diff --git a/tsdb/index/index.go b/tsdb/index/index.go
index fb68c71a2..d28be4f72 100644
--- a/tsdb/index/index.go
+++ b/tsdb/index/index.go
@@ -116,6 +116,12 @@ type Writer struct {
 	fbuf *bufio.Writer
 	pos  uint64
 
+	// Temporary file for posting offsets table.
+	fPO    *os.File
+	fbufPO *bufio.Writer
+	posPO  uint64
+	cntPO  uint64
+
 	toc   TOC
 	stage indexWriterStage
 
@@ -193,7 +199,13 @@ func NewWriter(ctx context.Context, fn string) (*Writer, error) {
 		return nil, errors.Wrap(err, "remove any existing index at path")
 	}
 
-	f, err := os.OpenFile(fn, os.O_CREATE|os.O_WRONLY, 0666)
+	// Main index file we are building.
+	f, err := os.OpenFile(fn, os.O_CREATE|os.O_RDWR, 0666)
+	if err != nil {
+		return nil, err
+	}
+	// Temporary file for posting offset table.
+	fPO, err := os.OpenFile(fn+"_tmp_po", os.O_CREATE|os.O_RDWR, 0666)
 	if err != nil {
 		return nil, err
 	}
@@ -202,11 +214,14 @@ func NewWriter(ctx context.Context, fn string) (*Writer, error) {
 	}
 
 	iw := &Writer{
-		ctx:   ctx,
-		f:     f,
-		fbuf:  bufio.NewWriterSize(f, 1<<22),
-		pos:   0,
-		stage: idxStageNone,
+		ctx:    ctx,
+		f:      f,
+		fbuf:   bufio.NewWriterSize(f, 1<<22),
+		pos:    0,
+		fPO:    fPO,
+		fbufPO: bufio.NewWriterSize(fPO, 1<<22),
+		posPO:  0,
+		stage:  idxStageNone,
 
 		// Reusable memory.
 		buf1: encoding.Encbuf{B: make([]byte, 0, 1<<22)},
@@ -564,39 +579,60 @@ func (w *Writer) writeLabelIndexesOffsetTable() error {
 
 // writePostingsOffsetTable writes the postings offset table.
 func (w *Writer) writePostingsOffsetTable() error {
-	startPos := w.pos
-	// Leave 4 bytes of space for the length, which will be calculated later.
-	if err := w.write([]byte("alen")); err != nil {
+	// Ensure everything is in the temporary file.
+	if err := w.fbufPO.Flush(); err != nil {
 		return err
 	}
-	w.crc32.Reset()
 
 	w.buf1.Reset()
-	w.buf1.PutBE32int(len(w.postings))
-	w.buf1.WriteToHash(w.crc32)
+	w.buf1.PutBE32int(int(w.posPO) + 4) // Length, including the count.
 	if err := w.write(w.buf1.Get()); err != nil {
 		return err
 	}
 
-	for _, e := range w.postings {
-		w.buf1.Reset()
-		w.buf1.PutUvarint(2)
-		w.buf1.PutUvarintStr(e.name)
-		w.buf1.PutUvarintStr(e.value)
-		w.buf1.PutUvarint64(e.offset)
-		w.buf1.WriteToHash(w.crc32)
-		if err := w.write(w.buf1.Get()); err != nil {
-			return err
-		}
-	}
-
-	// Write out the length.
 	w.buf1.Reset()
-	w.buf1.PutBE32int(int(w.pos - startPos - 4))
-	if err := w.writeAt(w.buf1.Get(), startPos); err != nil {
+	w.crc32.Reset()
+	w.buf1.PutBE32int(int(w.cntPO)) // Count.
+	w.buf1.WriteToHash(w.crc32)
+	if err := w.write(w.buf1.Get()); err != nil {
+		return err
+	}
+	// Copy temporary file into main index.
+	if _, err := w.fPO.Seek(0, 0); err != nil {
 		return err
 	}
 
+	buf := make([]byte, 1<<20)
+	l := 0
+	for {
+		n, err := w.fPO.Read(buf)
+		if err != nil && err != io.EOF {
+			return err
+		}
+		if n == 0 {
+			break
+		}
+		l += n
+		w.crc32.Write(buf[:n])
+		if err := w.write(buf[:n]); err != nil {
+			return err
+		}
+	}
+	if w.posPO != uint64(l) {
+		return errors.Errorf("wrote %d bytes to posting offset temporary file, but only read back %d", w.posPO, l)
+	}
+
+	// Cleanup temporary file.
+	name := w.fPO.Name()
+	if err := w.fPO.Close(); err != nil {
+		return err
+	}
+	w.fPO = nil
+	if err := os.Remove(name); err != nil {
+		return err
+	}
+
+	// Finally write the hash.
 	w.buf1.Reset()
 	w.buf1.PutHashSum(w.crc32)
 	return w.write(w.buf1.Get())
@@ -749,11 +785,18 @@ func (w *Writer) writePosting(name, value string, offs []uint32) error {
 		return err
 	}
 
-	w.postings = append(w.postings, postingsHashEntry{
-		name:   name,
-		value:  value,
-		offset: w.pos,
-	})
+	// Write out postings offset table to temporary file as we go.
+	w.buf1.Reset()
+	w.buf1.PutUvarint(2)
+	w.buf1.PutUvarintStr(name)
+	w.buf1.PutUvarintStr(value)
+	w.buf1.PutUvarint64(w.pos)
+	if n, err := w.fbufPO.Write(w.buf1.Get()); err != nil {
+		return err
+	} else {
+		w.posPO += uint64(n)
+	}
+	w.cntPO++
 
 	w.buf1.Reset()
 	w.buf1.PutBE32int(len(offs))
@@ -796,6 +839,11 @@ func (w *Writer) Close() error {
 			return err
 		}
 	}
+	if w.fPO != nil {
+		if err := w.fPO.Close(); err != nil {
+			return err
+		}
+	}
 	if err := w.fbuf.Flush(); err != nil {
 		return err
 	}