joaoh82 / rust_sqlite / 25563224520

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Commit Message

feat(engine): HNSW probe widened to cosine + dot via per-index metric (SQLR-28) (#113)

Phase 7d.2's `try_hnsw_probe` was L2-only, so any KNN query using
`vec_distance_cosine` or `vec_distance_dot` silently fell through to
brute-force even with an HNSW index attached. Surfaced by the SQLR-23
v2 W10 bench: the HNSW variant clocked ~181 ms vs ~129 ms for
brute-force because the cosine hot loop never touched the graph.

Lands as sub-phase 7d.4 — per-index distance metric, no file format
bump (the metric round-trips via the synthesized CREATE INDEX SQL in
`sqlrite_master`):

- New SQL surface: `CREATE INDEX … USING hnsw (col) WITH (metric =
  '<l2|cosine|dot>')`. Omitting the WITH clause defaults to L2,
  so pre-SQLR-28 catalogs round-trip byte-identical. Typo'd metric
  names error at CREATE INDEX time rather than silently defaulting
  to L2 — that silent fallback is exactly what we're fixing.
- New `SqlriteDialect` (wraps sqlparser's `SQLiteDialect`, only
  override is `supports_create_index_with_clause = true`).
- `HnswIndexEntry` grows a `metric: DistanceMetric` field; the load,
  rebuild, and dirty-rebuild paths all consume the per-entry metric
  instead of hard-coded L2.
- `try_hnsw_probe` widens to all three `vec_distance_*` functions
  and only fires when the index entry's metric matches the query
  function. Mismatch → brute-force fallback (correct, just slow).
- W10 bench bumped to v3; the HNSW variant creates the index
  `WITH (metric = 'cosine')`. v1/v2 numbers are not comparable.
- Tests: cosine + dot self-query through the optimizer,
  metric-mismatch fallback, unknown-metric rejection, WITH-on-btree
  rejection, save+reopen preserves cosine metric.

Unblocks SQLR-25 (republish v2/v3 bench numbers).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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