diff --git a/index.go b/index.go
index 11b3b3711..e16ab3c25 100644
--- a/index.go
+++ b/index.go
@@ -65,7 +65,7 @@ type memPostings struct {
 
 // Postings returns an iterator over the postings list for s.
 func (p *memPostings) get(t term) Postings {
-	return &listIterator{list: p.m[t], idx: -1}
+	return &listPostings{list: p.m[t], idx: -1}
 }
 
 // add adds a document to the index. The caller has to ensure that no
@@ -169,22 +169,22 @@ func (it *mergePostings) Err() error {
 	return nil
 }
 
-// listIterator implements the Iterator interface over a plain list.
-type listIterator struct {
+// listPostings implements the Postings interface over a plain list.
+type listPostings struct {
 	list []uint32
 	idx  int
 }
 
-func (it *listIterator) Value() uint32 {
+func (it *listPostings) Value() uint32 {
 	return it.list[it.idx]
 }
 
-func (it *listIterator) Next() bool {
+func (it *listPostings) Next() bool {
 	it.idx++
 	return it.idx < len(it.list)
 }
 
-func (it *listIterator) Seek(x uint32) bool {
+func (it *listPostings) Seek(x uint32) bool {
 	// Do binary search between current position and end.
 	it.idx = sort.Search(len(it.list)-it.idx, func(i int) bool {
 		return it.list[i+it.idx] >= x
@@ -192,7 +192,7 @@ func (it *listIterator) Seek(x uint32) bool {
 	return it.idx < len(it.list)
 }
 
-func (it *listIterator) Err() error {
+func (it *listPostings) Err() error {
 	return nil
 }
 
diff --git a/querier.go b/querier.go
index 33141c32a..b9ee62a81 100644
--- a/querier.go
+++ b/querier.go
@@ -491,6 +491,9 @@ func (it *chainedSeriesIterator) Seek(t int64) bool {
 }
 
 func (it *chainedSeriesIterator) Next() bool {
+	if it.cur == nil {
+		it.cur = it.series[it.i].Iterator()
+	}
 	if it.cur.Next() {
 		return true
 	}
diff --git a/querier_test.go b/querier_test.go
index 0da4d0608..8e1a6f736 100644
--- a/querier_test.go
+++ b/querier_test.go
@@ -2,11 +2,185 @@ package tsdb
 
 import (
 	"math/rand"
+	"sort"
 	"testing"
 
 	"github.com/stretchr/testify/require"
 )
 
