RandomSamplingFacetsCollector.amortizeFacetCounts caps each amortized label count at the docFreq of the label's drill-down term, so with a non-default FacetsConfig DrillDownTermsIndexing (NONE on any dimension, or FULL_PATH_ONLY / DIMENSION_AND_FULL_PATH on a hierarchical dimension) it silently returns 0, or a value below the true count

Finding LFV-425Area: facet / sampling (RandomSamplingFacetsCollector.amortizeFacetCounts x FacetsConfig drill-down term indexing)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-425

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 on a dimension whose FacetsConfig skips drill-down terms (DrillDownTermsIndexing NONE, or FULL_PATH_ONLY / DIMENSION_AND_FULL_PATH on a hierarchical dimension), call amortizeFacetCounts to scale the sampled counts up to estimates of the full counts, and expect what the default indexing gives: about sampled / samplingRate per label, capped by the label's real count. Instead each label is capped at reader.docFreq of its drill-down term, which these modes do not index at all (docFreq 0) or index only for the documents whose full path is that label, so a label seen 15 times in a 10% sample comes back as 0 and a label on 1000 documents comes back as 500. Nothing fails, and in the same FacetResult the amortized dimension total (for example 1000) no longer matches its children (all 0).

with DrillDownTermsIndexing.NONE, RandomSamplingFacetsCollector.amortizeFacetCounts caps a sampled label's count at the docFreq of a drill-down term that was never indexed.

Expected vs actual

Expected: 1 sampled hit / sampling rate 0.5 = 2 (both documents have dim/x). Actual: 0.

Files

• src/org/apache/lucene/facet/TestAmortizeWithoutDrillDownTerms.java (JUnit test)

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.TestAmortizeWithoutDrillDownTerms

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.