Reuse float histogram objects

This commit reduces the memory needed to query native histogram objects
by reusing existing HPoint instances.

Signed-off-by: Filip Petkovski <filip.petkovsky@gmail.com>

promql/engine.go

@@ -2078,21 +2078,27 @@ loop:
 		case chunkenc.ValNone:
 			break loop
 		case chunkenc.ValFloatHistogram, chunkenc.ValHistogram:
-			t, h := buf.AtFloatHistogram()
-			if value.IsStaleNaN(h.Sum) {
-				continue loop
-			}
+			t := buf.AtT()
 			// Values in the buffer are guaranteed to be smaller than maxt.
 			if t >= mintHistograms {
-				if ev.currentSamples >= ev.maxSamples {
-					ev.error(ErrTooManySamples(env))
-				}
-				point := HPoint{T: t, H: h}
 				if histograms == nil {
 					histograms = getHPointSlice(16)
 				}
-				histograms = append(histograms, point)
-				ev.currentSamples += point.size()
+				n := len(histograms)
+				if n < cap(histograms) {
+					histograms = histograms[:n+1]
+				} else {
+					histograms = append(histograms, HPoint{})
+				}
+				histograms[n].T, histograms[n].H = buf.AtFloatHistogram(histograms[n].H)
+				if value.IsStaleNaN(histograms[n].H.Sum) {
+					histograms = histograms[:n]
+					continue loop
+				}
+				if ev.currentSamples >= ev.maxSamples {
+					ev.error(ErrTooManySamples(env))
+				}
+				ev.currentSamples += histograms[n].size()
 			}
 		case chunkenc.ValFloat:
 			t, f := buf.At()
@@ -2108,7 +2114,12 @@ loop:
 				if floats == nil {
 					floats = getFPointSlice(16)
 				}
-				floats = append(floats, FPoint{T: t, F: f})
+				if n := len(floats); n < cap(floats) {
+					floats = floats[:n+1]
+					floats[n].T, floats[n].F = t, f
+				} else {
+					floats = append(floats, FPoint{T: t, F: f})
+				}
 			}
 		}
 	}

storage/buffer.go

@@ -74,7 +74,7 @@ func (b *BufferedSeriesIterator) PeekBack(n int) (sample chunks.Sample, ok bool)
 
 // Buffer returns an iterator over the buffered data. Invalidates previously
 // returned iterators.
-func (b *BufferedSeriesIterator) Buffer() chunkenc.Iterator {
+func (b *BufferedSeriesIterator) Buffer() *sampleRingIterator {
 	return b.buf.iterator()
 }
 
@@ -304,7 +304,7 @@ func (r *sampleRing) reset() {
 }
 
 // Returns the current iterator. Invalidates previously returned iterators.
-func (r *sampleRing) iterator() chunkenc.Iterator {
+func (r *sampleRing) iterator() *sampleRingIterator {
 	r.it.r = r
 	r.it.i = -1
 	return &r.it
@@ -374,9 +374,9 @@ func (it *sampleRingIterator) AtHistogram() (int64, *histogram.Histogram) {
 	return it.t, it.h
 }
 
-func (it *sampleRingIterator) AtFloatHistogram() (int64, *histogram.FloatHistogram) {
+func (it *sampleRingIterator) AtFloatHistogram(fh *histogram.FloatHistogram) (int64, *histogram.FloatHistogram) {
 	if it.fh == nil {
-		return it.t, it.h.ToFloat(nil)
+		return it.t, it.h.ToFloat(fh)
 	}
 	return it.t, it.fh
 }

storage/buffer_test.go

@@ -243,11 +243,11 @@ func TestBufferedSeriesIteratorMixedHistograms(t *testing.T) {
 	buf := it.Buffer()
 
 	require.Equal(t, chunkenc.ValFloatHistogram, buf.Next())
-	_, fh := buf.AtFloatHistogram()
+	_, fh := buf.AtFloatHistogram(nil)
 	require.Equal(t, histograms[0].ToFloat(nil), fh)
 
 	require.Equal(t, chunkenc.ValHistogram, buf.Next())
-	_, fh = buf.AtFloatHistogram()
+	_, fh = buf.AtFloatHistogram(nil)
 	require.Equal(t, histograms[1].ToFloat(nil), fh)
 }