Fix broken move to MRU

Author: simolus3

packages/web/src/db/adapters/options.ts

@@ -92,6 +92,8 @@ export interface WebSpecificOpenOptions {
   /**
    * If set to a value greater than zero, the worker will cache prepared statements to avoid preparing them every time
    * a query runs.
+   *
+   * Defaults to 0 (disabling the cache).
    */
   preparedStatementsCache?: number;
 }

packages/web/src/db/adapters/wa-sqlite/RawSqliteConnection.ts

@@ -222,7 +222,7 @@ export class RawSqliteConnection {
     }
   }
 
-  async *cachedStatements(api: SQLiteAPI, sql: string): AsyncIterable<number> {
+  private async *cachedStatements(api: SQLiteAPI, sql: string): AsyncIterable<number> {
     {
       const existing = this.statementCache?.lookup(sql);
       if (existing != null) {
@@ -242,7 +242,7 @@ export class RawSqliteConnection {
     } finally {
       // We can only cache statements if the sql text corresponds to a single statement, otherwise it's not clear what
       // portion of the original sql text to use as a key.
-      if (preparedStatements.length == 1 && this.statementCache) {
+      if (preparedStatements.length === 1 && this.statementCache) {
         const stmt = preparedStatements[0];
         // Don't cache EXPLAIN statements, their result becomes invalid after schema changes.
         if (this.sqlite3_stmt_isexplain(stmt) == 0) {

packages/web/src/db/adapters/wa-sqlite/StatementCache.ts

@@ -14,7 +14,8 @@ export class PreparedStatementCache {
   lookup(sql: string): number | null {
     const foundStatement = this.#statements.get(sql);
     if (foundStatement != null) {
-      // Insert again to mark it as used.
+      // Delete and re-insert to move to the end (most-recently-used position).
+      this.#statements.delete(sql);
       this.#statements.set(sql, foundStatement);
       return foundStatement;
     }

packages/web/tests/src/db/StatementCache.test.ts

@@ -18,6 +18,21 @@ test('statement cache', () => {
   expect(cache.lookup('SELECT 1')).not.toBeNull();
 });
 
+test('lookup promotes entry to most-recently-used', () => {
+  const cache = new PreparedStatementCache(3);
+  cache.addStatement('SELECT 0', 0);
+  cache.addStatement('SELECT 1', 1);
+  cache.addStatement('SELECT 2', 2);
+
+  // Access SELECT 0 so it becomes the most-recently-used entry.
+  expect(cache.lookup('SELECT 0')).toStrictEqual(0);
+
+  // Adding a fourth entry must evict SELECT 1 (oldest).
+  expect(cache.addStatement('SELECT 3', 3)).toStrictEqual(1);
+  expect(cache.lookup('SELECT 0')).not.toBeNull();
+  expect(cache.lookup('SELECT 1')).toBeNull();
+});
+
 test('does not cache explain statements', async () => {
   const db = generateTestDb({
     schema: new Schema({}),