RandomSamplingFacetsCollector never samples the first hit of a segment that starts mid-bin: createSample does not handle randomIndex == counter at the end of a segment, so a sampling bin that crosses a segment boundary loses its sample (or samples at a stale offset)
Finding LFV-432Area: facet / sampling (RandomSamplingFacetsCollector.createSample, cross-segment bin carry-over)Priority: lowImpact: minorNovelty: new
AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.
What it shows
You count facets with RandomSamplingFacetsCollector on a multi-segment index for a query with many more hits than sampleSize, and expect systematic sampling: one random hit out of every run of binSize consecutive hits, with a run that is cut by a segment boundary continued in the next segment, so every hit has about a 1/binSize chance to be sampled. Instead, when the hit picked for such a run is the first hit of the next segment, createSample leaves its carry-over state unset (it tests only randomIndex > counter and randomIndex < counter), so that run contributes no sample, or one at a stale offset, and the first hit of a segment that starts mid-bin is never sampled. Nothing fails; the sample and the amortized counts are just smaller than the reported sampling rate implies (4 hits in segments [1 | 3], sampleSize 2: half the seeds sample 1 hit instead of 2).
The test shows that RandomSamplingFacetsCollector never samples the first hit of a segment that starts mid-bin: 4 hits in segments [1 | 3], sampleSize 2 (bins {0,1} {2,3}), sampler seed 1 picks hit 1 for bin {0,1}, and that bin gets no sample.
Expected vs actual
Expected 2 sampled hits (one per bin). Actual: 1, because createSample handles only randomIndex > counter and < counter at the end of a segment, not ==.
Files
Run it
- Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
- Tested on Lucene main 645d4fe14a20 (2026-10-28): reproduced.
- Recorded outcome of run 1 on Lucene main 645d4fe14a20: 1 test(s) run, 1 failed.
- How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual).
- Seeds: a rerun with the same seed replays the same random choices. Assertions off (-da -dsa -Dtests.asserts=false) is the production-like setting.
# Lucene checkout at the tested commit, built with: ./gradlew --no-daemon assemble (JDK 25)
LUCENE=/path/to/lucene; V=12.0.0-SNAPSHOT
CP=$LUCENE/lucene/core/build/libs/lucene-core-$V.jar
CP=$CP:$LUCENE/lucene/codecs/build/libs/lucene-codecs-$V.jar
CP=$CP:$LUCENE/lucene/test-framework/build/libs/lucene-test-framework-$V.jar
CP=$CP:$LUCENE/lucene/facet/build/libs/lucene-facet-$V.jar
CP=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar # JUnit Platform console launcher (Maven Central)
javac -d classes -cp "$CP" $(find src -name '*.java')
# run 1: assertions on, seed DEADBEEF
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
-cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
execute --select-class org.apache.lucene.facet.TestRandomSamplingSegmentBoundaryThe module(s) facet also need their third-party runtime jars; ./gradlew -p lucene/<module> dependencies lists them.
Gradle alternative: copy the test into the matching module's src/test/ tree of the checkout and run ./gradlew -p lucene/<module> test --tests <class> -Ptests.seed=<seed> (add -Ptests.asserts=false for a run without assertions).
Feedback
Open an issue in lucene-ai-bug-hunt. Please do not open issues or pull requests in apache/lucene about this finding unless a Lucene committer agrees it should be filed.
Snapshot of 2026-11-03. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.