diff --git a/storage/buffer.go b/storage/buffer.go
index 2229e5259..38f559103 100644
--- a/storage/buffer.go
+++ b/storage/buffer.go
@@ -242,15 +242,16 @@ func (s fhSample) Type() chunkenc.ValueType {
 type sampleRing struct {
 	delta int64
 
-	// Lookback buffers. We use buf for mixed samples, but one of the three
+	// Lookback buffers. We use iBuf for mixed samples, but one of the three
 	// concrete ones for homogenous samples. (Only one of the four bufs is
 	// allowed to be populated!) This avoids the overhead of the interface
 	// wrapper for the happy (and by far most common) case of homogenous
 	// samples.
-	buf   []tsdbutil.Sample
-	fBuf  []fSample
-	hBuf  []hSample
-	fhBuf []fhSample
+	iBuf     []tsdbutil.Sample
+	fBuf     []fSample
+	hBuf     []hSample
+	fhBuf    []fhSample
+	bufInUse bufType
 
 	i int // Position of most recent element in ring buffer.
 	f int // Position of first element in ring buffer.
@@ -259,6 +260,16 @@ type sampleRing struct {
 	it sampleRingIterator
 }
 
+type bufType int
+
+const (
+	noBuf bufType = iota // Nothing yet stored in sampleRing.
+	iBuf
+	fBuf
+	hBuf
+	fhBuf
+)
+
 // newSampleRing creates a new sampleRing. If you do not know the prefereed
 // value type yet, use a size of 0 (in which case the provided typ doesn't
 // matter). On the first add, a buffer of size 16 will be allocated with the
@@ -278,7 +289,7 @@ func newSampleRing(delta int64, size int, typ chunkenc.ValueType) *sampleRing {
 	case chunkenc.ValFloatHistogram:
 		r.fhBuf = make([]fhSample, size)
 	default:
-		r.buf = make([]tsdbutil.Sample, size)
+		r.iBuf = make([]tsdbutil.Sample, size)
 	}
 	return r
 }
@@ -287,6 +298,7 @@ func (r *sampleRing) reset() {
 	r.l = 0
 	r.i = -1
 	r.f = 0
+	r.bufInUse = noBuf
 }
 
 // Returns the current iterator. Invalidates previously returned iterators.
@@ -310,18 +322,18 @@ func (it *sampleRingIterator) Next() chunkenc.ValueType {
 	if it.i >= it.r.l {
 		return chunkenc.ValNone
 	}
-	switch {
-	case len(it.r.fBuf) > 0:
+	switch it.r.bufInUse {
+	case fBuf:
 		s := it.r.atF(it.i)
 		it.t = s.t
 		it.f = s.f
 		return chunkenc.ValFloat
-	case len(it.r.hBuf) > 0:
+	case hBuf:
 		s := it.r.atH(it.i)
 		it.t = s.t
 		it.h = s.h
 		return chunkenc.ValHistogram
-	case len(it.r.fhBuf) > 0:
+	case fhBuf:
 		s := it.r.atFH(it.i)
 		it.t = s.t
 		it.fh = s.fh
@@ -372,8 +384,8 @@ func (it *sampleRingIterator) AtT() int64 {
 }
 
 func (r *sampleRing) at(i int) tsdbutil.Sample {
-	j := (r.f + i) % len(r.buf)
-	return r.buf[j]
+	j := (r.f + i) % len(r.iBuf)
+	return r.iBuf[j]
 }
 
