diff --git a/chunkenc/chunk.go b/chunkenc/chunk.go
index bc6c2010..71325843 100644
--- a/chunkenc/chunk.go
+++ b/chunkenc/chunk.go
@@ -58,9 +58,10 @@ type Appender interface {
 
 // Iterator iterates over the data of a time series.
 type Iterator interface {
-	// Seek advances the iterator forward to the given timestamp.
-	// If there's no value exactly at t, it advances to the first value
-	// after t.
+	// Seek advances the iterator forward to the sample with the timestamp t or first value after t.
+	// If the current iterator points to the sample with timestamp after t already,
+	// Seek should not advance the iterator.
+	// Seek returns false if there is no such sample with the timestamp equal or larger than t.
 	Seek(t int64) bool
 	// At returns the current timestamp/value pair.
 	At() (int64, float64)
diff --git a/chunkenc/chunk_test.go b/chunkenc/chunk_test.go
index f6577b42..dd7b6615 100644
--- a/chunkenc/chunk_test.go
+++ b/chunkenc/chunk_test.go
@@ -17,7 +17,6 @@ import (
 	"fmt"
 	"io"
 	"math/rand"
-	"reflect"
 	"testing"
 
 	"github.com/prometheus/tsdb/testutil"
@@ -35,28 +34,23 @@ func TestChunk(t *testing.T) {
 		t.Run(fmt.Sprintf("%v", enc), func(t *testing.T) {
 			for range make([]struct{}, 1) {
 				c := nc()
-				if err := testChunk(c); err != nil {
-					t.Fatal(err)
-				}
+				testChunk(t, c)
 			}
 		})
 	}
 }
 
-func testChunk(c Chunk) error {
+func testChunk(t *testing.T, c Chunk) {
 	app, err := c.Appender()
-	if err != nil {
-		return err
-	}
+	testutil.Ok(t, err)
 
-	var exp []pair
+	var all []pair
 	var (
 		ts = int64(1234123324)
 		v  = 1243535.123
 	)
 	for i := 0; i < 300; i++ {
 		ts += int64(rand.Intn(10000) + 1)
-		// v = rand.Float64()
 		if i%2 == 0 {
 			v += float64(rand.Intn(1000000))
 		} else {
@@ -67,29 +61,51 @@ func testChunk(c Chunk) error {
 		// appending to a partially filled chunk.
 		if i%10 == 0 {
 			app, err = c.Appender()
-			if err != nil {
-				return err
-			}
+			testutil.Ok(t, err)
 		}
 
 		app.Append(ts, v)
-		exp = append(exp, pair{t: ts, v: v})
-		// fmt.Println("appended", len(c.Bytes()), c.Bytes())
+		all = append(all, pair{t: ts, v: v})
 	}
 
-	it := c.Iterator(nil)
-	var res []pair
-	for it.Next() {
-		ts, v := it.At()
-		res = append(res, pair{t: ts, v: v})
+	// 1. Expand iterator in simple case.
+	it1 := c.Iterator(nil)
+	var res1 []pair
+	for it1.Next() {
+		ts, v := it1.At()
+		res1 = append(res1, pair{t: ts, v: v})
 	}
-	if it.Err() != nil {
-		return it.Err()
+	testutil.Ok(t, it1.Err())
+	testutil.Equals(t, all, res1)
+
+	// 2. Expand second iterator while reusing first one.
+	it2 := c.Iterator(it1)
+	var res2 []pair
+	for it2.Next() {
+		ts, v := it2.At()
+		res2 = append(res2, pair{t: ts, v: v})
 	}
-	if !reflect.DeepEqual(exp, res) {
-		return fmt.Errorf("unexpected result\n\ngot: %v\n\nexp: %v", res, exp)
+	testutil.Ok(t, it2.Err())
+	testutil.Equals(t, all, res2)
+
+	// 3. Test Iterator Seek.
+	mid := len(all) / 2
+
+	it3 := c.Iterator(nil)
+	var res3 []pair
+	testutil.Equals(t, true, it3.Seek(all[mid].t))
+	// Below ones should not matter.
+	testutil.Equals(t, true, it3.Seek(all[mid].t))
+	testutil.Equals(t, true, it3.Seek(all[mid].t))
+	ts, v = it3.At()
+	res3 = append(res3, pair{t: ts, v: v})
+
+	for it3.Next() {
+		ts, v := it3.At()
+		res3 = append(res3, pair{t: ts, v: v})
 	}
-	return nil
+	testutil.Ok(t, it3.Err())
+	testutil.Equals(t, all[mid:], res3)
 }
 
 func benchmarkIterator(b *testing.B, newChunk func() Chunk) {
diff --git a/chunkenc/xor.go b/chunkenc/xor.go
index 7d31c4c5..86c05f0f 100644
--- a/chunkenc/xor.go
+++ b/chunkenc/xor.go
@@ -242,16 +242,16 @@ type xorIterator struct {
 }
 
 func (it *xorIterator) Seek(t int64) bool {
-	if t >= it.t {
+	if it.err != nil {
 		return false
 	}
 
-	for it.Next() {
-		if t >= it.t {
-			return true
+	for t > it.t || it.numRead == 0 {
+		if !it.Next() {
+			return false
 		}
 	}
-	return false
+	return true
 }
 
 func (it *xorIterator) At() (int64, float64) {
diff --git a/querier_test.go b/querier_test.go
index cc48c95d..5ef27b75 100644
--- a/querier_test.go
+++ b/querier_test.go
@@ -924,7 +924,7 @@ func TestSeriesIterator(t *testing.T) {
 				if tc.success {
 					// Init the list and then proceed to check.
 					remaining := exp.Next()
-					testutil.Assert(t, remaining == true, "")
+					testutil.Assert(t, remaining, "")
 
 					for remaining {
 						sExp, eExp := exp.At()
@@ -1034,7 +1034,7 @@ func TestSeriesIterator(t *testing.T) {
 					if tc.success {
 						// Init the list and then proceed to check.
 						remaining := exp.Next()
-						testutil.Assert(t, remaining == true, "")
+						testutil.Assert(t, remaining, "")
 
 						for remaining {
 							sExp, eExp := exp.At()
@@ -1101,8 +1101,8 @@ func TestChunkSeriesIterator_DoubleSeek(t *testing.T) {
 	}
 
 	res := newChunkSeriesIterator(chkMetas, nil, 2, 8)
-	testutil.Assert(t, res.Seek(1) == true, "")
-	testutil.Assert(t, res.Seek(2) == true, "")
+	testutil.Assert(t, res.Seek(1), "")
+	testutil.Assert(t, res.Seek(2), "")
 	ts, v := res.At()
 	testutil.Equals(t, int64(2), ts)
 	testutil.Equals(t, float64(2), v)
@@ -1119,12 +1119,12 @@ func TestChunkSeriesIterator_SeekInCurrentChunk(t *testing.T) {
 
 	it := newChunkSeriesIterator(metas, nil, 1, 7)
 
-	testutil.Assert(t, it.Next() == true, "")
+	testutil.Assert(t, it.Next(), "")
 	ts, v := it.At()
 	testutil.Equals(t, int64(1), ts)
 	testutil.Equals(t, float64(2), v)
 
-	testutil.Assert(t, it.Seek(4) == true, "")
+	testutil.Assert(t, it.Seek(4), "")
 	ts, v = it.At()
 	testutil.Equals(t, int64(5), ts)
 	testutil.Equals(t, float64(6), v)