Facets#getTopDims on taxonomy and SortedSetDocValues facets admits a multi-valued dimension without requireDimCount even when the query has no hits in it, so the result list contains null (AssertionError with -ea) and a dimension that has hits can be pushed out (released since 9.2.0)

Finding LFV-368Area: facet / getTopDims of taxonomy and SortedSetDocValues facets (TaxonomyFacets#getTopDims and AbstractSortedSetDocValueFacetCounts#getTopDims, shared by FastTaxonomyFacetCounts, the taxonomy int and float association facets, SortedSetDocValuesFacetCounts and ConcurrentSortedSetDocValuesFacetCounts)Priority: mediumImpact: silent wrong resultsNovelty: new

Finding page for LFV-368

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 facet dimensions a and b as multi-valued (FacetsConfig#setMultiValued, requireDimCount left at its default false), index one document with a=a1 and one with b=b1, search for the second document only and call getTopDims(10, 10), expecting [b], the same as getAllDims(10), as the Facets#getTopDims javadoc says. Taxonomy and SortedSetDocValues facets instead return [null, b] (with -ea, getTopDims throws AssertionError at Lucene's own 'assert facetResult != null'), and SSDV getTopDims(1, 10) returns [null], dropping b, the only dimension with a hit. For such dimensions getTopDims uses the placeholder value -1 without checking for hits, the admission test 'value != 0' lets -1 through, and the per-dimension result of a dimension without hits is null.

The test shows that Facets#getTopDims on taxonomy (FastTaxonomyFacetCounts) and SortedSetDocValues facets admits a multi-valued dimension without requireDimCount that has no hits for the query, so its null result lands in the list.

Expected vs actual

Expected the getAllDims(10) prefix [b] for both; actual taxonomy getTopDims(10, 10) = [null, b] and SSDV getTopDims(1, 10) = [null] with -da, and Lucene's own assert facetResult != null throws AssertionError (TaxonomyFacets.java:686) with -ea.

Files

src/org/apache/lucene/facet/TestGetTopDimsDimWithoutHits.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.TestGetTopDimsDimWithoutHits

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.