docs(adr): ADR-0071 correction — matrix already implemented, refocus on multi-hop (#2293)

A code check before implementing the proposed matrix "P1" found matrix/across
pivoting already shipped end-to-end (DatasetMatrixTable in plugin-report +
ReportDefaultInspector type=matrix columns-across + report.form.ts), so the
ADR-0021 D2 matrix follow-up is satisfied — the first draft's "flattened" premise
was stale. Refocus ADR-0071 on the one remaining gap, multi-hop joins, which
native-sql-strategy.ts still rejects ("Only the first dotted hop is supported").
Adds a Correction section citing the matrix implementation.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: os-zhuang

docs/adr/0071-dataset-semantic-layer-depth.md

@@ -1,8 +1,29 @@
-# ADR-0071: Dataset semantic-layer depth — multi-hop joins and matrix pivoting
+# ADR-0071: Dataset semantic-layer depth — multi-hop joins
 
 ## Status
 
-Proposed
+Proposed. (Refocused — see **Correction** below. The first merged draft also
+proposed matrix/across pivoting; a code check found that already implemented, so
+this ADR now covers only the remaining gap: multi-hop joins.)
+
+## Correction (2026-06-24)
+
+The first merged draft listed **matrix / across pivoting** as an open gap (a
+proposed "P1"). That was wrong — a check of the current code found matrix
+pivoting **already implemented end-to-end**:
+
+- **Render** — `packages/plugin-report/src/DatasetReportRenderer.tsx` →
+  `DatasetMatrixTable` does a true cross-tab: one dataset query over
+  `[...rows, ...columns]`, pivoted client-side into row × column × measure cells,
+  with server-supplied row/column subtotals + grand total and drill-down. Covered
+  by `DatasetReportRenderer.test.tsx`.
+- **Author** — `packages/app-shell/src/views/metadata-admin/inspectors/ReportDefaultInspector.tsx`
+  offers `type: 'matrix'` and a dedicated "columns across" dimension editor;
+  `packages/spec/src/ui/report.form.ts` reveals `columns` when `type == 'matrix'`.
+
+The ADR-0021 D2 matrix follow-up is therefore **satisfied**; the stale "flattened"
+note came from the ADR-0021 era and predates that renderer. This ADR is refocused
+on the one follow-up that remains open: **multi-hop joins**.
 
 ## Context
 
@@ -11,74 +32,59 @@ to `include` (joins **derived** from the object graph — no hand-written `ON`
 clauses), and named `dimensions` / `measures` that presentations bind by name.
 RLS/tenant scoping is enforced per joined object at runtime (D-C, fail-closed).
 
-Two capability gaps remain, both flagged as follow-ups in ADR-0021:
-
-1. **Single-hop joins only.** `Dataset.include` is `string[]` of relationship
-   **names** (lookup / master_detail fields on the *base* object), and a
-   dimension/measure `field` uses a one-dot `relationship.field` path.
-   `native-sql-strategy.qualifyAndRegisterJoin` splits on the **first** dot:
-   `account.region` → `LEFT JOIN account` + column `"account"."region"`. A
-   two-hop path like `account.owner.region` is parsed as alias=`account`,
-   column=`owner.region` → invalid SQL. You cannot group/aggregate by a field
-   two relationships away (e.g. `opportunity → account → owner → region`).
-
-2. **Matrix / "across" is flattened, not pivoted.** `Report` already declares
-   `rows` (down) and `columns` (across) for `type: 'matrix'`, and the dataset
-   query can group by both. But the dataset report renderer shows a **flat**
-   table (rows ∪ columns as combined groupings) — it does not pivot the
-   `columns` dimension into a 2-D grid (ADR-0021 D2 follow-up). A true matrix
-   (rows × distinct-column-values × measure cells) isn't rendered.
+**Single-hop joins only.** `Dataset.include` is `string[]` of relationship
+**names** (lookup / master_detail fields on the *base* object), and a
+dimension/measure `field` uses a one-dot `relationship.field` path.
+`native-sql-strategy.ts qualifyAndRegisterJoin` splits on the **first** dot — the
+code comment is explicit: *"Only the first dotted hop is supported (single-level
+relation)"* (`const [alias, ...rest] = rawSql.split('.')`). So `account.region`
+works, but a two-hop path like `account.owner.region` is parsed as
+alias=`account`, column=`owner.region` → invalid SQL. You cannot group/aggregate
+by a field two relationships away (e.g. `opportunity → account → owner → region`).
 
