Tighten sqlite3 parser reference facade

[mmkal]

Summary

This stacked PR extends the parser-backed reference facade from PR #111 so partial-index where expressions are analyzed by sqlite3-parser instead of the planner token-scanning raw SQL.

The net effect is narrower direct drop-column planning: function names and collation names inside a partial index no longer look like dropped-column references, while real column references in the where expression still force the conservative table rebuild path.

Base PR: #111 (issue-110-sqlite3-parser-schemadiff).

Before / after

Before, the planner used sqlMentionsIdentifier(index.where, column.name), so these valid drops could be misread as partial-index dependencies:

create table t(x text, lower text);
create index t_x_partial on t(x) where lower(x) = 'a';

create table t(x text, nocase text);
create index t_x_partial on t(x) where x collate nocase = 'a';

Those now stay on the direct path:

alter table t drop column lower;
alter table t drop column nocase;

A true reference still rebuilds the table:

create table t(x int, y int);
create index t_x_partial on t(x) where y > 0;

-- rebuilding table "t": column "y" dropped (cannot drop in place)
alter table t rename to __sqlfu_old_t;
create table t(x int);
insert into t(x) select x from __sqlfu_old_t;
drop table __sqlfu_old_t;

Implementation

• Add indexWhereReferenceFacts and indexWhereReferencesDroppedColumns to packages/sqlfu/src/schemadiff/sqlite/references.ts.
• Keep all sqlite3-parser imports inside references.ts.
• Replace the remaining plan.ts partial-index token scan with the sqlfu-owned facade helper.
• Extend drop-column.sql schemadiff fixtures for lower(x), collate nocase, and where y > 0.

Tests

• pnpm --filter sqlfu exec vitest run test/schemadiff/fixtures.test.ts
• pnpm --filter sqlfu typecheck
• pnpm exec oxfmt --check packages/sqlfu/src/schemadiff/sqlite/references.ts packages/sqlfu/src/schemadiff/sqlite/plan.ts

---

Note

Medium Risk
Changes SQLite drop-column vs rebuild decisions in schemadiff; incorrect reference extraction could pick the wrong migration path, though behavior is covered by fixture tests.

Overview
Partial-index WHERE clauses are now analyzed through the sqlite3-parser facade in references.ts instead of token-scanning index.where in the planner. Direct ALTER TABLE … DROP COLUMN eligibility uses indexWhereReferencesDroppedColumns, which parses CreateIndexStmt and only treats real column references as blockers.

Behavior change: dropping columns named like SQL builtins (e.g. lower, nocase) no longer misfires when those names appear only as a function or COLLATE in the partial index predicate; a genuine reference such as where y > 0 still forces a table rebuild. New schemadiff fixtures document those cases.

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commit: b62ca54