docs: handoff 21 — ADR-0028 complete
ADR-0028 (query plans / `explain`) is fully implemented; the handoff-16 design trio (ADR-0026 / 0027 / 0028) is now closed. - handoff-21: session summary, the two deliberate deviations from handoff-20's plan, test coverage, open clusters. - requirements.md: QA1 / QA2 ticked. - CLAUDE.md: the `EXPLAIN QUERY PLAN` deferred-items line updated to "implemented per ADR-0028".
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# Session handoff — 2026-05-19 (21)
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Twenty-first handover. This session **finished ADR-0028** —
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query plans / `explain`. Steps 2–5 (planned in full in
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handoff-20 §3) are implemented, tested and committed; step 1
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landed in the previous session. **ADR-0028 is complete**, and
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with it the handoff-16 design trio (ADR-0026 / ADR-0027 /
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ADR-0028) is fully implemented.
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## State at handoff
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**Branch:** `main`. Working tree clean. **3 feature commits**
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this session (plus this handoff); all local — push
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asynchronously, not blocking.
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```
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<this file> docs: handoff 21 — ADR-0028 complete
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ae99276 explain: typing-surface matrix cells (ADR-0028 step 5)
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a7d459f explain: styled plan tree + annotation taxonomy (step 4)
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d17addd explain: `explain` command end to end (steps 2–3)
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c1fcf28 docs: handoff 20 — ADR-0028 step 1 done, 2-5 planned
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03d8a09 ui: styled-output-line mechanism (ADR-0028 step 1)
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```
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**Tests:** **1172 passing, 0 failing, 1 ignored** (`cargo
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test`). The ignored test is the long-standing `` ```ignore ``
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doc-test in `src/friendly/mod.rs`. Typing-surface matrix:
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**174 cells** (was 161 — +13 for `explain`).
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**Clippy:** clean (`cargo clippy --all-targets -- -D
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warnings`, nursery group).
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## §1. What ADR-0028 delivered
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The `explain` prefix command — `explain show data …`,
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`explain update …`, `explain delete from …` — captures a
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query's plan via `EXPLAIN QUERY PLAN` and renders it as an
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annotated, span-styled tree. `EXPLAIN QUERY PLAN` never
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executes the wrapped statement, so explaining a destructive
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`update` / `delete` is safe and changes nothing.
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- **Grammar** (`src/dsl/grammar/data.rs`): a new `EXPLAIN`
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`CommandNode` whose shape is a `Choice` over the three
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explainable query shapes, reached through `Subgrammar` — the
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inner command is parsed, completed and hinted exactly as it
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is standalone. `Command::Explain { query: Box<Self> }`.
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- **Worker** (`src/db.rs`): SQL construction split out of
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`do_query_data` / `do_update` / `do_delete` into
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`build_query_data_sql` / `build_update_sql` /
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`build_delete_sql`, so `EXPLAIN QUERY PLAN` runs the exact
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same statement. `Request::ExplainPlan` / `do_explain_plan`
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capture the plan; `QueryPlan` / `ExplainRow` carry it back.
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- **Display SQL**: the executed statement with `?N` params
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inlined as standard-SQL literals (`<>` for inequality,
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double-quoted idents, the implicit `ORDER BY <pk>` that
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`limit` adds).
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- **Render** (`src/output_render.rs`): `render_explain_plan`
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draws the box-drawing tree; `PLAN_TAXONOMY` classifies each
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node's `detail` and only the category-bearing keyword run
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carries a semantic colour (efficient / expensive /
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automatic-index). An automatic-index node also gets the
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`← add an index?` advice tag.
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- **Catalog**: `parse.usage.explain` plus `help.data.explain`
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— `explain` appears in the in-app `help` listing.
