IndexIVF helper APIs ignore quantizer_params and return inconsistent results

Summary

In FAISS v1.15.0, IndexIVF::search forwards SearchParametersIVF.quantizer_params to the coarse quantizer.

However, search_and_reconstruct and search_and_return_codes call the coarse quantizer without forwarding quantizer_params.

When an IVF index uses an HNSW coarse quantizer, these APIs can therefore return different results even when they receive the same SearchParametersIVF object.

Environment

• FAISS: v1.15.0
• CPU
• IVF index with an HNSW coarse quantizer
• Metric: L2
• nprobe=1
• One FAISS/OpenMP thread
• Nested parameter: SearchParametersHNSW(efSearch=1)

Reproduction

Run the deterministic reproduction script:

python repro.py

The script uses fixed float32 arithmetic-grid inputs, does not generate random data, and passes the same SearchParametersIVF object to:

1. search
2. search_and_reconstruct
3. search_and_return_codes

The same input vectors, index, thread configuration, and search parameters are used for all three calls.

Actual behavior

The result is stable across 10 fresh processes:

• search returns ID 28
• search_and_reconstruct returns ID 22
• search_and_return_codes returns ID 22

Expected behavior

All three APIs should honor the same SearchParametersIVF object, including its nested quantizer_params.

They should use the same HNSW coarse-quantizer search configuration and return consistent IDs and distances for the same query.

Additional validation

• With the default HNSW setting efSearch=16, all three paths return ID 22
• With an IndexFlatL2 coarse quantizer, the results agree
• An independent float64 brute-force oracle confirms that ID 22 is the unique nearest neighbor
• The divergence is reproduced in 10/10 independent processes
• The input hashes and output values are identical across all reproductions

Root cause

The normal IndexIVF::search path forwards the nested parameter:

quantizer->search(
    sub_n,
    sub_x,
    cur_nprobe,
    coarse_dis.get(),
    idx.get(),
    params ? params->quantizer_params : nullptr);

Relevant source:

https://github.com/facebookresearch/faiss/blob/v1.15.0/faiss/IndexIVF.cpp#L336-L342

However, search_and_reconstruct uses:

quantizer->search(n, x, cur_nprobe, coarse_dis.get(), idx.get());

Relevant source:

https://github.com/facebookresearch/faiss/blob/v1.15.0/faiss/IndexIVF.cpp#L1128-L1148

Similarly, search_and_return_codes uses:

quantizer->search(n, x, cur_nprobe, coarse_dis.get(), idx.get());

Relevant source:

https://github.com/facebookresearch/faiss/blob/v1.15.0/faiss/IndexIVF.cpp#L1183-L1204

The public SearchParametersIVF structure contains the quantizer_params field:

https://github.com/facebookresearch/faiss/blob/v1.15.0/faiss/IndexIVF.h#L68-L95

The official FAISS documentation also explicitly documents nested quantizer parameters for IVF-HNSW indexes:

https://github.com/facebookresearch/faiss/wiki/Setting-search-parameters-for-one-query

Suggested fix

Update both helper methods to forward the nested parameters:

quantizer->search(
    n,
    x,
    cur_nprobe,
    coarse_dis.get(),
    idx.get(),
    params ? params->quantizer_params : nullptr);

Suggested regression test

Add a test using an IVF index with an HNSW coarse quantizer and a deterministic nested SearchParametersHNSW object. Verify that:

• search
• search_and_reconstruct
• search_and_return_codes

return identical IDs and distances when called with the same SearchParametersIVF.

Impact

This issue makes the helper APIs silently ignore per-query HNSW search parameters. Applications that rely on per-query tuning may receive different neighbors, reconstructed vectors, or returned codes depending on which API they call.