GroupingSearch.setIgnoreDocsWithoutGroupField(true) is applied only to the first pass, so totalGroupCount, getAllMatchingGroups() and getAllGroupHeads() still include the null group

Finding LFV-234Area: grouping / GroupingSearch (all-groups and all-group-heads passes)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 build a GroupingSearch with setIgnoreDocsWithoutGroupField(true) and setAllGroups(true) or setAllGroupHeads(true), and some documents that match your query have no value in the group field. You expect those documents to form no group at all, as the setter's javadoc says ('ignore documents that don't have the group field instead of putting them in a null group'), so that totalGroupCount, documented as usable 'for accurate pagination', equals the number of groups that can be returned. Instead only the returned top groups honour the flag: totalGroupCount and getAllMatchingGroups() still count the null group (3 instead of 2 in the repro), and getAllGroupHeads() still marks the best document without the field as a group head.

Expected vs actual

• Expected: You expect those documents to form no group at all, as the setter's javadoc says ('ignore documents that don't have the group field instead of putting them in a null group'), so that totalGroupCount, documented as usable 'for accurate pagination', equals the number of groups that can be returned.
• Actual: Only the returned top groups honour the flag: totalGroupCount and getAllMatchingGroups() still count the null group (3 instead of 2 in the repro), and getAllGroupHeads() still marks the best document without the field as a group head.

Files

• src/org/apache/lucene/search/grouping/TestIgnoreDocsWithoutGroupFieldAllGroups.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/grouping/build/libs/lucene-grouping-$V.jar
CP=$CP:$LUCENE/lucene/queries/build/libs/lucene-queries-$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.search.grouping.TestIgnoreDocsWithoutGroupFieldAllGroups

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.