Summary

For classic histograms and NHCB (custom buckets) where the first bucket has a non-positive upper boundary (-Inf, x] with x<=0, both BucketQuantile and HistogramQuantile place all observations at the upper boundary x. BucketFraction and HistogramFraction were inconsistent: they treated observations as being at -Inf (contributing to every fraction query whose lower bound fell inside the bucket).

The upshot is that if there's a bucket (-Inf, x] where x<=0, for example (-Inf, -3] and there are 10 observations in it out of 100 observations total, then the 10% quantile is -3, since quantile assumes that observations are at the upper boundary. However without the fix the histogram_fraction treats them as being at -Inf, meaning that the fraction from for example -4 to -3 is zero. Which is a contradiction with quantile at -3.

• Fix BucketFraction (classic histograms) to match BucketQuantile: observations in the first bucket with upperBound <= 0 are assumed to be at the upper boundary.
• Fix HistogramFraction (native histograms) to match HistogramQuantile: same heuristic for the first NHCB bucket with b.Upper <= 0.
• Observations in the first NHCB bucket with a positive upper boundary retain the existing behaviour (assume lower boundary of 0).
• Add missing test cases for histogram_fraction on NHCB histograms with non-positive first bucket upper boundaries.
• Add cross-consistency checks between histogram_quantile and histogram_fraction for exponential native histograms.

Test plan

• go test ./promql/... passes
• New tests in histograms.test cover classic histogram first bucket with non-positive upper boundary
• New tests in native_histograms.test cover NHCB first bucket with non-positive upper boundary
• Consistency cross-checks added for exponential native histograms (single_histogram, negative_histogram)

[BUGFIX] PromQL: `histogram_fraction` first-bucket heuristic is now consistent with `histogram_quantile`: observations in the first bucket with a non-positive upper boundary are assumed to be at the upper boundary, not at -Inf.

Coded with Claude Sonnet 4.6.

Signed-off-by: György Krajcsovits [email protected]