kNN and vector-similarity searches that hit the IndexSearcher timeout before collecting any hit return 0 hits with IndexSearcher.timedOut() == false, so the timed-out search looks like a search without matches

Finding LFV-115Area: search / IndexSearcher timeout reporting for vector queries (AbstractKnnVectorQuery rewrite, AbstractVectorSimilarityQuery scorerSupplier)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 set a timeout with IndexSearcher#setTimeout and run a kNN query (KnnFloatVectorQuery or KnnByteVectorQuery) or an approximate FloatVectorSimilarityQuery/ByteVectorSimilarityQuery, and the timeout fires before the vector search has found its first hit. You expect what a TermQuery gives under the same timeout: 0 hits with IndexSearcher#timedOut() == true, the documented signal ('Returns true if any search hit the timeout') that the answer is incomplete. Instead the search returns 0 hits, TotalHits '0 hits' (EQUAL_TO) and timedOut() == false, so the timed-out search cannot be told apart from a search that has no matches; IndexSearcher#count of a kNN query behaves the same way.

Expected vs actual

• Expected: You expect what a TermQuery gives under the same timeout: 0 hits with IndexSearcher#timedOut() == true, the documented signal ('Returns true if any search hit the timeout') that the answer is incomplete.
• Actual: The search returns 0 hits, TotalHits '0 hits' (EQUAL_TO) and timedOut() == false, so the timed-out search cannot be told apart from a search that has no matches; IndexSearcher#count of a kNN query behaves the same way.

Files

• src/org/apache/lucene/search/TestVectorQueryTimeoutReporting.java (JUnit test)
• src2/org/apache/lucene/search/TestVectorQueryTimeoutReporting.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=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar   # JUnit Platform console launcher (Maven Central)

javac -d classes-src -cp "$CP" $(find src -name '*.java')
javac -d classes-src2 -cp "$CP" $(find src2 -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-src:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.search.TestVectorQueryTimeoutReporting
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes-src2:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.search.TestVectorQueryTimeoutReporting

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.