+type mockSeriesIterator struct {
+	seek   func(int64) bool
+	values func() (int64, float64)
+	next   func() bool
+	err    func() error
+}
+
+func (m *mockSeriesIterator) Seek(t int64) bool        { return m.seek(t) }
+func (m *mockSeriesIterator) Values() (int64, float64) { return m.values() }
+func (m *mockSeriesIterator) Next() bool               { return m.next() }
+func (m *mockSeriesIterator) Err() error               { return m.err() }
+
+type mockSeries struct {
+	labels   func() Labels
+	iterator func() SeriesIterator
+}
+
+func (m *mockSeries) Labels() Labels           { return m.labels() }
+func (m *mockSeries) Iterator() SeriesIterator { return m.iterator() }
+
+type listSeriesIterator struct {
+	list []sample
+	idx  int
+}
+
+func newListSeriesIterator(list []sample) *listSeriesIterator {
+	return &listSeriesIterator{list: list, idx: -1}
+}
+
+func (it *listSeriesIterator) Values() (int64, float64) {
+	s := it.list[it.idx]
+	return s.t, s.v
+}
+
+func (it *listSeriesIterator) Next() bool {
+	it.idx++
+	return it.idx < len(it.list)
+}
+
+func (it *listSeriesIterator) Seek(t int64) bool {
+	// Do binary search between current position and end.
+	it.idx = sort.Search(len(it.list)-it.idx, func(i int) bool {
+		s := it.list[i+it.idx]
+		return s.t >= t
+	})
+	return it.idx < len(it.list)
+}
+
+func (it *listSeriesIterator) Err() error {
+	return nil
+}
+
+// func TestChainedSeriesIterator(t *testing.T) {
+
+// 	cases := []struct {
+// 		series []Series
+// 	}{}
+
+// 	for _, c := range cases {
+
+// 	}
+// }
+
+type mockSeriesSet struct {
+	next   func() bool
+	series func() Series
+	err    func() error
+}
+
+func (m *mockSeriesSet) Next() bool     { return m.next() }
+func (m *mockSeriesSet) Series() Series { return m.series() }
+func (m *mockSeriesSet) Err() error     { return m.err() }
+
+func newListSeriesSet(list []Series) *mockSeriesSet {
+	i := -1
+	return &mockSeriesSet{
+		next: func() bool {
+			i++
+			return i < len(list)
+		},
+		series: func() Series {
+			return list[i]
+		},
+		err: func() error { return nil },
+	}
+}
+
+func TestShardSeriesSet(t *testing.T) {
+	newSeries := func(l map[string]string, s []sample) Series {
+		return &mockSeries{
+			labels:   func() Labels { return LabelsFromMap(l) },
+			iterator: func() SeriesIterator { return newListSeriesIterator(s) },
+		}
+	}
+
+	cases := []struct {
+		// The input sets in order (samples in series in b are strictly
+		// after those in a).
+		a, b SeriesSet
+		// The composition of a and b in the shard series set must yield
+		// results equivalent to the result series set.
+		exp SeriesSet
+	}{
+		{
+			a: newListSeriesSet([]Series{
+				newSeries(map[string]string{
+					"a": "a",
+				}, []sample{
+					{t: 1, v: 1},
+				}),
+			}),
+			b: newListSeriesSet([]Series{
+				newSeries(map[string]string{
+					"a": "a",
+				}, []sample{
+					{t: 2, v: 2},
+				}),
+				newSeries(map[string]string{
+					"b": "b",
+				}, []sample{
+					{t: 1, v: 1},
+				}),
+			}),
+			exp: newListSeriesSet([]Series{
+				newSeries(map[string]string{
+					"a": "a",
+				}, []sample{
+					{t: 1, v: 1},
+					{t: 2, v: 2},
+				}),
+				newSeries(map[string]string{
+					"b": "b",
+				}, []sample{
+					{t: 1, v: 1},
+				}),
+			}),
+		},
+	}
+
+Outer:
+	for _, c := range cases {
+		res := newShardSeriesSet(c.a, c.b)
+
+		for {
+			eok, rok := c.exp.Next(), res.Next()
+			require.Equal(t, eok, rok, "next")
+
+			if !eok {
+				continue Outer
+			}
+			sexp := c.exp.Series()
+			sres := res.Series()
+
+			require.Equal(t, sexp.Labels(), sres.Labels(), "labels")
+
+			smplExp, errExp := expandSeriesIterator(sexp.Iterator())
+			smplRes, errRes := expandSeriesIterator(sres.Iterator())
+
+			require.Equal(t, errExp, errRes, "samples error")
+			require.Equal(t, smplExp, smplRes, "samples")
+		}
+	}
+}
+
+func expandSeriesIterator(it SeriesIterator) (r []sample, err error) {
+	for it.Next() {
+		t, v := it.Values()
+		r = append(r, sample{t: t, v: v})
+	}
+
+	return r, it.Err()
+}
+
 func TestCompareLabels(t *testing.T) {
 	cases := []struct {
 		a, b []Label
diff --git a/reader.go b/reader.go
index ff5beb8a1..4ef550f42 100644
--- a/reader.go
+++ b/reader.go
@@ -364,7 +364,7 @@ func (r *indexReader) Postings(name, value string) (Postings, error) {
 		b = b[4:]
 	}
 
-	return &listIterator{list: l, idx: -1}, nil
+	return &listPostings{list: l, idx: -1}, nil
 }
 
 type stringTuples struct {