Give stored fields priority over doc values during fetch

[vim345]

Summary

When a retrieved field is both stored and has doc values, fetch reads it twice or from the wrong place, depending on the fetch path:

• FillDocsTask (default fetch and parallel fetch by document) reads it twice per hit. fetchSlice fills it from doc values, then the stored-document read fills it again and overwrites the doc-values result.
• FillFieldsTask (parallel fetch by field) checks doc values first, so it returns doc values while the other paths return stored values.

This PR makes stored fields take priority everywhere, as discussed in review. Stored values keep the original order and duplicates, while doc values come back sorted (and de-duplicated for SORTED_SET). The changes:

• fetchSlice skips the doc-values read for stored fields. They are filled once, from the stored document.
• FillFieldsTask.getFieldForHit checks the stored case before the doc-values case.

Fields that are only stored or only have doc values are unaffected.

Behavior change

Default fetch and parallel fetch by document return exactly what they did before, minus the wasted doc-values read. Parallel fetch by field (parallelFetchByField: true) now returns stored values for these fields. For multi-valued fields that means original order instead of doc-values order, matching the other paths.

Tests

• FillDocsTaskTest:
  • stored-only and stored + doc-values fields are both selected for the stored read;
  • fetchSlice returns the stored values (b, a, b) for a stored + doc-values field and never calls getDocValues.
• StoredDocValuesFetchTest: a multi-valued ATOM field that is stored + doc-valued returns stored values (b, a, b), and a stored-only field returns its values. It runs three ways: the default fetch, parallel fetch by document, and parallel fetch by field. Each parallel variant first asserts that its fetch config took effect. Against main, only the by-field run fails (it returned a, b).
• ./gradlew :test passes, including spotless.

Side note: MultiSegmentParallel*FetchTest set parallel fetch options as YAML keys that nothing reads, so MultiSegmentParallelFieldsFetchTest never runs FillFieldsTask. Worth a follow-up.

🤖 Generated with Claude Code

[aprudhomme]

One thing that is useful about stored fields is that it preserves ordering, where doc values get sorted. It may be better to give stored fields priority when both are enabled, to allow retrieval of the unsorted values.

[vim345]

Good point, thanks. I changed it so stored fields take priority whenever both are enabled, although not a single field enables them both right now.