diff --git a/promql/functions.go b/promql/functions.go
index 0d3ec41951..433c06f023 100644
--- a/promql/functions.go
+++ b/promql/functions.go
@@ -548,7 +548,7 @@ func linearRegression(samples []model.SamplePair, interceptTime model.Time) (slo
 
 	slope = covXY / varX
 	intercept = sumY/n - slope*sumX/n
-	return
+	return slope, intercept
 }
 
 // === deriv(node model.ValMatrix) Vector ===
diff --git a/storage/local/chunk.go b/storage/local/chunk.go
index b843fa3a71..2e9fcf4cff 100644
--- a/storage/local/chunk.go
+++ b/storage/local/chunk.go
@@ -260,42 +260,52 @@ type chunk interface {
 
 // A chunkIterator enables efficient access to the content of a chunk. It is
 // generally not safe to use a chunkIterator concurrently with or after chunk
-// mutation. The error returned by any of the methods is always the last error
-// encountered by the iterator, i.e. once an error has been encountered, no
-// method will ever return a nil error again. In general, errors signal data
-// corruption in the chunk and require quarantining.
+// mutation.
 type chunkIterator interface {
 	// Gets the last timestamp in the chunk.
 	lastTimestamp() (model.Time, error)
-	// Gets the value that is closest before the given time. In case a value
-	// exists at precisely the given time, that value is returned. If no
-	// applicable value exists, ZeroSamplePair is returned.
-	valueAtOrBeforeTime(model.Time) (model.SamplePair, error)
-	// Gets all values contained within a given interval.
-	rangeValues(metric.Interval) ([]model.SamplePair, error)
 	// Whether a given timestamp is contained between first and last value
 	// in the chunk.
 	contains(model.Time) (bool, error)
-	// scan, value, and err implement a bufio.Scanner-like interface.  The
-	// scan method advances the iterator to the next value in the chunk and
-	// returns true if that worked. In that case, the value method will
-	// return the sample pair the iterator has advanced to. If the scan
-	// method returns false, either an error has occured or the end of the
-	// chunk has been reached. In the former case, the err method will
-	// return the error. In the latter case, the err method will return nil.
-	// Upon creation, the iterator is at position "minus one". After the
-	// first scan call, value will return the 1st value in the
-	// chunk. valueAtOrBeforeTime and rangeValues all modify the iterator
-	// position, too. They behave as if their return values were retrieved
-	// after a scan call, i.e. calling the value or err methods after a call
-	// to those methods will retrieve the same return value again (or the
-	// last value in the range in case of rangeValues), and subsequent scan
-	// calls will advance the iterator from there.
+	// Scans the next value in the chunk. Directly after the iterator has
+	// been created, the next value is the first value in the
+	// chunk. Otherwise, it is the value following the last value scanned or
+	// found (by one of the find... methods). Returns false if either the
+	// end of the chunk is reached or an error has occurred.
 	scan() bool
+	// Finds the most recent value at or before the provided time. Returns
+	// false if either the chunk contains no value at or before the provided
+	// time, or an error has occurred.
+	findAtOrBefore(model.Time) bool
+	// Finds the oldest value at or after the provided time. Returns false
+	// if either the chunk contains no value at or after the provided time,
+	// or an error has occurred.
+	findAtOrAfter(model.Time) bool
+	// Returns the last value scanned (by the scan method) or found (by one
+	// of the find... methods). It returns ZeroSamplePair before any of
+	// those methods were called.
 	value() model.SamplePair
+	// Returns the last error encountered. In general, an error signal data
+	// corruption in the chunk and requires quarantining.
 	err() error
 }
 
+// rangeValues is a utility function that retrieves all values within the given
+// range from a chunkIterator.
+func rangeValues(it chunkIterator, in metric.Interval) ([]model.SamplePair, error) {
+	result := []model.SamplePair{}
+	if !it.findAtOrAfter(in.OldestInclusive) {
+		return result, it.err()
+	}
+	for !it.value().Timestamp.After(in.NewestInclusive) {
+		result = append(result, it.value())
+		if !it.scan() {
+			break
+		}
+	}
+	return result, it.err()
+}
+
 func transcodeAndAdd(dst chunk, src chunk, s model.SamplePair) ([]chunk, error) {
 	chunkOps.WithLabelValues(transcode).Inc()
 
@@ -374,46 +384,6 @@ func (it *indexAccessingChunkIterator) lastTimestamp() (model.Time, error) {
 	return it.acc.timestampAtIndex(it.len - 1), it.acc.err()
 }
 
