Make namedVector compose with multivector in ann-benchmark

[rlmanrique]

Summary

Running the benchmarker with both --namedVector and -m (multivector/Colbert) failed at import time:

go run main.go ann-benchmark -v lotte-recreation-reduced-vl-10000.hdf5 \
  -m 128 --indexType hnsw -d cosine --namedVector vector1
...
INFO[0000] Error for index 0: inconsistent vector lengths: 2432 != 2560
INFO[0000] Error for index 1: inconsistent vector lengths: 1664 != 2560
INFO[0000] Error for index 5: inconsistent vector lengths: 1920 != 2560

Root cause

The cfg.NamedVector != "" branches in both the ingestion and query paths unconditionally overwrote any multivector payload with a single flat encodeVector(vec). For a Colbert dataset with variable token counts (19/13/15/16/20 × 128), each document encoded to a different length and Weaviate rejected every doc whose length differed from the first object in the batch.

The schema-creation branch had the matching gap: when NamedVector != "", the multivector / muvera block was never added, so Weaviate created a plain single-vector HNSW index regardless of -m.

Changes

File	Function	Fix
cmd/ann_benchmark.go	writeChunk	Emit MULTI_FP32 under cfg.NamedVector when both flags set, instead of clobbering
cmd/ann_benchmark.go	createSchema	Inject the multivector / muvera block into the named vector index config when -m > 0
cmd/random_vectors.go	nearVectorQueryGrpc	Query path sends MULTI_FP32 with Targets.TargetVectors=[cfg.NamedVector] when both are set
cmd/ann_benchmark.go	enableCompression	Check NamedVector before MultiVectorDimensions so compression updates land on the right VectorConfig key

Single-vector + namedVector and multivector-only paths are unchanged.

Test plan

• CGO_ENABLED=1 go build ./...
• CGO_ENABLED=1 go test ./cmd/...
• Repro that originally failed: go run main.go ann-benchmark -v lotte-recreation-reduced-vl-10000.hdf5 -m 128 --indexType hnsw -d cosine --namedVector vector1 — ingest completes, queries return non-zero recall
• Regression: ann-benchmark with only -m (no --namedVector) still works
• Regression: ann-benchmark with only --namedVector (no -m) still works

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca