From 3e96d902e0c6b388a938c78af7362d5bfe49e39b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Gy=C3=B6rgy=20Krajcsovits?= <gyorgy.krajcsovits@grafana.com>
Date: Thu, 18 Sep 2025 13:12:18 +0200
Subject: [PATCH] feat(remote): reduce resolution of native histograms on
 remote read
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If a sample read through remote read has too high resolution,
reduce it to the maximum allowed.

This is a slow path, but we only expect it to happen if the server
side is newer version that allows higher resolution.

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>
---
 storage/remote/codec.go      | 20 +++++++++++++++++---
 storage/remote/codec_test.go | 28 ++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/storage/remote/codec.go b/storage/remote/codec.go
index 2042e12199..2031d6101d 100644
--- a/storage/remote/codec.go
+++ b/storage/remote/codec.go
@@ -472,7 +472,7 @@ func (c *concreteSeriesIterator) Seek(t int64) chunkenc.ValueType {
 }
 
 func validateHistogramSchema(h *prompb.Histogram) error {
-	if histogram.IsValidSchema(h.Schema) {
+	if histogram.IsAcceptibleSchema(h.Schema) {
 		return nil
 	}
 	return fmt.Errorf("invalid histogram schema %d", h.Schema)
@@ -500,14 +500,28 @@ func (c *concreteSeriesIterator) AtHistogram(*histogram.Histogram) (int64, *hist
 		panic("iterator is not on an integer histogram sample")
 	}
 	h := c.series.histograms[c.histogramsCur]
-	return h.Timestamp, h.ToIntHistogram()
+	mh := h.ToIntHistogram()
+	if mh.Schema > histogram.ExponentialSchemaMax && mh.Schema <= histogram.ExponentialSchemaMaxReserved {
+		// This is a very slow path, but it should only happen if the
+		// sample is from a newer Prometheus version that supports higher
+		// resolution.
+		mh.ReduceResolution(histogram.ExponentialSchemaMax)
+	}
+	return h.Timestamp, mh
 }
 
 // AtFloatHistogram implements chunkenc.Iterator.
 func (c *concreteSeriesIterator) AtFloatHistogram(*histogram.FloatHistogram) (int64, *histogram.FloatHistogram) {
 	if c.curValType == chunkenc.ValHistogram || c.curValType == chunkenc.ValFloatHistogram {
 		fh := c.series.histograms[c.histogramsCur]
-		return fh.Timestamp, fh.ToFloatHistogram() // integer will be auto-converted.
+		mfh := fh.ToFloatHistogram() // integer will be auto-converted.
+		if mfh.Schema > histogram.ExponentialSchemaMax && mfh.Schema <= histogram.ExponentialSchemaMaxReserved {
+			// This is a very slow path, but it should only happen if the
+			// sample is from a newer Prometheus version that supports higher
+			// resolution.
+			mfh.ReduceResolution(histogram.ExponentialSchemaMax)
+		}
+		return fh.Timestamp, mfh
 	}
 	panic("iterator is not on a histogram sample")
 }
diff --git a/storage/remote/codec_test.go b/storage/remote/codec_test.go
index 396acd0a5f..da96519b70 100644
--- a/storage/remote/codec_test.go
+++ b/storage/remote/codec_test.go
@@ -592,6 +592,34 @@ func TestConcreteSeriesIterator_InvalidHistogramSamples(t *testing.T) {
 	}
 }
 
+func TestConcreteSeriesIterator_ReducesHighResolutionHistograms(t *testing.T) {
+	for _, schema := range []int32{9, 52} {
+		t.Run(fmt.Sprintf("schema=%d", schema), func(t *testing.T) {
+			h := testHistogram.Copy()
+			h.Schema = schema
+			fh := h.ToFloat(nil)
+			series := &concreteSeries{
+				labels: labels.FromStrings("foo", "bar"),
+				histograms: []prompb.Histogram{
+					prompb.FromIntHistogram(1, h),
+					prompb.FromFloatHistogram(2, fh),
+				},
+			}
+			it := series.Iterator(nil)
+			require.Equal(t, chunkenc.ValHistogram, it.Next())
+			_, gotH := it.AtHistogram(nil)
+			require.Equal(t, histogram.ExponentialSchemaMax, gotH.Schema)
+			_, gotFH := it.AtFloatHistogram(nil)
+			require.Equal(t, histogram.ExponentialSchemaMax, gotFH.Schema)
+			require.Equal(t, chunkenc.ValFloatHistogram, it.Next())
+			_, gotFH = it.AtFloatHistogram(nil)
+			require.Equal(t, histogram.ExponentialSchemaMax, gotFH.Schema)
+			require.Equal(t, chunkenc.ValNone, it.Next())
+			require.NoError(t, it.Err())
+		})
+	}
+}
+
 func TestFromQueryResultWithDuplicates(t *testing.T) {
 	ts1 := prompb.TimeSeries{
 		Labels: []prompb.Label{