Fusion min-max normalises over every keyword match, so hybrid_search asked
BM25 for a ranked list of the whole corpus: a hash insert per posting, then a
sort of every match, then the merge sorted every candidate again to keep k.
- BM25Index::scores returns every match unsorted, accumulated in a dense array
(contributions are strictly positive, so zero means untouched). search() is
built on it with the bounded heap.
- merge_vector_keyword partitions out its top k (select_nth) and orders only
those, with the same score-then-id order.
- Both hybrid paths use scores().
Rankings are identical (equivalence tests for both changes). p50 0.24 -> 0.07
ms (1K), 2.1 -> 0.49 ms (10K), 23 -> 4.65 ms (100K).
The harness gains --fusion-study, which measured the alternative — capping the
keyword pool — and found it changes the top-10 for most queries (overlap
0.83-0.92, different #1 for 10-35%) for only a 2x saving. Not adopted.
Co-Authored-By: Claude Fable 5.1 <[email protected]>
hybrid_search rebuilt the BM25 index from scratch (re-tokenising every record)
and rewrote the whole .h5 file on every single query, so a query cost O(store
size) in both CPU and disk I/O. Steady-state p50 per the search harness:
5.5 -> 0.24 ms (1K), 49 -> 2.1 ms (10K), 884 -> 23 ms (100K).
- BM25Index is incremental: add_document / remove_document keep it exactly
equivalent to a fresh build over the same live documents (property test: 60
random op sequences compared against BM25Index::build after every step). IDF
moves to query time since it depends on the live document count. Top-k uses
a bounded heap, ties break by doc id (results were HashMap-ordered), and the
"WAND" code that computed a bound and then discarded it is removed.
- HDF5Memory keeps one index for its lifetime, built lazily. Appends are
picked up by ensure_bm25_fresh whatever path added them; delete and in-place
update report themselves; compaction drops the index. A test drives every
mutation and compares against a fresh build.
- A query no longer calls flush(). Activation boosts are marked dirty and
persisted by the next checkpoint, including a best-effort one on drop so a
search-only session keeps them (approved behaviour change). Activation
weights are capped at 16.0; they previously grew without bound.
The archived mission branch's BM25 cache was reviewed and not used: it was
invalidated by every write, so interleaved save/search still rebuilt per
query, and it changed the default fusion weights.
Co-Authored-By: Claude Fable 5.1 <[email protected]>
top_k_scores.sort_by(...) ran over the full k-sized buffer for every
matching document that beat the running threshold (twice in the full
branch), plus another full sort on first reaching k results —
O(m·k log k) for m matching documents. Replace the Vec<f32> buffer
with a BinaryHeap<Reverse<HeapScore>> min-heap of size k, giving
O(m log k). Existing wand_returns_same_results_as_exhaustive test
confirms results are unchanged.
INT-11
