")
-}
-
-func (ss stringset) slice() []string {
-	slice := make([]string, 0, len(ss))
-	for k := range ss {
-		slice = append(slice, k)
-	}
-	sort.Strings(slice)
-	return slice
-}
diff --git a/index.go b/index.go
index 064f31f23..f236410a5 100644
--- a/index.go
+++ b/index.go
@@ -1,38 +1,86 @@
 package tsdb
 
-import "sort"
+import (
+	"sort"
+	"strings"
+)
 
 // Index provides read access to an inverted index.
 type Index interface {
 	Postings(ref uint32) Iterator
 }
 
-// memIndex is an inverted in-memory index.
 type memIndex struct {
 	lastID uint32
-	m      map[string][]uint32
+
+	forward  map[uint32]*chunkDesc // chunk ID to chunk desc
+	values   map[string]stringset  // label names to possible values
+	postings *memPostings          // postings lists for terms
+}
+
+// newMemIndex returns a new in-memory  index.
+func newMemIndex() *memIndex {
+	return &memIndex{
+		lastID:   0,
+		forward:  make(map[uint32]*chunkDesc),
+		values:   make(map[string]stringset),
+		postings: &memPostings{m: make(map[string][]uint32)},
+	}
+}
+
+func (ix *memIndex) numSeries() int {
+	return len(ix.forward)
+}
+
+func (ix *memIndex) Postings(s string) Iterator {
+	return ix.postings.get(s)
+}
+
+func (ix *memIndex) add(chkd *chunkDesc) {
+	// Add each label pair as a term to the inverted index.
+	terms := make([]string, 0, len(chkd.lset))
+	b := make([]byte, 0, 64)
+
+	for _, l := range chkd.lset {
+		b = append(b, l.Name...)
+		b = append(b, sep)
+		b = append(b, l.Value...)
+
+		terms = append(terms, string(b))
+		b = b[:0]
+
+		// Add to label name to values index.
+		valset, ok := ix.values[l.Name]
+		if !ok {
+			valset = stringset{}
+			ix.values[l.Name] = valset
+		}
+		valset.set(l.Value)
+	}
+	ix.lastID++
+	id := ix.lastID
+
+	ix.postings.add(id, terms...)
+
+	// Store forward index for the returned ID.
+	ix.forward[id] = chkd
+}
+
+type memPostings struct {
+	m map[string][]uint32
 }
 
 // Postings returns an iterator over the postings list for s.
-func (ix *memIndex) Postings(s string) Iterator {
-	return &listIterator{list: ix.m[s], idx: -1}
+func (p *memPostings) get(s string) Iterator {
+	return &listIterator{list: p.m[s], idx: -1}
 }
 
 // add adds a document to the index. The caller has to ensure that no
 // term argument appears twice.
-func (ix *memIndex) add(terms ...string) uint32 {
-	ix.lastID++
-
+func (p *memPostings) add(id uint32, terms ...string) {
 	for _, t := range terms {
-		ix.m[t] = append(ix.m[t], ix.lastID)
+		p.m[t] = append(p.m[t], id)
 	}
-
-	return ix.lastID
-}
-
-// newMemIndex returns a new in-memory index.
-func newMemIndex() *memIndex {
-	return &memIndex{m: make(map[string][]uint32)}
 }
 
 // Iterator provides iterative access over a postings list.
@@ -47,11 +95,6 @@ type Iterator interface {
 	Value() uint32
 }
 
-// compressIndex returns a compressed index for the given input index.
-func compressIndex(ix Index) {
-
-}
-
 // Intersect returns a new iterator over the intersection of the
 // input iterators.
 func Intersect(its ...Iterator) Iterator {
@@ -133,3 +176,27 @@ func (it *listIterator) Seek(x uint32) bool {
 	})
 	return it.idx < len(it.list)
 }
+
+type stringset map[string]struct{}
+
+func (ss stringset) set(s string) {
+	ss[s] = struct{}{}
+}
+
+func (ss stringset) has(s string) bool {
+	_, ok := ss[s]
+	return ok
+}
+
+func (ss stringset) String() string {
+	return strings.Join(ss.slice(), ",")
+}
+
+func (ss stringset) slice() []string {
+	slice := make([]string, 0, len(ss))
+	for k := range ss {
+		slice = append(slice, k)
+	}
+	sort.Strings(slice)
+	return slice
+}