 ## Prior art / industry alignment
 
 - **Curated join graph, not arbitrary SQL.** Salesforce *report types* pre-wire
-  a primary object + up to 3 related objects; Looker *Explores* declare the
-  joins a developer blesses; Power BI / Tableau model **relationships** and the
-  engine resolves the path; Cube (this platform's runtime) resolves **transitive
-  joins** from pairwise relationship declarations. All keep joins governed and
-  derived from the object graph — exactly ADR-0021's D-C stance. This ADR stays
-  in that lane (no `ON` clauses).
+  a primary object + up to 3 related objects; Looker *Explores* declare blessed
+  joins; Power BI / Tableau model **relationships** and the engine resolves the
+  path; Cube (this platform's runtime) resolves **transitive joins** from
+  pairwise relationship declarations. All keep joins governed and derived from
+  the object graph — ADR-0021's D-C stance. This ADR stays in that lane (no `ON`).
 - **Spanning depth.** Salesforce report types reach 4 objects (≈3 to-one hops);
-  SOQL allows 5 levels of parent traversal. A small, fixed depth covers the vast
+  SOQL allows 5 levels of parent traversal. A small fixed depth covers the vast
   majority of real reports.
-- **The fan-out lesson.** Looker (*symmetric aggregates*) and Tableau (*relationships
-  vs. joins*) both exist because joining across a **to-many** edge and then
-  aggregating **double-counts**. The robust answer for a governed, AI-authored,
-  RLS-scoped layer is to **not** traverse to-many in the join chain at all (see
-  Decision A) rather than to build symmetric-aggregate machinery now.
-- **Matrix.** Salesforce matrix reports allow ≤2 row groupings + ≤2 column
-  groupings; Power BI's matrix visual pivots with overflow handling. P1 targets
-  the single-column-grouping MVP with a display-side overflow cap.
-
-## Decision
+- **The fan-out lesson.** Looker (*symmetric aggregates*) and Tableau
+  (*relationships vs. joins*) exist because joining across a **to-many** edge and
+  then aggregating **double-counts**. The robust answer for a governed,
+  AI-authored, RLS-scoped layer is to **not** traverse to-many in the join chain
+  (see Decision) rather than build symmetric-aggregate machinery now.
 
-### A. Multi-hop joins (framework: spec + compiler + strategy + RLS)
+## Decision — multi-hop joins (framework: spec + compiler + strategy + RLS)
 
 - **To-one only (the correctness boundary).** Multi-hop traversal is restricted
   to **to-one** relationships — `lookup` / `master_detail` (child→parent), which
   is exactly what `include` holds today. To-one chains **never fan out**, so
   existing `SUM`/`COUNT`/etc. stay correct with **zero** symmetric-aggregate
   machinery. Traversing a **to-many** (child) relationship inside a dataset's
   join chain is explicitly **out of scope** (it needs symmetric aggregates /
-  sub-query rollups — a separate feature). Consequently the depth cap below is a
+  sub-query rollups — a separate feature). The depth cap below is therefore a
   **performance/complexity** guard, not a correctness one.
 - **Spec.** `Dataset.include` accepts relationship **paths** — dotted chains of
   to-one relationship field names from the base object (`'account'`,
   `'account.owner'`). A dimension/measure `field` may reference any field
   reachable along a **declared** path (`account.owner.region`). The dotted path
   **is** the join alias, so paths are **self-disambiguating** (no named-join
-  disambiguation like Power BI active/inactive or Looker aliased joins). A depth
+  disambiguation as in Power BI active/inactive or Looker aliased joins). A depth
   cap (default **3 hops** → 4 objects, Salesforce-report-type parity) bounds join
   count/perf; undeclared paths are still rejected (D-C unchanged).
 - **Compiler** (`dataset-compiler`). Expand each `include` path into the ordered
-  join chain; `cube.joins` is keyed by the full path alias (`account.owner`) and
-  carries `{ parentAlias, fkField, targetObject }` so the chain is reconstructable.
+  join chain; `cube.joins` keyed by the full path alias (`account.owner`),
+  carrying `{ parentAlias, fkField, targetObject }` so the chain is reconstructable.
 - **Strategy** (`native-sql-strategy.qualifyAndRegisterJoin`). For `a.b.c`,
-  register the chain (`base → a` on alias `a`; `a → b` on alias `a.b`) and
-  qualify the column as `"a.b"."c"`. Emit `LEFT JOIN`s (outer — base rows
-  without a related record still appear, the report-friendly default) in
-  dependency order. Base columns stay qualified with the base table so shared
-  column names remain unambiguous across all hops.
+  register the chain (`base → a` on alias `a`; `a → b` on alias `a.b`) and qualify
+  the column as `"a.b"."c"`. Emit `LEFT JOIN`s (outer — base rows without a
+  related record still appear, the report-friendly default) in dependency order.
+  Base columns stay qualified with the base table so shared names stay unambiguous.
 - **RLS (D-C, fail-closed).** Apply the tenant read-scope to **every** object in
   the chain, not just the first hop. The strategy already scopes per joined
   alias; generalize the loop to each hop's target object.
@@ -87,57 +93,36 @@ Two capability gaps remain, both flagged as follow-ups in ADR-0021:
   `account.owner.region`; the include editor shows/edits paths; the existing
   `missingRelationship` author-time validation generalizes to paths.
 
