Taxonomy Facets#getTopDims returns another dimension's children, childCount and value under a multi-valued dimension's name, because replacing the bottom of the top-dims queue updates dim and value but not dimOrd (released since 9.2.0)

Finding LFV-363Area: facet / taxonomy facets (TaxonomyFacets#getTopDims, shared by FastTaxonomyFacetCounts and the int and float association facets)Priority: mediumImpact: silent wrong resultsNovelty: new

Finding page for LFV-363

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

What it shows

You mark taxonomy facet dims as multi-valued, run a query and call getTopDims(1, 10), and expect the same as getAllDims(10).subList(0, 1), as the Facets#getTopDims javadoc says. When the winning dim is visited after the dim it pushes out of the internal top-dims queue (dim x with 2 hits, created first, and dim y with 1 hit, both with requireDimCount), Lucene returns 'dim=x value=1 childCount=1 y1 (1)' instead of 'dim=x value=2 childCount=2 x1 (1) x2 (1)', that is y's children and count under x's name. The queue-replacement branch sets bottomDim.dim and bottomDim.value but never bottomDim.dimOrd, and the children of multi-valued dims are then computed from that stale ordinal.

The test shows that taxonomy getTopDims(1, 10) returns dim y's value, childCount and children under dim x's name when x (multi-valued, requireDimCount, created first, 2 hits) replaces y (1 hit) in the top-dims queue, because the replacement keeps y's dimOrd.

Expected vs actual

Expected (Facets#getTopDims javadoc: same as getAllDims, first topNDims): dim=x value=2 childCount=2 x1 (1) x2 (1); actual (-ea and -da): dim=x value=1 childCount=1 y1 (1).

Files

• src/org/apache/lucene/facet/taxonomy/TestTaxonomyGetTopDimsMultiValued.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.taxonomy.TestTaxonomyGetTopDimsMultiValued

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.