-// valueAtOrBeforeTime implements chunkIterator.
-func (it *indexAccessingChunkIterator) valueAtOrBeforeTime(t model.Time) (model.SamplePair, error) {
-	i := sort.Search(it.len, func(i int) bool {
-		return it.acc.timestampAtIndex(i).After(t)
-	})
-	if i == 0 || it.acc.err() != nil {
-		return ZeroSamplePair, it.acc.err()
-	}
-	it.pos = i - 1
-	it.lastValue = model.SamplePair{
-		Timestamp: it.acc.timestampAtIndex(i - 1),
-		Value:     it.acc.sampleValueAtIndex(i - 1),
-	}
-	return it.lastValue, it.acc.err()
-}
-
-// rangeValues implements chunkIterator.
-func (it *indexAccessingChunkIterator) rangeValues(in metric.Interval) ([]model.SamplePair, error) {
-	oldest := sort.Search(it.len, func(i int) bool {
-		return !it.acc.timestampAtIndex(i).Before(in.OldestInclusive)
-	})
-	newest := sort.Search(it.len, func(i int) bool {
-		return it.acc.timestampAtIndex(i).After(in.NewestInclusive)
-	})
-	if oldest == it.len || it.acc.err() != nil {
-		return nil, it.acc.err()
-	}
-
-	result := make([]model.SamplePair, 0, newest-oldest)
-	for i := oldest; i < newest; i++ {
-		it.pos = i
-		it.lastValue = model.SamplePair{
-			Timestamp: it.acc.timestampAtIndex(i),
-			Value:     it.acc.sampleValueAtIndex(i),
-		}
-		result = append(result, it.lastValue)
-	}
-	return result, it.acc.err()
-}
-
 // contains implements chunkIterator.
 func (it *indexAccessingChunkIterator) contains(t model.Time) (bool, error) {
 	return !t.Before(it.acc.timestampAtIndex(0)) &&
@@ -433,6 +403,38 @@ func (it *indexAccessingChunkIterator) scan() bool {
 	return it.acc.err() == nil
 }
 
+// findAtOrBefore implements chunkIterator.
+func (it *indexAccessingChunkIterator) findAtOrBefore(t model.Time) bool {
+	i := sort.Search(it.len, func(i int) bool {
+		return it.acc.timestampAtIndex(i).After(t)
+	})
+	if i == 0 || it.acc.err() != nil {
+		return false
+	}
+	it.pos = i - 1
+	it.lastValue = model.SamplePair{
+		Timestamp: it.acc.timestampAtIndex(i - 1),
+		Value:     it.acc.sampleValueAtIndex(i - 1),
+	}
+	return true
+}
+
+// findAtOrAfter implements chunkIterator.
+func (it *indexAccessingChunkIterator) findAtOrAfter(t model.Time) bool {
+	i := sort.Search(it.len, func(i int) bool {
+		return !it.acc.timestampAtIndex(i).Before(t)
+	})
+	if i == it.len || it.acc.err() != nil {
+		return false
+	}
+	it.pos = i
+	it.lastValue = model.SamplePair{
+		Timestamp: it.acc.timestampAtIndex(i),
+		Value:     it.acc.sampleValueAtIndex(i),
+	}
+	return true
+}
+
 // value implements chunkIterator.
 func (it *indexAccessingChunkIterator) value() model.SamplePair {
 	return it.lastValue
diff --git a/storage/local/preload.go b/storage/local/preload.go
index b0113bd6be..fb6a21f64b 100644
--- a/storage/local/preload.go
+++ b/storage/local/preload.go
@@ -30,7 +30,7 @@ func (p *memorySeriesPreloader) PreloadRange(
 	fp model.Fingerprint,
 	from model.Time, through model.Time,
 ) SeriesIterator {
-	cds, iter := p.storage.preloadChunksForRange(fp, from, through, false)
+	cds, iter := p.storage.preloadChunksForRange(fp, from, through)
 	p.pinnedChunkDescs = append(p.pinnedChunkDescs, cds...)
 	return iter
 }
@@ -40,7 +40,7 @@ func (p *memorySeriesPreloader) PreloadInstant(
 	fp model.Fingerprint,
 	timestamp model.Time, stalenessDelta time.Duration,
 ) SeriesIterator {
-	cds, iter := p.storage.preloadChunksForRange(fp, timestamp.Add(-stalenessDelta), timestamp, true)
+	cds, iter := p.storage.preloadChunksForInstant(fp, timestamp.Add(-stalenessDelta), timestamp)
 	p.pinnedChunkDescs = append(p.pinnedChunkDescs, cds...)
 	return iter
 }
diff --git a/storage/local/series.go b/storage/local/series.go
index f76d5ee27b..0dcad859a8 100644
--- a/storage/local/series.go
+++ b/storage/local/series.go
@@ -399,30 +399,40 @@ func (s *memorySeries) newIterator(pinnedChunkDescs []*chunkDesc, quarantine fun
 	}
 }
 
