feat: ADR-0035 4c — DROP TABLE [IF EXISTS]
Add advanced-mode SQL `DROP TABLE [IF EXISTS] <name>` -> SqlDropTable, executing through the existing do_drop_table (cascade / inbound- relationship refusal / metadata cleanup) — full parity with the simple `drop table`. The only new behaviour is `IF EXISTS` as a no-op-with-note: a new DropOutcome::Skipped mirroring CreateOutcome::Skipped (journalled, no snapshot), rendered via a new ddl.drop_skipped_absent note + DslDropSkipped event. - Grammar: SQL_DROP_TABLE node (entry `drop`, shape `table [if exists] <name> [;]`), registered Advanced. SQL-first dispatch: `drop table T` -> SqlDropTable in advanced; `drop column`/`relationship`/`index`/ `constraint` fall back to the simple `drop` node (and still execute). - Worker: Request::SqlDropTable + db.sql_drop_table; the if-exists-and- absent arm journals + replies Skipped without a snapshot, else snapshot_then(do_drop_table) -> Dropped. - Completion: advanced `drop ` now surfaces the SQL `table` (the shared-entry-word behaviour from `create`); test split into simple (full DSL list) + advanced (SQL surface). Known shared-entry-word completion unevenness (advanced `drop ` offers only `table`; partial `drop rel` returns an empty list) deferred to 4i (merge candidate sets for shared entry words) along with a flagged user request to visually distinguish simple- vs advanced-mode completions in the hint UI — tracked in ADR §13 4i (d)/(e), the 4c plan, and the completion test. The DSL drops still parse + execute via fallback. 10 new tests (parse/builder + Tier-3: drop existing + one-undo-step + restore, IF EXISTS skip + journal, plain-absent error, inbound refusal). Docs: ADR-0035 Status/§13, README, requirements.md Q1. Tests: 1805 passing, 0 failing, 1 ignored. Clippy clean.
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# Plan: ADR-0035 Phase 4, sub-phase 4c — `DROP TABLE [IF EXISTS]`
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Add advanced-mode SQL `DROP TABLE [IF EXISTS] <name>` → `SqlDropTable`.
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Reuses the existing `do_drop_table` (cascade/inbound-refusal +
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metadata cleanup), so it has full parity with the simple `drop table`;
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the only new behaviour is `IF EXISTS` as a **no-op-that-succeeds-with-a-
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note**, mirroring 4a's `CreateOutcome::Skipped` with a `DropOutcome`
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(ADR-0035 §4/§13). Small slice.
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## 1. Baseline
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- Tests: **1795 passing, 0 failing, 1 ignored**; clippy clean. Branch
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`main`, last commit `76d6059` (4b). 4c starts here.
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## 2. Decisions (from ADR §4/§13 + the 4a precedent — not re-litigated)
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1. **`IF EXISTS` → no-op-with-note** (universal cross-vendor idiom,
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already in scope per the 4a `IF [NOT] EXISTS` decision): dropping an
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absent table succeeds with a note instead of the plain "no such
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table" error. Mirrors `CreateOutcome::Skipped` via a new
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`DropOutcome { Dropped, Skipped }`. The skip is **journalled** (no
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snapshot), exactly as the create-skip is.
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2. **Table only.** SQL `DROP TABLE` drops a table; `DROP INDEX` is 4c's
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sibling 4d (`SqlDropIndex`); column/constraint drops are `ALTER TABLE`
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(4e+). `drop column`/`drop relationship`/`drop index` keep falling
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back to the simple `drop` node.
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3. **Dispatch (ADR-0033 Amendment 1).** `drop` is a shared entry word;
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the SQL node is tried first in advanced mode. `drop table T` parses
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as `SqlDropTable` in advanced mode (equivalent execution to the
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simple `DropTable` — both call `do_drop_table`); the simple form is
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the only one in simple mode.
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4. **Cascade / inbound-relationship refusal parity** — reuse
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`do_drop_table` unchanged (it already refuses dropping a table with
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inbound relationships and cleans `__rdbms_*` metadata, incl. the 4a.3
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`table_checks` rows). One undo step (`snapshot_then`).
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## 3. Phase 1 — Requirements checklist (4c)
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- [ ] `DROP TABLE <name>` parses in advanced mode → `SqlDropTable`.
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- [ ] `DROP TABLE IF EXISTS <name>` sets the `if_exists` flag.
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- [ ] Dropping an existing table removes it + its metadata (parity with
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simple `drop table`); one undo step; `undo` restores it.
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- [ ] `IF EXISTS` on an **absent** table → success with a note, no
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error, no snapshot; the line is journalled.
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- [ ] Plain `DROP TABLE` on an absent table → the existing "no such
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table" error (unchanged).
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- [ ] Dropping a table with **inbound relationships** is refused
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(existing `do_drop_table` semantics), in advanced mode too.
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- [ ] `drop column` / `drop relationship` / `drop index` still parse as
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the simple forms in advanced mode (fallback).
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- [ ] Engine-neutral note wording.
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### Testing
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- [ ] **Tier 1** (builder): `drop table <name>` → `SqlDropTable{if_exists:
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false}`; `drop table if exists <name>` → `if_exists: true`; the simple
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`drop column`/`drop relationship` forms still parse (fallback).
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- [ ] **Tier 3** (`tests/sql_drop_table.rs`): drop existing → gone +
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one undo step + undo restores; `IF EXISTS` absent → skipped + note +
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journalled; plain absent → error; inbound-relationship refusal.
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- [ ] **Catalog** lockstep + vocab audit for the new note key.
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## 4. Architecture & design
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### 4.1 Grammar (`src/dsl/grammar/ddl.rs`)
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- `IF_EXISTS_OPT` = `Optional(Seq[Word("if"), Word("exists")])`.
