test(index): make test_query_delta_indices deterministic

[wombatu-kun]

Problem

test_query_delta_indices (rust/lance/src/index/append.rs) is a recurring flake on its IVF_HNSW_SQ case, previously tracked by #2125 and partially addressed by #6953. It surfaced again as an unrelated CI failure on #7310: https://github.com/lance-format/lance/actions/runs/27660888625/job/81804843234 (assertion left == right failed, left: [116, 1000], right: [0, 1000]).

The test writes 1000 random vectors, appends the same vectors with shifted ids (so row i and row i+1000 share a vector), builds two delta index segments, and asserts the top-2 nearest to vector[0] are exactly [0, 1000] (one exact hit per segment). The IVF_HNSW_SQ build is non-deterministic at three points that all feed the trained centroids -> partition split -> per-partition HNSW graph -> whether the approximate search keeps the exact match:

• input vectors come from the unseeded generate_random_array;
• IVF centroid training samples a random 512-of-1000 subset with unseeded RNGs;
• k-means centroid init is unseeded.

So the search occasionally drops the exact match (id 0) and returns a near neighbor instead.

Fix

Test-only change, no production behavior change:

• seed the input vectors with generate_random_array_with_seed;
• supply fixed IVF centroids for the SQ case via IvfBuildParams::try_with_centroids, which skips IVF centroid training entirely (both the random sample and the unseeded k-means init).

HNSW level assignment is already seeded (HNSW_LEVEL_RNG_SEED) and SQ does not sample at this size (sample_size 256 * 2^8 >> 1000), so with seeded data and fixed centroids the whole build is reproducible. The 2 IVF partitions, nprobes(2), all structural assertions, and the exact [0, 1000] assertion are unchanged. The ivf_pq case is left as-is; it was never the flaky case.

Tests

Ran test_query_delta_indices (both parametrized cases) 300x with zero failures; the SQ case now returns [0, 1000] identically on every run.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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