+// preloadChunksForInstant preloads chunks for the latest value in the given
+// range. If the last sample saved in the memorySeries itself is the latest
+// value in the given range, it will in fact preload zero chunks and just take
+// that value.
+func (s *memorySeries) preloadChunksForInstant(
+	fp model.Fingerprint,
+	from model.Time, through model.Time,
+	mss *memorySeriesStorage,
+) ([]*chunkDesc, SeriesIterator, error) {
+	// If we have a lastSamplePair in the series, and thas last samplePair
+	// is in the interval, just take it in a singleSampleSeriesIterator. No
+	// need to pin or load anything.
+	lastSample := s.lastSamplePair()
+	if !through.Before(lastSample.Timestamp) &&
+		!from.After(lastSample.Timestamp) &&
+		lastSample != ZeroSamplePair {
+		iter := &boundedIterator{
+			it:    &singleSampleSeriesIterator{samplePair: lastSample},
+			start: model.Now().Add(-mss.dropAfter),
+		}
+		return nil, iter, nil
+	}
+	// If we are here, we are out of luck and have to delegate to the more
+	// expensive method.
+	return s.preloadChunksForRange(fp, from, through, mss)
+}
+
 // preloadChunksForRange loads chunks for the given range from the persistence.
 // The caller must have locked the fingerprint of the series.
 func (s *memorySeries) preloadChunksForRange(
 	fp model.Fingerprint,
 	from model.Time, through model.Time,
-	lastSampleOnly bool,
 	mss *memorySeriesStorage,
 ) ([]*chunkDesc, SeriesIterator, error) {
-	// If we have to preload for only one sample, and we have a
-	// lastSamplePair in the series, and thas last samplePair is in the
-	// interval, just take it in a singleSampleSeriesIterator. No need to
-	// pin or load anything.
-	if lastSampleOnly {
-		lastSample := s.lastSamplePair()
-		if !through.Before(lastSample.Timestamp) &&
-			!from.After(lastSample.Timestamp) &&
-			lastSample != ZeroSamplePair {
-			iter := &boundedIterator{
-				it:    &singleSampleSeriesIterator{samplePair: lastSample},
-				start: model.Now().Add(-mss.dropAfter),
-			}
-			return nil, iter, nil
-		}
-	}
 	firstChunkDescTime := model.Latest
 	if len(s.chunkDescs) > 0 {
 		firstChunkDescTime = s.chunkDescs[0].firstTime()
@@ -547,12 +557,13 @@ func (it *memorySeriesIterator) ValueAtOrBeforeTime(t model.Time) model.SamplePa
 			return ZeroSamplePair
 		}
 		if containsT {
-			value, err := it.chunkIt.valueAtOrBeforeTime(t)
-			if err != nil {
-				it.quarantine(err)
-				return ZeroSamplePair
+			if it.chunkIt.findAtOrBefore(t) {
+				return it.chunkIt.value()
 			}
-			return value
+			if it.chunkIt.err() != nil {
+				it.quarantine(it.chunkIt.err())
+			}
+			return ZeroSamplePair
 		}
 	}
 
@@ -570,12 +581,13 @@ func (it *memorySeriesIterator) ValueAtOrBeforeTime(t model.Time) model.SamplePa
 		return ZeroSamplePair
 	}
 	it.chunkIt = it.chunkIterator(l - i)
-	value, err := it.chunkIt.valueAtOrBeforeTime(t)
-	if err != nil {
-		it.quarantine(err)
-		return ZeroSamplePair
+	if it.chunkIt.findAtOrBefore(t) {
+		return it.chunkIt.value()
 	}
-	return value
+	if it.chunkIt.err() != nil {
+		it.quarantine(it.chunkIt.err())
+	}
+	return ZeroSamplePair
 }
 
