From b69ec3f39c59545a30480b05b9fa979809153b66 Mon Sep 17 00:00:00 2001
From: Linas Medziunas <linas.medziunas@gmail.com>
Date: Wed, 28 Jan 2026 15:11:10 +0200
Subject: [PATCH] Only track signs of surviving buckets

Signed-off-by: Linas Medziunas <linas.medziunas@gmail.com>
---
 promql/engine.go                              | 45 ++++++++-----------
 .../testdata/native_histograms.test           |  6 +--
 2 files changed, 21 insertions(+), 30 deletions(-)

diff --git a/promql/engine.go b/promql/engine.go
index 1036ed0d5e..039c5fce7b 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -3243,9 +3243,18 @@ func trimHistogram(trimmedHist *histogram.FloatHistogram, rhs float64, isUpperTr
 	updatedCount := 0.0
 	origSum := trimmedHist.Sum
 	removedSum := 0.0
-	hasPositive, hasNegative := false, false
 	isCustomBucket := trimmedHist.UsesCustomBuckets()
 
+	var hasPositive, hasNegative bool
+	trackBucketSigns := func(bucket histogram.Bucket[float64]) {
+		if bucket.Lower < 0 {
+			hasNegative = true
+		}
+		if bucket.Upper > 0 {
+			hasPositive = true
+		}
+	}
+
 	if isUpperTrim {
 		// Calculate the fraction to keep for buckets that contain the trim value.
 		// For TRIM_UPPER, we keep observations below the trim point (rhs).
@@ -3255,18 +3264,6 @@ func trimHistogram(trimmedHist *histogram.FloatHistogram, rhs float64, isUpperTr
 			if bucket.Count == 0 {
 				continue
 			}
-			if isCustomBucket {
-				if bucket.Upper <= 0 {
-					hasNegative = true
-				} else {
-					if bucket.Lower < 0 {
-						hasNegative = true
-					}
-					hasPositive = true
-				}
-			} else {
-				hasPositive = true
-			}
 
 			keepCount, bucketMidpoint := computeBucketTrim(bucket, rhs, isUpperTrim, true, isCustomBucket)
 
@@ -3274,10 +3271,12 @@ func trimHistogram(trimmedHist *histogram.FloatHistogram, rhs float64, isUpperTr
 			case bucket.Upper <= rhs:
 				// Bucket is entirely below the trim point - keep all.
 				updatedCount += bucket.Count
+				trackBucketSigns(bucket)
 			case bucket.Lower < rhs:
 				// Bucket contains the trim point - interpolate.
 				removedCount := bucket.Count - keepCount
 				removedSum += removedCount * bucketMidpoint
+				trackBucketSigns(bucket)
 
 				updatedCount += keepCount
 				trimmedHist.PositiveBuckets[i] = keepCount
@@ -3296,19 +3295,20 @@ func trimHistogram(trimmedHist *histogram.FloatHistogram, rhs float64, isUpperTr
 			if bucket.Count == 0 {
 				continue
 			}
-			hasNegative = true
 
 			keepCount, bucketMidpoint := computeBucketTrim(bucket, rhs, isUpperTrim, false, isCustomBucket)
 
 			switch {
 			case bucket.Upper <= rhs:
 				updatedCount += bucket.Count
+				trackBucketSigns(bucket)
 			case bucket.Lower < rhs:
 				removedCount := bucket.Count - keepCount
 				removedSum += removedCount * bucketMidpoint
 
 				trimmedHist.NegativeBuckets[i] = keepCount
 				updatedCount += keepCount
+				trackBucketSigns(bucket)
 			default:
 				bucketMidpoint = -math.Sqrt(bucket.Lower * bucket.Upper)
 				removedSum += bucket.Count * bucketMidpoint
@@ -3323,30 +3323,20 @@ func trimHistogram(trimmedHist *histogram.FloatHistogram, rhs float64, isUpperTr
 			if bucket.Count == 0 {
 				continue
 			}
-			if isCustomBucket {
-				if bucket.Upper <= 0 {
-					hasNegative = true
-				} else {
-					if bucket.Lower < 0 {
-						hasNegative = true
-					}
-					hasPositive = true
-				}
-			} else {
-				hasPositive = true
-			}
 