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## §2. Decisions / deviations from handoff-20's plan
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handoff-20 §3 was followed closely. The three gotchas it
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flagged all held — `static` wrappers (`EXPLAIN_SHOW_DATA`
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etc.) for the `Subgrammar` `&'static` requirement; the
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role-based `build_show_data` extraction; steps 2+3 combined
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into one commit because the `Command::Explain` variant breaks
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every exhaustive `match Command`. Two deliberate deviations:
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1. **Display SQL via param-inlining, not a parallel
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`compile_operand`.** handoff-20 suggested a second
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literal-rendering variant of the WHERE compiler. Instead,
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`inline_params_for_display` (`src/db.rs`) takes the
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executed SQL and substitutes each `?N` with its bound
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literal in a single quote-aware scan. This is structurally
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guaranteed to match the executed statement and avoids a
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second expression compiler that could drift from the
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first. (`<>`, double-quoted idents and the implicit
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`ORDER BY` all come for free — they are already in the
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executed SQL.)
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2. **`help_id: Some("data.explain")`, not `None`.**
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handoff-20 planned no help entry; at the user's request
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`explain` now has a `help.data.explain` catalog entry and
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appears in `help` like every other command.
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Other notes:
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- `render_explain_plan` returns `Vec<OutputLine>` (styled).
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Commit A built the tree with plain lines; commit B enriched
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the *same* function with the taxonomy + styled runs — no
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duplicated tree-walking logic.
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- `explain` is **not** written to `history.log`
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(`do_explain_plan` takes no `source`) — it is a read-only
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diagnostic, and `EXPLAIN QUERY PLAN` does not execute.
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- The plan tree renders wholly in the neutral foreground
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colour (not the `System` green) so connectors / names stay
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neutral per ADR-0028 §6; only category keywords are
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coloured. Every plan line therefore carries a `styled_runs`
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payload, including the display-SQL line.
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## §3. Test coverage added
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- **Parse** (`src/dsl/grammar/data.rs` `explain_tests`): each
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wrapped form parses to `Command::Explain`; `explain` of an
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incomplete inner command is the same parse error; `explain
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show table` is rejected (`explain` covers `show data` only).
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- **Worker** (`src/db.rs`): scan vs. index-search plans;
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`explain delete` / `explain update` leave data untouched;
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display SQL inlines literals, quotes idents, shows the
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implicit `ORDER BY`, writes `<>`; missing-table errors.
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- **Render** (`src/output_render.rs`): the taxonomy classes,
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the automatic-index tag, neutral fallback, a cyclic-parent
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guard.
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- **App** (`src/app.rs`): the `DslExplainSucceeded` handler
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renders the `[ok]` header, display SQL and tree.
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- **Typing-surface matrix**: 13 cells in
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`tests/typing_surface/explain.rs`.
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## §4. What's next
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ADR-0028 closes the handoff-16 design trio. No feature is in
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flight. Open clusters, unchanged from handoff-16/17/18/19/20
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(prioritisation is a **user decision — ask**):
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- Snapshot / undo / replay `U`-series (designed in ADR-0006).
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- Constraints `C3`; m:n convenience `C4`; modify-relationship
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`C3a`; column drop/rename/type-change grammar `C1` (the
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rebuild primitive exists).
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- Friendly-error layer `H1` (partial); strong syntax-help in
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parse errors `H1a`.
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- Session-log / Markdown export `V4` — would be the first
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real reuse of the `OutputLine` styled-runs mechanism beyond
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the plan renderer.
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- `SD1`; CI workflow `TT5`; readline shortcuts `I1b`;
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multi-line input `I1`; `TU1`.
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## §5. How to take over
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1. **Read this file**, then `CLAUDE.md` (working-style
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rules), then `docs/requirements.md` (per-item progress —
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`QA1` / `QA2` now ticked).
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2. **Run `cargo test`** — 1172 passing, 0 failing, 1 ignored.
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3. **Run `cargo clippy --all-targets -- -D warnings`** —
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clean.
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4. Pick the next cluster *with the user* — §4 has no default.
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### Note on the typing-surface matrix
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`tests/typing_surface/` is **174 cells**. The matrix-snapshot
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discipline from handoff-17/18/19/20 stands: a failing cell
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with *correct* new behaviour → update its snapshot; with
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*wrong* behaviour → the cell earned its keep. `cargo insta`
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is not installed on this machine — regenerate snapshots with
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`INSTA_UPDATE=always cargo test --test typing_surface_matrix
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<filter>` and review the written `.snap` files before
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committing.
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