# M2.3 — Schema + sqlx migrations

| Field | Value |
|---|---|
| Phase | M2 — Projections |
| Size | M — 1–3 days |
| Status | ⬜ Not started |
| Flags | — |
| Spec | inlined below |
| Blocks | M2.2 |

## Goal

The tables the provenance graph lives in, with the constraints that make a
malformed graph impossible rather than merely unlikely.

## Facts (inlined — no spec read needed)

```sql
CREATE TABLE memory_node (
  id        BIGSERIAL PRIMARY KEY,
  level     TEXT NOT NULL CHECK (level IN ('L0','L1','L2')),
  project   TEXT NOT NULL,
  query_id  TEXT,                       -- NULL at L2
  run_id    TEXT NOT NULL,
  t         INT  NOT NULL,
  source    TEXT,                       -- set at L0
  text      TEXT NOT NULL,
  sha256    TEXT NOT NULL UNIQUE,       -- content identity, from M0.2
  embedding vector(768) NOT NULL,
  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
);
CREATE TABLE memory_edge (
  child_sha  TEXT NOT NULL REFERENCES memory_node(sha256) ON DELETE CASCADE,
  parent_sha TEXT NOT NULL REFERENCES memory_node(sha256) ON DELETE CASCADE,
  PRIMARY KEY (child_sha, parent_sha)
);
CREATE INDEX ON memory_node USING hnsw (embedding vector_cosine_ops);
CREATE INDEX ON memory_node (project, level);
```

`sha256 UNIQUE` is what makes rebuild idempotent — re-inserting identical content
is a conflict to ignore, not a duplicate row. It is also why the hash must exclude
run ids and timestamps (M0.2).

`ON DELETE CASCADE` on both edge columns: dropping a node should not leave
dangling edges. Rebuild drops everything anyway, but a partial cleanup should not
be able to corrupt the graph.

`query_id` is NULL at L2 by design — L2 spans queries. Enforce it:
`CHECK ((level = 'L2') = (query_id IS NULL))`.

Cosine distance, not L2: these are normalised text embeddings and cosine is what
the model was trained for. `vector_cosine_ops` must match the operator the query
uses (`<=>`), or the index is silently ignored and every query is a seq scan.

## Steps

1. `migrations/0001_init.sql` with the above, plus the `query_id`/level CHECK.
2. `CREATE EXTENSION IF NOT EXISTS vector;` first — the declarative extension
   (M2.2) should have run, and this makes local dev work too.
3. Wire `sqlx::migrate!()` and run at startup.
4. `sqlx prepare` for offline compile-time query checking in CI.
5. Add a `schema_version` sanity query the repo layer asserts on connect.
6. Document that changing the embedding model is a migration, because the column
   width is part of the schema.

## Acceptance

- Migrations apply to a clean database and are idempotent.
- Inserting a duplicate `sha256` conflicts rather than duplicating.
- An `L2` row with a non-null `query_id` is rejected by the CHECK.
- The HNSW index is used by a cosine-distance query.

## Verify

**Harness:** a disposable database — `sqlx::test` or testcontainers with the same
image tag as production, `ghcr.io/cloudnative-pg/postgresql:16.2`.

**Integration test** — `tests/it_schema.rs`:
1. `a1_migrate_clean` — apply to an empty database, assert both tables exist.
2. `a2_migrate_idempotent` — apply twice, assert no error.
3. `a3_sha_unique` — insert the same sha twice, assert a unique violation.
4. `a4_level_check` — `level='L3'` rejected; `level='L2'` with a `query_id`
   rejected; `level='L1'` without one rejected.
5. `a5_edge_fk` — an edge referencing a missing sha is rejected.
6. `a6_cascade` — delete a node, assert its edges are gone.
7. `a7_hnsw_is_used` — `EXPLAIN` a `ORDER BY embedding <=> $1 LIMIT 10` query and
   assert the plan contains `Index Scan` on the HNSW index, not `Seq Scan`.

**Command:** `cargo test -p mem-store schema`

**False pass:**
- Testing the schema against SQLite or plain Postgres without pgvector. It will
  accept `vector(768)` as an unknown type in some configurations and every
  vector assertion becomes meaningless. Use the production image.
- Omitting assertion 7. An index created with the wrong opclass exists, reports
  healthy, and is never used — queries just get slower as the table grows, which
  reads as a scaling problem rather than a wrong index.

## Traps

- `vector_l2_ops` with a `<=>` query, or the reverse. The index is silently
  ignored. This is the single most common pgvector mistake.
- Making `query_id` NOT NULL because L1 always has one. L2 then cannot be stored,
  and the workaround is a sentinel string that pollutes every group-by.

---

Background: [DESIGN.md](../DESIGN.md) — pgvector schema