 func (r *sampleRing) atF(i int) fSample {
@@ -397,91 +409,113 @@ func (r *sampleRing) atFH(i int) fhSample {
 // from this package (fSample, hSample, fhSample), call one of the specialized
 // methods addF, addH, or addFH for better performance.
 func (r *sampleRing) add(s tsdbutil.Sample) {
-	if len(r.buf) == 0 {
+	if r.bufInUse == noBuf {
+		// First sample.
+		switch s := s.(type) {
+		case fSample:
+			r.bufInUse = fBuf
+			r.fBuf = addF(s, r.fBuf, r)
+		case hSample:
+			r.bufInUse = hBuf
+			r.hBuf = addH(s, r.hBuf, r)
+		case fhSample:
+			r.bufInUse = fhBuf
+			r.fhBuf = addFH(s, r.fhBuf, r)
+		}
+		return
+	}
+	if r.bufInUse != iBuf {
 		// Nothing added to the interface buf yet. Let's check if we can
 		// stay specialized.
 		switch s := s.(type) {
 		case fSample:
-			if len(r.hBuf)+len(r.fhBuf) == 0 {
+			if r.bufInUse == fBuf {
 				r.fBuf = addF(s, r.fBuf, r)
 				return
 			}
 		case hSample:
-			if len(r.fBuf)+len(r.fhBuf) == 0 {
+			if r.bufInUse == hBuf {
 				r.hBuf = addH(s, r.hBuf, r)
 				return
 			}
 		case fhSample:
-			if len(r.fBuf)+len(r.hBuf) == 0 {
+			if r.bufInUse == fhBuf {
 				r.fhBuf = addFH(s, r.fhBuf, r)
 				return
 			}
 		}
 		// The new sample isn't a fit for the already existing
 		// ones. Copy the latter into the interface buffer where needed.
-		switch {
-		case len(r.fBuf) > 0:
+		switch r.bufInUse {
+		case fBuf:
 			for _, s := range r.fBuf {
-				r.buf = append(r.buf, s)
+				r.iBuf = append(r.iBuf, s)
 			}
 			r.fBuf = nil
-		case len(r.hBuf) > 0:
+		case hBuf:
 			for _, s := range r.hBuf {
-				r.buf = append(r.buf, s)
+				r.iBuf = append(r.iBuf, s)
 			}
 			r.hBuf = nil
-		case len(r.fhBuf) > 0:
+		case fhBuf:
 			for _, s := range r.fhBuf {
-				r.buf = append(r.buf, s)
+				r.iBuf = append(r.iBuf, s)
 			}
 			r.fhBuf = nil
 		}
+		r.bufInUse = iBuf
 	}
-	r.buf = addSample(s, r.buf, r)
+	r.iBuf = addSample(s, r.iBuf, r)
 }
 
 // addF is a version of the add method specialized for fSample.
 func (r *sampleRing) addF(s fSample) {
-	switch {
-	case len(r.buf) > 0:
-		// Already have interface samples. Add to the interface buf.
-		r.buf = addSample(s, r.buf, r)
-	case len(r.hBuf)+len(r.fhBuf) > 0:
+	switch r.bufInUse {
+	case fBuf: // Add to existing fSamples.
+		r.fBuf = addF(s, r.fBuf, r)
+	case noBuf: // Add first sample.
+		r.fBuf = addF(s, r.fBuf, r)
+		r.bufInUse = fBuf
+	case iBuf: // Already have interface samples. Add to the interface buf.
+		r.iBuf = addSample(s, r.iBuf, r)
+	default:
 		// Already have specialized samples that are not fSamples.
 		// Need to call the checked add method for conversion.
 		r.add(s)
-	default:
-		r.fBuf = addF(s, r.fBuf, r)
 	}
 }
 
 // addH is a version of the add method specialized for hSample.
 func (r *sampleRing) addH(s hSample) {
-	switch {
-	case len(r.buf) > 0:
-		// Already have interface samples. Add to the interface buf.
-		r.buf = addSample(s, r.buf, r)
-	case len(r.fBuf)+len(r.fhBuf) > 0:
-		// Already have samples that are not hSamples.
+	switch r.bufInUse {
+	case hBuf: // Add to existing hSamples.
+		r.hBuf = addH(s, r.hBuf, r)
+	case noBuf: // Add first sample.
+		r.hBuf = addH(s, r.hBuf, r)
+		r.bufInUse = hBuf
+	case iBuf: // Already have interface samples. Add to the interface buf.
+		r.iBuf = addSample(s, r.iBuf, r)
+	default:
+		// Already have specialized samples that are not hSamples.
 		// Need to call the checked add method for conversion.
 		r.add(s)
-	default:
-		r.hBuf = addH(s, r.hBuf, r)
 	}
 }
 
