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Fix minimum required bucket count for exponential histogram merging #133711

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Merged
merged 6 commits into from
Sep 29, 2025
Merged

Fix minimum required bucket count for exponential histogram merging #133711

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6e23ae4
Fix minimum required bucket count for exponential histogram merging
JonasKunz Aug 28, 2025
751026d
Merge branch 'main' into histo-minimum-buckets
JonasKunz Aug 28, 2025
4de4de0
Replace with IllegalArgumentException
JonasKunz Aug 29, 2025
86ff128
Merge branch 'main' into histo-minimum-buckets
JonasKunz Aug 29, 2025
c8ec9fc
Merge branch 'main' into histo-minimum-buckets
JonasKunz Aug 29, 2025
ea69d63
Merge branch 'main' into histo-minimum-buckets
JonasKunz Aug 29, 2025
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3 changes: 2 additions & 1 deletion ...gram/src/main/java/org/elasticsearch/exponentialhistogram/ExponentialHistogramMerger.java
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Expand Up @@ -69,7 +69,8 @@ private ExponentialHistogramMerger(int bucketLimit, ExponentialHistogramCircuitB

// Only intended for testing, using this in production means an unnecessary reduction of precision
private ExponentialHistogramMerger(int bucketLimit, int maxScale, ExponentialHistogramCircuitBreaker circuitBreaker) {
this.bucketLimit = bucketLimit;
// We need at least four buckets to represent any possible distribution
this.bucketLimit = Math.max(4, bucketLimit);
this.maxScale = maxScale;
this.circuitBreaker = circuitBreaker;
downscaleStats = new DownscaleStats();
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22 changes: 22 additions & 0 deletions ...src/test/java/org/elasticsearch/exponentialhistogram/ExponentialHistogramMergerTests.java
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Expand Up @@ -146,6 +146,28 @@ public void testUpscalingDoesNotExceedIndexLimits() {
}
}

public void testMinimumBucketCountBounded() {
FixedCapacityExponentialHistogram a = createAutoReleasedHistogram(2);
a.tryAddBucket(-1, 1, false);
a.tryAddBucket(0, 1, false);

FixedCapacityExponentialHistogram b = createAutoReleasedHistogram(2);
b.tryAddBucket(-1, 1, true);
b.tryAddBucket(0, 1, true);

// What we try here is not possible: the provided buckets are not mergeable to three buckets
try (ReleasableExponentialHistogram merged = ExponentialHistogram.merge(3, breaker(), a, b)) {
FixedCapacityExponentialHistogram expectedResult = createAutoReleasedHistogram(4);
expectedResult.tryAddBucket(-1, 1, false);
expectedResult.tryAddBucket(0, 1, false);
expectedResult.tryAddBucket(-1, 1, true);
expectedResult.tryAddBucket(0, 1, true);

assertBucketsEqual(merged.negativeBuckets(), expectedResult.negativeBuckets());
assertBucketsEqual(merged.positiveBuckets(), expectedResult.positiveBuckets());
}
}

/**
* Verify that the resulting histogram is independent of the order of elements and therefore merges performed.
*/
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