ParentsChildrenBlockJoinQuery: advance() does not count the matching children it skips, so conjunctions, sorted searches and explain return children beyond childLimitPerParent

Finding LFV-1048Area: join / block join iterator (ParentsChildrenBlockJoinQuery advance() and Weight#explain; reached through ConjunctionDISI, sorted-search competitive iterators and IndexSearcher.explain)Priority: mediumImpact: silent wrong resultsNovelty: new

Finding page for LFV-1048

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

What it shows

You index the blocks [a, b, p1] and [c, p2] and build a ParentsChildrenBlockJoinQuery with childLimitPerParent 1, which on its own matches a and c, the first matching child of each parent. You expect the BooleanQuery +join #id:b to match nothing and explain(join, b) to say it is not a match. Instead the conjunction returns b and explain reports b as a match: advance(target) starts counting the limit at the target instead of at the block's first child, so it returns a child that nextDoc() never returns, which breaks the DocIdSetIterator#advance contract.

Expected vs actual

• Expected: count(+join #id:b) = 0, because the join alone matches only a and c (count(join) = 2).
• Actual: count(+join #id:b) = 1: AssertionError: expected:<0> but was:<1>, with assertions on and off.

Files

• src/org/apache/lucene/search/join/TestParentsChildrenChildLimitAdvance.java (JUnit test)

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main 645d4fe14a20 (2026-10-28): 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/join/build/libs/lucene-join-$V.jar
CP=$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 off (production-like), seed 5EED1048
java -da -dsa -Dtests.asserts=false -Xmx2g --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=5EED1048 \
     -cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.search.join.TestParentsChildrenChildLimitAdvance

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).

Files shown on the finding page

• TestParentsChildrenChildLimitAdvance.java

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.