RandomSamplingFacetsCollector samples one hit per (int) (1 / samplingRate) hits but amortizes by the untruncated samplingRate, so amortized facet counts are up to 2x too high (1999 hits with sampleSize 1000: every hit is sampled, yet every count is doubled)

Finding LFV-437Area: facet / sampling (RandomSamplingFacetsCollector: sampling rate vs bin size in amortizeFacetCounts and getSamplingRate)Priority: mediumImpact: silent wrong resultsNovelty: known upstream

AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.

What it shows

You sample facet counts with RandomSamplingFacetsCollector and sampleSize 1000 on a query with 1999 hits, call amortizeFacetCounts to scale the sampled counts back up, and expect estimates of the true counts; here the sample happens to contain every hit, so the right answer is the exact counts 667/666/666. Instead you get 1333/1331/1331 and a dimension total of 3996 for 1999 hits, with no error: the collector computes samplingRate = 1000 / 1999 = 0.50025 but samples one hit per bin of (int) (1 / samplingRate) = 1 hit, and it amortizes, and reports through getSamplingRate(), with the untruncated rate. In general the counts are too high by totalHits / (sampleSize * binSize), between 1 and 2, worst just below twice the sample size.

Expected vs actual

Expected: The test passes: it asserts the documented behaviour.
Actual: You get 1333/1331/1331 and a dimension total of 3996 for 1999 hits, with no error: the collector computes samplingRate = 1000 / 1999 = 0.50025 but samples one hit per bin of (int) (1 / samplingRate) = 1 hit, and it amortizes, and reports through getSamplingRate(), with the untruncated rate.

Files

src/org/apache/lucene/facet/TestRandomSamplingBinSize.java (JUnit test)
src2/org/apache/lucene/facet/TestRandomSamplingBinSize.java (JUnit test)

Run it

Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
Tested on Lucene main ea57360912ed (2026-10-24): reproduced.
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-src -cp "$CP" $(find src -name '*.java')
javac -d classes-src2 -cp "$CP" $(find src2 -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-src:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.facet.TestRandomSamplingBinSize
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes-src2:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.facet.TestRandomSamplingBinSize

The 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 2026-11-04. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.