Cache prepared statements per database connection

tests/unit/db-statements.test.ts

@@ -0,0 +1,45 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { newFixture, type Fixture } from '../helpers/fixture.js';
+
+let fixture: Fixture;
+beforeEach(async () => {
+    vi.resetModules();
+    fixture = await newFixture([]);
+});
+afterEach(async () => { await fixture.cleanup(); });
+
+describe('prepared statement caching', () => {
+    it('reuses statements with fresh parameters and remains usable after rollback', async () => {
+        const { db } = await import('../../src/lib/server/db/index.js');
+        const insertSql = 'INSERT INTO settings (key, value) VALUES (?, ?)';
+        const selectSql = 'SELECT value FROM settings WHERE key = ?';
+        const insert = db().prepare(insertSql);
+        const select = db().prepare(selectSql);
+        insert.run('cache-first', 'one');
+        expect(() => db().transaction(() => {
+            db().prepare(insertSql).run('cache-rollback', 'discarded');
+            throw new Error('rollback');
+        })()).toThrow('rollback');
+        db().prepare(insertSql).run('cache-second', 'two');
+
+        expect(db().prepare(insertSql)).toBe(insert);
+        expect(db().prepare(selectSql)).toBe(select);
+        expect(select.get('cache-first')).toEqual({ value: 'one' });
+        expect(select.get('cache-second')).toEqual({ value: 'two' });
+        expect(select.get('cache-rollback')).toBeUndefined();
+    });
+
+    it('prepares fresh statements when the connection is reopened', async () => {
+        const { db, closeDb } = await import('../../src/lib/server/db/index.js');
+        const sql = 'SELECT value FROM settings WHERE key = ?';
+        const oldStatement = db().prepare(sql);
+        db().prepare('INSERT INTO settings (key, value) VALUES (?, ?)').run('cache-persisted', 'saved');
+        closeDb();
+
+        const statement = db().prepare(sql);
+        expect(statement).not.toBe(oldStatement);
+        expect(statement.database).not.toBe(oldStatement.database);
+        expect(statement.get('cache-persisted')).toEqual({ value: 'saved' });
+        expect(db().prepare(sql)).toBe(statement);
+    });
+});