 // RangeValues implements SeriesIterator.
@@ -602,7 +614,7 @@ func (it *memorySeriesIterator) RangeValues(in metric.Interval) []model.SamplePa
 		if c.firstTime().After(in.NewestInclusive) {
 			break
 		}
-		chValues, err := it.chunkIterator(i + j).rangeValues(in)
+		chValues, err := rangeValues(it.chunkIterator(i+j), in)
 		if err != nil {
 			it.quarantine(err)
 			return nil
diff --git a/storage/local/storage.go b/storage/local/storage.go
index e6527085be..bc380bd6bd 100644
--- a/storage/local/storage.go
+++ b/storage/local/storage.go
@@ -736,46 +736,76 @@ func (s *memorySeriesStorage) getOrCreateSeries(fp model.Fingerprint, m model.Me
 	return series, nil
 }
 
+// getSeriesForRange is a helper method for preloadChunksForRange and preloadChunksForInstant.
+func (s *memorySeriesStorage) getSeriesForRange(
+	fp model.Fingerprint,
+	from model.Time, through model.Time,
+) *memorySeries {
+	series, ok := s.fpToSeries.get(fp)
+	if ok {
+		return series
+	}
+	has, first, last, err := s.persistence.hasArchivedMetric(fp)
+	if err != nil {
+		log.With("fingerprint", fp).With("error", err).Error("Archive index error while preloading chunks.")
+		return nil
+	}
+	if !has {
+		s.invalidPreloadRequestsCount.Inc()
+		return nil
+	}
+	if last.Before(from) || first.After(through) {
+		return nil
+	}
+	metric, err := s.persistence.archivedMetric(fp)
+	if err != nil {
+		log.With("fingerprint", fp).With("error", err).Error("Archive index error while preloading chunks.")
+		return nil
+	}
+	series, err = s.getOrCreateSeries(fp, metric)
+	if err != nil {
+		// getOrCreateSeries took care of quarantining already.
+		return nil
+	}
+	return series
+}
+
 func (s *memorySeriesStorage) preloadChunksForRange(
 	fp model.Fingerprint,
 	from model.Time, through model.Time,
-	lastSampleOnly bool,
 ) ([]*chunkDesc, SeriesIterator) {
 	s.fpLocker.Lock(fp)
 	defer s.fpLocker.Unlock(fp)
 
-	series, ok := s.fpToSeries.get(fp)
-	if !ok {
-		has, first, last, err := s.persistence.hasArchivedMetric(fp)
-		if err != nil {
-			log.With("fingerprint", fp).With("error", err).Error("Archive index error while preloading chunks.")
-			return nil, nopIter
-		}
-		if !has {
-			s.invalidPreloadRequestsCount.Inc()
-			return nil, nopIter
-		}
-		if from.Before(last) && through.After(first) {
-			metric, err := s.persistence.archivedMetric(fp)
-			if err != nil {
-				log.With("fingerprint", fp).With("error", err).Error("Archive index error while preloading chunks.")
-				return nil, nopIter
-			}
-			series, err = s.getOrCreateSeries(fp, metric)
-			if err != nil {
-				log.With("fingerprint", fp).With("error", err).Error("Error while retrieving series.")
-				return nil, nopIter
-			}
-		} else {
-			return nil, nopIter
-		}
+	series := s.getSeriesForRange(fp, from, through)
+	if series == nil {
+		return nil, nopIter
 	}
-	cds, it, err := series.preloadChunksForRange(fp, from, through, lastSampleOnly, s)
+	cds, iter, err := series.preloadChunksForRange(fp, from, through, s)
 	if err != nil {
 		s.quarantineSeries(fp, series.metric, err)
 		return nil, nopIter
 	}
-	return cds, it
+	return cds, iter
+}
+
+func (s *memorySeriesStorage) preloadChunksForInstant(
+	fp model.Fingerprint,
+	from model.Time, through model.Time,
+) ([]*chunkDesc, SeriesIterator) {
+	s.fpLocker.Lock(fp)
+	defer s.fpLocker.Unlock(fp)
+
+	series := s.getSeriesForRange(fp, from, through)
+	if series == nil {
+		return nil, nopIter
+	}
+	cds, iter, err := series.preloadChunksForInstant(fp, from, through, s)
+	if err != nil {
+		s.quarantineSeries(fp, series.metric, err)
+		return nil, nopIter
+	}
+	return cds, iter
 }
 
 func (s *memorySeriesStorage) handleEvictList() {
diff --git a/storage/local/storage_test.go b/storage/local/storage_test.go
index ab5c2a9ed5..01f30c28b1 100644
--- a/storage/local/storage_test.go
+++ b/storage/local/storage_test.go
@@ -495,12 +495,12 @@ func TestDropMetrics(t *testing.T) {
 		t.Errorf("unexpected number of fingerprints: %d", len(fps2))
 	}
 
