With an executor, grouped search breaks ties by slice number instead of doc ID, so tied groups and tied documents differ from sequential search

Finding LFV-238Area: grouping / collector managers (FirstPassGroupingCollectorManager, TopGroupsCollectorManager and BlockGroupingCollectorManager reduce through the cross-shard merges SearchGroup.merge, TopGroups.merge and TopGroups.mergeBlockGroups; GroupingSearch)Priority: lowImpact: 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 run a grouped search (GroupingSearch, or the grouping collector managers) on an IndexSearcher that has an executor, and several groups or several documents tie on the sort, for example because every hit scores 1.0. You expect the tie to be broken by doc ID, as Sort's javadoc says and as the same search without an executor does, so the top group and the top documents are the same either way. Instead the per-slice results are combined with merges written for real shards, which break ties by shard index, here the slice number: the search returns doc 1 instead of doc 0, or group g1 instead of g0, and the choice changes whenever the slicing changes.

Expected vs actual

• Expected: You expect the tie to be broken by doc ID, as Sort's javadoc says and as the same search without an executor does, so the top group and the top documents are the same either way.
• Actual: The per-slice results are combined with merges written for real shards, which break ties by shard index, here the slice number: the search returns doc 1 instead of doc 0, or group g1 instead of g0, and the choice changes whenever the slicing changes.

Files

• src/org/apache/lucene/search/grouping/TestConcurrentGroupingDocTieOrder.java (JUnit test)
• src/org/apache/lucene/search/grouping/TestConcurrentGroupingGroupTieOrder.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.TestConcurrentGroupingDocTieOrder \
       --select-class org.apache.lucene.search.grouping.TestConcurrentGroupingGroupTieOrder

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.