Add TODOs and some minor tweaks

Signed-off-by: beorn7 <beorn@grafana.com>

promql/engine.go

@@ -1767,7 +1767,7 @@ func (ev *evaluator) matrixIterSlice(it *storage.BufferedSeriesIterator, mint, m
 			}
 		}
 	}
-	// The seeked sample might also be in the range.
+	// The sought sample might also be in the range.
 	if ok {
 		if it.ChunkEncoding() == chunkenc.EncHistogram {
 			t, h := it.HistogramValues()

promql/value.go

@@ -87,6 +87,7 @@ type Point struct {
 }
 
 func (p Point) String() string {
+	// TODO(beorn7): Support Histogram.
 	v := strconv.FormatFloat(p.V, 'f', -1, 64)
 	return fmt.Sprintf("%v @[%v]", v, p.T)
 }
@@ -299,11 +300,9 @@ func (ssi *storageSeriesIterator) At() (t int64, v float64) {
 	return p.T, p.V
 }
 
-// AtHistogram always returns (0, histogram.Histogram{}) because there is no
-// support for histogram values yet.
-// TODO(beorn7): Fix that for histogram support in PromQL.
 func (ssi *storageSeriesIterator) AtHistogram() (int64, histogram.Histogram) {
-	return 0, histogram.Histogram{}
+	p := ssi.points[ssi.curr]
+	return p.T, *p.H
 }
 
 func (ssi *storageSeriesIterator) ChunkEncoding() chunkenc.Encoding {

storage/buffer.go

@@ -230,8 +230,6 @@ func (it *sampleRingIterator) At() (int64, float64) {
 	return it.r.at(it.i)
 }
 
-// AtHistogram always returns (0, histogram.Histogram{}) because there is no
-// support for histogram values yet.
 func (it *sampleRingIterator) AtHistogram() (int64, histogram.Histogram) {
 	return it.r.atHistogram(it.i)
 }

storage/buffer_test.go

@@ -216,11 +216,11 @@ func newFakeSeriesIterator(nsamples, step int64) *fakeSeriesIterator {
 }
 
 func (it *fakeSeriesIterator) At() (int64, float64) {
-	return it.idx * it.step, 123 // value doesn't matter
+	return it.idx * it.step, 123 // Value doesn't matter.
 }
 
 func (it *fakeSeriesIterator) AtHistogram() (int64, histogram.Histogram) {
-	return it.idx * it.step, histogram.Histogram{} // value doesn't matter
+	return it.idx * it.step, histogram.Histogram{} // Value doesn't matter.
 }
 
 func (it *fakeSeriesIterator) ChunkEncoding() chunkenc.Encoding {

storage/series.go

@@ -91,10 +91,9 @@ func (it *listSeriesIterator) At() (int64, float64) {
 	return s.T(), s.V()
 }
 
-// AtHistogram always returns (0, histogram.Histogram{}) because there is no
-// support for histogram values yet.
 func (it *listSeriesIterator) AtHistogram() (int64, histogram.Histogram) {
-	return 0, histogram.Histogram{}
+	s := it.samples.Get(it.idx)
+	return s.T(), *s.H()
 }
 
 func (it *listSeriesIterator) ChunkEncoding() chunkenc.Encoding {

tsdb/chunkenc/chunk.go

@@ -89,6 +89,8 @@ type Appender interface {
 // Iterator iterates over the samples of a time series, in timestamp-increasing order.
 type Iterator interface {
 	// Next advances the iterator by one.
+	// TODO(beorn7): Perhaps this should return if the next value is a float or a histogram
+	// to make it easier calling the right method (At vs AtHistogram)?
 	Next() bool
 	// Seek advances the iterator forward to the first sample with the timestamp equal or greater than t.
 	// If current sample found by previous `Next` or `Seek` operation already has this property, Seek has no effect.
@@ -100,6 +102,7 @@ type Iterator interface {
 	At() (int64, float64)
 	// AtHistogram returns the current timestamp/histogram pair.
 	// Before the iterator has advanced AtHistogram behaviour is unspecified.
+	// TODO(beorn7): Maybe return *histogram.Histogram? It's a fairly large struct.
 	AtHistogram() (int64, histogram.Histogram)
 	// Err returns the current error. It should be used only after iterator is
 	// exhausted, that is `Next` or `Seek` returns false.

tsdb/tsdbutil/buffer.go

@@ -166,6 +166,7 @@ func (it *sampleRingIterator) At() (int64, float64) {
 }
 
 func (it *sampleRingIterator) AtHistogram() (int64, histogram.Histogram) {
+	// TODO(beorn7): Add proper histogram support.
 	return 0, histogram.Histogram{}
 }
 

tsdb/tsdbutil/buffer_test.go

@@ -153,7 +153,11 @@ func (it *listSeriesIterator) At() (int64, float64) {
 }
 
 func (it *listSeriesIterator) AtHistogram() (int64, histogram.Histogram) {
-	return 0, histogram.Histogram{}
+	s := it.list[it.idx]
+	if s.h == nil {
+		return s.t, histogram.Histogram{}
+	}
+	return s.t, *s.h
 }
 
 func (it *listSeriesIterator) ChunkEncoding() chunkenc.Encoding {