[Feature](lance) Support multi-vector columns and queries #66494

Parent

Tracked by #66340. Single-vector search was introduced by #65730.

Scope

• Map List<FixedSizeList<...>> Lance columns for multi-vector use.
• Accept multiple sub-vectors in one logical query.
• Implement Multi-vector Search.
• Define aggregation, scoring, filtering, Top-K, and _distance semantics.
• Do not implicitly expand the scope to joint search across multiple vector columns.

Completion criteria

• FE type analysis, serialization, and BE execution are implemented.
• Tests cover supported element types, dimensions, nullability, and scoring behavior.
• User documentation describes the query format and result semantics.
• Representative multi-vector datasets and indexes are verified.

Completion evidence — 2026-09-30

The issue remains closed; the previously stale checklist is now synchronized with the merged implementation #68028 and English/Chinese documentation apache/doris-website#4148.

#68028 records FE/BE and native Lance-C coverage, independent scoring checks, representative indexed/unindexed multi-vector fixtures, and successful local SQL suites; its recorded External Regression status is successful. The documented contract covers Float16/Float32/Float64, row-level scoring, null/empty handling, filtering, Top-K/offset, refinement, and resource limits. This is one logical query with multiple subvectors, not independent batch queries (#68394) or joint scoring across multiple vector columns.

The implementation merged into branch-4.1, which satisfies the parent tracker's branch-completion convention. This audit did not rerun those tests or make a published-release availability claim.