-	_, it := s.preloadChunksForRange(fpList[0], model.Earliest, model.Latest, false)
+	_, it := s.preloadChunksForRange(fpList[0], model.Earliest, model.Latest)
 	if vals := it.RangeValues(metric.Interval{OldestInclusive: insertStart, NewestInclusive: now}); len(vals) != 0 {
 		t.Errorf("unexpected number of samples: %d", len(vals))
 	}
 
-	_, it = s.preloadChunksForRange(fpList[1], model.Earliest, model.Latest, false)
+	_, it = s.preloadChunksForRange(fpList[1], model.Earliest, model.Latest)
 	if vals := it.RangeValues(metric.Interval{OldestInclusive: insertStart, NewestInclusive: now}); len(vals) != N {
 		t.Errorf("unexpected number of samples: %d", len(vals))
 	}
@@ -522,12 +522,12 @@ func TestDropMetrics(t *testing.T) {
 		t.Errorf("unexpected number of fingerprints: %d", len(fps3))
 	}
 
-	_, it = s.preloadChunksForRange(fpList[0], model.Earliest, model.Latest, false)
+	_, it = s.preloadChunksForRange(fpList[0], model.Earliest, model.Latest)
 	if vals := it.RangeValues(metric.Interval{OldestInclusive: insertStart, NewestInclusive: now}); len(vals) != 0 {
 		t.Errorf("unexpected number of samples: %d", len(vals))
 	}
 
-	_, it = s.preloadChunksForRange(fpList[1], model.Earliest, model.Latest, false)
+	_, it = s.preloadChunksForRange(fpList[1], model.Earliest, model.Latest)
 	if vals := it.RangeValues(metric.Interval{OldestInclusive: insertStart, NewestInclusive: now}); len(vals) != 0 {
 		t.Errorf("unexpected number of samples: %d", len(vals))
 	}
@@ -746,7 +746,7 @@ func testValueAtOrBeforeTime(t *testing.T, encoding chunkEncoding) {
 
 	fp := model.Metric{}.FastFingerprint()
 
-	_, it := s.preloadChunksForRange(fp, model.Earliest, model.Latest, false)
+	_, it := s.preloadChunksForRange(fp, model.Earliest, model.Latest)
 
 	// #1 Exactly on a sample.
 	for i, expected := range samples {
@@ -820,7 +820,7 @@ func benchmarkValueAtOrBeforeTime(b *testing.B, encoding chunkEncoding) {
 
 	fp := model.Metric{}.FastFingerprint()
 
-	_, it := s.preloadChunksForRange(fp, model.Earliest, model.Latest, false)
+	_, it := s.preloadChunksForRange(fp, model.Earliest, model.Latest)
 
 	b.ResetTimer()
 
@@ -898,7 +898,7 @@ func testRangeValues(t *testing.T, encoding chunkEncoding) {
 
 	fp := model.Metric{}.FastFingerprint()
 
-	_, it := s.preloadChunksForRange(fp, model.Earliest, model.Latest, false)
+	_, it := s.preloadChunksForRange(fp, model.Earliest, model.Latest)
 
 	// #1 Zero length interval at sample.
 	for i, expected := range samples {
@@ -1050,7 +1050,7 @@ func benchmarkRangeValues(b *testing.B, encoding chunkEncoding) {
 
 	fp := model.Metric{}.FastFingerprint()
 
-	_, it := s.preloadChunksForRange(fp, model.Earliest, model.Latest, false)
+	_, it := s.preloadChunksForRange(fp, model.Earliest, model.Latest)
 
 	b.ResetTimer()
 
@@ -1096,7 +1096,7 @@ func testEvictAndPurgeSeries(t *testing.T, encoding chunkEncoding) {
 
 	// Drop ~half of the chunks.
 	s.maintainMemorySeries(fp, 10000)
-	_, it := s.preloadChunksForRange(fp, model.Earliest, model.Latest, false)
+	_, it := s.preloadChunksForRange(fp, model.Earliest, model.Latest)
 	actual := it.RangeValues(metric.Interval{
 		OldestInclusive: 0,
 		NewestInclusive: 100000,
@@ -1114,7 +1114,7 @@ func testEvictAndPurgeSeries(t *testing.T, encoding chunkEncoding) {
 
 	// Drop everything.
 	s.maintainMemorySeries(fp, 100000)
-	_, it = s.preloadChunksForRange(fp, model.Earliest, model.Latest, false)
+	_, it = s.preloadChunksForRange(fp, model.Earliest, model.Latest)
 	actual = it.RangeValues(metric.Interval{
 		OldestInclusive: 0,
 		NewestInclusive: 100000,