 			keepCount, bucketMidpoint := computeBucketTrim(bucket, rhs, isUpperTrim, true, isCustomBucket)
 
 			switch {
 			case bucket.Lower >= rhs:
 				updatedCount += bucket.Count
+				trackBucketSigns(bucket)
 			case bucket.Upper > rhs:
 				removedCount := bucket.Count - keepCount
 				removedSum += removedCount * bucketMidpoint
 
 				trimmedHist.PositiveBuckets[i] = keepCount
 				updatedCount += keepCount
+				trackBucketSigns(bucket)
 			default:
 				if !isCustomBucket {
 					bucketMidpoint = math.Sqrt(bucket.Lower * bucket.Upper)
@@ -3361,19 +3351,20 @@ func trimHistogram(trimmedHist *histogram.FloatHistogram, rhs float64, isUpperTr
 			if bucket.Count == 0 {
 				continue
 			}
-			hasNegative = true
 
 			keepCount, bucketMidpoint := computeBucketTrim(bucket, rhs, isUpperTrim, false, isCustomBucket)
 
 			switch {
 			case bucket.Lower >= rhs:
 				updatedCount += bucket.Count
+				trackBucketSigns(bucket)
 			case bucket.Upper > rhs:
 				removedCount := bucket.Count - keepCount
 				removedSum += removedCount * bucketMidpoint
 
 				trimmedHist.NegativeBuckets[i] = keepCount
 				updatedCount += keepCount
+				trackBucketSigns(bucket)
 			default:
 				bucketMidpoint = -math.Sqrt(bucket.Lower * bucket.Upper)
 				removedSum += bucket.Count * bucketMidpoint
diff --git a/promql/promqltest/testdata/native_histograms.test b/promql/promqltest/testdata/native_histograms.test
index c62983f8b2..7f2585ef6e 100644
--- a/promql/promqltest/testdata/native_histograms.test
+++ b/promql/promqltest/testdata/native_histograms.test
@@ -1897,7 +1897,7 @@ eval instant at 1m h_test >/ 0
   h_test {{schema:0 sum:127.28553390593274 count:30.5 z_bucket:0.5 z_bucket_w:0.001 buckets:[2 4 8 16]}}
 
 eval instant at 1m h_test </ 0
-  h_test {{schema:0 sum:3.5418471982869164 count:3.5 z_bucket:0.5 z_bucket_w:0.001 n_buckets:[1 2]}}
+  h_test {{schema:0 sum:0.0 count:3.5 z_bucket:0.5 z_bucket_w:0.001 n_buckets:[1 2]}}
 
 # Exponential buckets: trim uses exponential interpolation if cutoff is inside a bucket
 eval instant at 1m h_test_2 </ 1.13
@@ -1923,7 +1923,7 @@ eval instant at 1m h_test >/ -1
   h_test{} {{count:32 sum:126.57842712474618 z_bucket:1 z_bucket_w:0.001 buckets:[2 4 8 16] n_buckets:[1]}}
 
 eval instant at 1m h_test </ -1
-  h_test{} {{count:2 sum:4.24895397947347 z_bucket_w:0.001 n_offset:1 n_buckets:[2]}}
+  h_test{} {{count:2 sum:0.0 z_bucket_w:0.001 n_offset:1 n_buckets:[2]}}
 
 # Custom buckets: trim on bucket boundary without interpolation
 eval instant at 1m cbh </ 13
@@ -2018,7 +2018,7 @@ eval instant at 1m cbh_two_buckets_split_at_zero </ 10.0
 
 # Skip (0; +Inf] bucket (100).
 eval instant at 1m cbh_two_buckets_split_at_zero </ 0.0
-  cbh_two_buckets_split_at_zero {{schema:-53 sum:33.0 count:1 custom_values:[0] buckets:[1 0]}}
+  cbh_two_buckets_split_at_zero {{schema:-53 sum:0.0 count:1 custom_values:[0] buckets:[1 0]}}
 
 # Skip both buckets (1, 100).
 eval instant at 1m cbh_two_buckets_split_at_zero </ -10.0