IndexHNSW ignores SearchParameters.sel when bounded_queue=False #5504

Summary

When searching an IndexHNSW index with SearchParameters.sel, the ID selector works correctly when bounded_queue=True.

However, when bounded_queue=False, the selector is ignored and the search may return IDs that are explicitly excluded.

This violates the contract of SearchParameters.sel: when it is non-null, only IDs accepted by the selector should be returned.

Bug type

Incorrect result

Environment

FAISS version: 1.14.2
Python: 3.x
OS: Windows
Installation: pip faiss-cpu
Index: IndexHNSWFlat
Metric: METRIC_L2

Minimal reproducer

import faiss
import numpy as np

xb = np.array(
    [
        [0.0, 0.0],
        [10.0, 0.0],
        [20.0, 0.0],
        [30.0, 0.0],
    ],
    dtype=np.float32,
)

xq = np.array([[0.0, 0.0]], dtype=np.float32)

index = faiss.IndexHNSWFlat(2, 4, faiss.METRIC_L2)
index.hnsw.efConstruction = 40
index.add(xb)

# Only ID 1 is allowed.
selector = faiss.IDSelectorRange(1, 2)

params = faiss.SearchParametersHNSW(
    efSearch=40,
    bounded_queue=False,
    sel=selector,
)

D, I = index.search(xq, 1, params=params)

print("labels:", I.tolist())
print("distances:", D.tolist())

assert I.tolist() == [[1]]
assert D.tolist() == [[100.0]]

Expected behavior

Because the selector only allows ID 1, the result should be:

labels: [[1]]
distances: [[100.0]]

Every returned label must satisfy:

sel->is_member(label)

This requirement should not depend on the value of bounded_queue.

Actual behavior

With bounded_queue=False, the result is:

labels: [[0]]
distances: [[0.0]]

ID 0 is not accepted by the selector, but it is still returned.

Comparison

Using the same index, data, query, and selector, but changing the parameter to:

bounded_queue=True

returns the correct result:

labels: [[1]]
distances: [[100.0]]

Why this is a bug

This is not normal approximate-nearest-neighbor recall behavior.

Even if HNSW fails to find the best allowed candidate, it must not return an ID that is explicitly rejected by the selector. The selector is a hard validity constraint on returned results, not a ranking preference.

In this example:

• ID 0 is explicitly excluded;
• ID 1 is the only allowed ID;
• filtering works when bounded_queue=True;
• filtering is ignored when bounded_queue=False.

This indicates a queue-path-specific ID filtering bug.

Impact

Applications that rely on SearchParameters.sel for permission filtering, tenant isolation, metadata filtering, or deleted-record filtering may receive IDs that should not be returned when bounded_queue=False.

This can cause filtering rules to be bypassed and restricted or cross-tenant data to appear in search results.