 // addFH is a version of the add method specialized for fhSample.
 func (r *sampleRing) addFH(s fhSample) {
-	switch {
-	case len(r.buf) > 0:
-		// Already have interface samples. Add to the interface buf.
-		r.buf = addSample(s, r.buf, r)
-	case len(r.fBuf)+len(r.hBuf) > 0:
-		// Already have samples that are not fhSamples.
+	switch r.bufInUse {
+	case fhBuf: // Add to existing fhSamples.
+		r.fhBuf = addFH(s, r.fhBuf, r)
+	case noBuf: // Add first sample.
+		r.fhBuf = addFH(s, r.fhBuf, r)
+		r.bufInUse = fhBuf
+	case iBuf: // Already have interface samples. Add to the interface buf.
+		r.iBuf = addSample(s, r.iBuf, r)
+	default:
+		// Already have specialized samples that are not fhSamples.
 		// Need to call the checked add method for conversion.
 		r.add(s)
-	default:
-		r.fhBuf = addFH(s, r.fhBuf, r)
 	}
 }
 
@@ -701,15 +735,15 @@ func (r *sampleRing) reduceDelta(delta int64) bool {
 		return true
 	}
 
-	switch {
-	case len(r.fBuf) > 0:
+	switch r.bufInUse {
+	case fBuf:
 		genericReduceDelta(r.fBuf, r)
-	case len(r.hBuf) > 0:
+	case hBuf:
 		genericReduceDelta(r.hBuf, r)
-	case len(r.fhBuf) > 0:
+	case fhBuf:
 		genericReduceDelta(r.fhBuf, r)
 	default:
-		genericReduceDelta(r.buf, r)
+		genericReduceDelta(r.iBuf, r)
 	}
 	return true
 }
@@ -733,12 +767,12 @@ func (r *sampleRing) nthLast(n int) (tsdbutil.Sample, bool) {
 		return fSample{}, false
 	}
 	i := r.l - n
-	switch {
-	case len(r.fBuf) > 0:
+	switch r.bufInUse {
+	case fBuf:
 		return r.atF(i), true
-	case len(r.hBuf) > 0:
+	case hBuf:
 		return r.atH(i), true
-	case len(r.fhBuf) > 0:
+	case fhBuf:
 		return r.atFH(i), true
 	default:
 		return r.at(i), true
@@ -751,15 +785,15 @@ func (r *sampleRing) samples() []tsdbutil.Sample {
 	k := r.f + r.l
 	var j int
 
-	switch {
-	case len(r.buf) > 0:
-		if k > len(r.buf) {
-			k = len(r.buf)
+	switch r.bufInUse {
+	case iBuf:
+		if k > len(r.iBuf) {
+			k = len(r.iBuf)
 			j = r.l - k + r.f
 		}
-		n := copy(res, r.buf[r.f:k])
-		copy(res[n:], r.buf[:j])
-	case len(r.fBuf) > 0:
+		n := copy(res, r.iBuf[r.f:k])
+		copy(res[n:], r.iBuf[:j])
+	case fBuf:
 		if k > len(r.fBuf) {
 			k = len(r.fBuf)
 			j = r.l - k + r.f
@@ -770,7 +804,7 @@ func (r *sampleRing) samples() []tsdbutil.Sample {
 		for i, s := range resF {
 			res[i] = s
 		}
-	case len(r.hBuf) > 0:
+	case hBuf:
 		if k > len(r.hBuf) {
 			k = len(r.hBuf)
 			j = r.l - k + r.f
@@ -781,7 +815,7 @@ func (r *sampleRing) samples() []tsdbutil.Sample {
 		for i, s := range resH {
 			res[i] = s
 		}
-	case len(r.fhBuf) > 0:
+	case fhBuf:
 		if k > len(r.fhBuf) {
 			k = len(r.fhBuf)
 			j = r.l - k + r.f