-### B. Matrix pivot (mostly objectui renderer)
-
-- **Query unchanged.** Group by `rows ∪ columns` — the across dimension is just
-  another `groupBy`.
-- **Renderer** (`DatasetReportRenderer`). When `type === 'matrix'` and `columns`
-  is non-empty, **pivot** the flat result into a 2-D grid: distinct `columns`
-  values become column headers, `rows` go down the side, measure(s) fill the
-  cells. Cap distinct column values at a **display** ceiling (default **50**,
-  with a "+N more — refine with a filter" notice) — a render-side guard, not a
-  query limit. MVP = a single column grouping; ≤2 row/col groupings (Salesforce
-  parity) is a fast-follow.
-- **Spec unchanged** (`rows` / `columns` already exist on `Report`).
-
 ## Consequences
 
-- New capability: analytics two-plus to-one hops deep, and true matrix reports.
-- Cost/risk: multi-hop joins multiply `LEFT JOIN`s (perf) and widen the RLS
-  surface (each hop scoped) — bounded by the depth cap + declared-path allowlist.
-  No fan-out risk by construction (to-one only). Matrix column cardinality is
-  bounded by the display cap.
-- **Backward compatible.** Single-hop `include` and flat rows keep working
-  unchanged; the depth cap and "flat unless `matrix` + `columns`" defaults
-  preserve current behaviour.
+- New capability: analytics two-plus to-one hops deep.
+- Cost/risk: multi-hop multiplies `LEFT JOIN`s (perf) and widens the RLS surface
+  (each hop scoped) — bounded by the depth cap + declared-path allowlist. No
+  fan-out risk by construction (to-one only).
+- **Backward compatible.** Single-hop `include` keeps working unchanged; the
+  depth cap default preserves current behaviour.
 
 ## Phasing
 
-- **P1 — matrix pivot (objectui-only, lowest risk).** Render the matrix grid from
-  the existing `rows`/`columns`. No spec/query change. Ships independently.
-- **P2 — multi-hop (framework).** Spec path support + compiler chain + strategy
+- **P1 — multi-hop (framework).** Spec path support + compiler chain + strategy
   chained joins + per-hop RLS, behind the depth cap, **to-one only**. Gated by
   the ADR-0021 reconciliation harness (old-vs-new numbers).
-- **P3 — objectui catalog/UI.** Multi-hop field picker + include-path editor +
+- **P2 — objectui catalog/UI.** Multi-hop field picker + include-path editor +
   path-aware author-time validation.
 
 ## Alternatives considered
 
-- **Arbitrary join predicates / raw SQL** — rejected (ADR-0021 D-C: joins are
-  derived from the object graph, no `ON` clauses; keeps datasets reviewable and
-  RLS-safe). Matches Salesforce/Cube/Power BI, not the developer-only Looker
-  `sql_on`.
+- **Arbitrary join predicates / raw SQL** — rejected (ADR-0021 D-C: joins derived
+  from the object graph, no `ON` clauses; reviewable and RLS-safe). Matches
+  Salesforce/Cube/Power BI, not the developer-only Looker `sql_on`.
 - **To-many traversal with symmetric aggregates** — deferred. Correct
   cross-to-many aggregation (Looker-style symmetric aggregates) is a separate,
   larger feature; the to-one boundary delivers the common case safely first.
-- **Edge-graph + automatic path resolution** (declare relationship edges, let the
-  engine find the path — Cube/Power BI style) — rejected for v1 in favour of
-  explicit dotted paths: smaller delta, self-disambiguating, and per-path
-  RLS-auditable.
-- **Materialized / precomputed pivots** — out of scope; query-time pivot is
-  sufficient at expected cardinalities (bounded by the display cap).
+- **Edge-graph + automatic path resolution** (Cube/Power BI style) — rejected for
+  v1 in favour of explicit dotted paths: smaller delta, self-disambiguating, and
+  per-path RLS-auditable.
 
 ## Related
 
-- ADR-0021 (analytics dataset semantic layer) — the foundation; D2 / join
-  follow-ups this ADR addresses.
+- ADR-0021 (analytics dataset semantic layer) — the foundation; the matrix D2
+  follow-up (now confirmed implemented) and the join follow-up this ADR addresses.