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- `SQL_DROP_TABLE_SHAPE` = `Seq[Word("table"), IF_EXISTS_OPT,
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TABLE_NAME_EXISTING, Optional(Punct(';'))]` (mirrors the simple
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`DROP_TABLE` shape + the optional `IF EXISTS` + trailing `;`).
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- `pub static SQL_DROP_TABLE: CommandNode { entry: "drop", shape,
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ast_builder: build_sql_drop_table, help_id: "ddl.sql_drop_table",
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usage_ids: ["parse.usage.sql_drop_table"] }`.
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- `build_sql_drop_table`: `name = require_ident("table_name")`,
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`if_exists = path.contains_word("if")` (the `if` only appears in the
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`IF EXISTS` prefix; mirror the create builder's `if_not_exists`).
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### 4.2 Command (`src/dsl/command.rs`)
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`SqlDropTable { name: String, if_exists: bool }`. Verb label
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`"drop table"`; `target_table()` → `name` (add to the existing match
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arms alongside `DropTable`).
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### 4.3 Worker (`src/db.rs`)
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- `DropOutcome { Dropped, Skipped }` (the drop peer of `CreateOutcome`;
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`Skipped` needs no payload — the runtime renders the note from the
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command's name).
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- `Request::SqlDropTable { name, if_exists, source, reply:
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oneshot<Result<DropOutcome>> }` + `db.sql_drop_table` method.
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- Dispatch arm: if `if_exists && !user_table_exists(name)` → journal the
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line (no snapshot) and reply `Skipped` (mirror the create-skip arm);
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else `snapshot_then(… do_drop_table(name) … → Dropped)`.
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- `do_drop_table` unchanged.
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### 4.4 Runtime + event + app (`src/runtime.rs`, `src/event.rs`, `src/app.rs`)
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- `Command::SqlDropTable` → `db.sql_drop_table` → map `Dropped` →
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`CommandOutcome::Schema(None)` (same as simple drop), `Skipped` → a new
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`CommandOutcome::SchemaDropSkipped`.
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- `CommandOutcome::SchemaDropSkipped` → `AppEvent::DslDropSkipped {
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command }`; the app handler notes `ddl.drop_skipped_absent` with
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`command.target_table()` (mirror `DslCreateSkipped`). No structure to
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render (the table is gone / never existed).
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### 4.5 Friendly catalog
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- New key `ddl.drop_skipped_absent: "table '{name}' doesn't exist —
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skipped (no changes made)"` + a `keys.rs` `("ddl.drop_skipped_absent",
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&["name"])` entry. Engine-neutral. Help/usage skeleton body for
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`ddl.sql_drop_table` + `parse.usage.sql_drop_table` (a fresh node — its
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keys must exist; unlike the deferred *refresh* of the create skeleton,
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these are new keys the node references, so they land now).
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## 5. Out of 4c scope
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- `DROP INDEX` (4d → `SqlDropIndex`); `ALTER TABLE` (4e–4h).
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- **Shared-entry-word completion (deferred to 4i, user-confirmed
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2026-05-25).** Adding the SQL `drop` node makes advanced-mode `drop `
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completion surface only `table`; a partial DSL subcommand keyword
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(`drop rel`) returns an empty list (mid-word dead end). The DSL drops
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still parse + execute via fallback — only the completion hint is
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affected. This is the pre-existing shared-entry-word model (same as
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`create`/`insert`/`update`/`delete`), exposed here because `drop` has
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five distinct DSL subcommands with no SQL equivalent. 4i merges the
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candidate sets for shared entry words; **the user also wants to
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discuss visually distinguishing simple- vs advanced-mode completions
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in the hint UI (likely by colour)** before/with that work. Tracked in
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ADR-0035 §13 4i (d)/(e) and the advanced-mode completion test in
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`src/completion.rs`.
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## 6. Devil's Advocate review of this plan
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- **Reuse vs fork?** `do_drop_table` is the single executor; the SQL
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path differs only at the `IF EXISTS` branch (mirrors the create-skip).
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✓
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- **Dispatch fallback?** A builder test asserts `drop column`/`drop
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relationship` still parse simple in advanced mode. ✓
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- **One undo step + restore?** `snapshot_then` wrap + a dedicated undo
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test (drop → undo → table back, with its data/metadata). ✓
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- **No-op journalled, not snapshotted?** Mirrors the create-skip arm
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exactly; a journalling test. ✓
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- **Engine-neutral?** The note is a catalog key under the vocab audit;
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the plain-absent error is the existing `do_drop_table` path (unchanged,
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already engine-neutral). ✓
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## 7. Implementation sequence (test-first)
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1. **Command + grammar + builder** — Tier-1 builder tests (parse +
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`if_exists` + simple fallback) → red → add `SqlDropTable`, the grammar
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node + shape + builder, REGISTRY entry, thread `Request`/method/
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dispatch/runtime (drop execution can land here too) → green.
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2. **Worker no-op-with-note** — Tier-3 (`tests/sql_drop_table.rs`): drop
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existing, `IF EXISTS` skip + journal, plain-absent error, inbound
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refusal, one undo step + restore → red → add `DropOutcome` + the
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dispatch arm + the `SchemaDropSkipped`/`DslDropSkipped`/app-note path
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→ green.
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3. **Catalog** — add the note + help/usage keys + bodies; lockstep +
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vocab audit → green.
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4. **Full sweep** — `cargo test` (no regression from 1795) + clippy.
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5. **Docs** — ADR-0035 Status/§13 4c; README; `requirements.md` Q1.
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Propose commit; wait for approval.
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## 8. Exit gate
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- All §3 items satisfied; four tiers green, zero skips; no regression
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from the 1795 baseline; written DA pass; clippy clean.
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