Since 10.4.0 Lucene opens indexes created by Lucene 8 once Lucene 9 has rewritten every segment, but without Lucene 9's field-consistency leniency for them (LUCENE-10518), so FieldExistsQuery returns documents that lack the doc value (or throws) and IndexWriter refuses the index

Finding LFV-490Area: search / FieldExistsQuery and index / FieldInfos field-schema consistency on Lucene-8-created indexes (SegmentInfos per-segment open gate, #14607)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 have an index first created by Lucene 8 that Lucene 9 later rewrote completely (every live segment is 9.x), in which a field is indexed in every document but has a doc value in only some (Lucene 9 allowed that only for such 8.x-created indexes), and you open it with Lucene 10.4.0 or later, which accepts it since #14607. You expect FieldExistsQuery on that field to return only the documents with the doc value, as Lucene 9.12.3 does and Lucene 9's own TestFieldExistsQuery#testOldIndices asserts, and IndexWriter to open whatever DirectoryReader.open accepts. Instead, once the documents share a segment FieldExistsQuery silently rewrites to MatchAllDocsQuery and also returns the document without the doc value (hits [d1, d2] instead of [d1], count 2 instead of 1), while they sit in separate segments it throws IllegalStateException, and IndexWriter and FieldInfos.getMergedFieldInfos throw IllegalArgumentException 'cannot change field "my_field" from doc values type=BINARY to inconsistent doc values type=NONE'.

Expected vs actual

Files

Run it

# 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/backward-codecs/build/libs/lucene-backward-codecs-$V.jar
CP=$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)

# fixtures legacy8-exists.zip and legacy8-exists-merged.zip: lucene-core-8.11.4.jar, lucene-core-9.12.3.jar and
# lucene-backward-codecs-9.12.3.jar (Maven Central)
javac -d fixture-classes-8 -cp lucene-core-8.11.4.jar fixture-src/CreateWith8x.java
javac -d fixture-classes-9 -cp lucene-core-9.12.3.jar:lucene-backward-codecs-9.12.3.jar fixture-src/ReindexWith9x.java
for mode in exists exists-merged; do
  mkdir -p fixture-$mode
  java -cp fixture-classes-8:lucene-core-8.11.4.jar CreateWith8x fixture-$mode
  java -cp fixture-classes-9:lucene-core-9.12.3.jar:lucene-backward-codecs-9.12.3.jar ReindexWith9x fixture-$mode $mode
  (cd fixture-$mode && rm -f write.lock && zip -q -X ../src/org/apache/lucene/backward_index/legacy8-$mode.zip *)
done

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:src:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.backward_index.TestPre9CreatedIndexSchemaConsistency

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.