perf(library): trim scan ingestion writes

Fresh staging tables hold no path keys, so the first row for an id no longer issues a delete. Removing scanned paths now runs in one transaction with a subselect instead of committing a lookup and two deletes per chunk.

lib/services/library_database.dart

@@ -909,6 +909,7 @@ class LibraryDatabase {
 
     try {
       final batch = db.batch();
+      final stagedIds = <String>{};
       for (final json in items) {
         final id = json['id'] as String?;
         if (id == null || id.trim().isEmpty) {
@@ -924,6 +925,8 @@ class LibraryDatabase {
           _incrementalStagePathKeysTable,
           id,
           json['filePath'] as String?,
+          // The stage starts empty: only a repeated id has keys to replace.
+          replaceExisting: !stagedIds.add(id),
         );
       }
       await batch.commit(noResult: true);
@@ -1056,6 +1059,7 @@ class LibraryDatabase {
         pending = 0;
       }
 
+      final stagedIds = <String>{};
       await for (final json in items) {
         final id = json['id'] as String?;
         if (id == null || id.trim().isEmpty) {
@@ -1071,6 +1075,8 @@ class LibraryDatabase {
           _scanStagePathKeysTable,
           id,
           json['filePath'] as String?,
+          // The stage starts empty: only a repeated id has keys to replace.
+          replaceExisting: !stagedIds.add(id),
         );
         streamed++;
         pending++;
@@ -2166,31 +2172,26 @@ class LibraryDatabase {
     final db = await database;
     var totalDeleted = 0;
     const chunkSize = 500;
-    for (var i = 0; i < filePaths.length; i += chunkSize) {
-      final end = (i + chunkSize < filePaths.length)
-          ? i + chunkSize
-          : filePaths.length;
-      final chunk = filePaths.sublist(i, end);
-      final placeholders = List.filled(chunk.length, '?').join(',');
-      final rows = await db.rawQuery(
-        'SELECT id FROM library WHERE file_path IN ($placeholders)',
-        chunk,
-      );
-      final ids = rows
-          .map((row) => row['id'] as String)
-          .toList(growable: false);
-      if (ids.isNotEmpty) {
-        final idPlaceholders = List.filled(ids.length, '?').join(',');
-        await db.rawDelete(
-          'DELETE FROM library_path_keys WHERE item_id IN ($idPlaceholders)',
-          ids,
+    // One commit for the whole removal; a rescan that dropped a folder can
+    // otherwise pay several WAL commits per chunk.
+    await db.transaction((txn) async {
+      for (var i = 0; i < filePaths.length; i += chunkSize) {
+        final end = (i + chunkSize < filePaths.length)
+            ? i + chunkSize
+            : filePaths.length;
+        final chunk = filePaths.sublist(i, end);
+        final placeholders = List.filled(chunk.length, '?').join(',');
+        await txn.rawDelete(
+          'DELETE FROM library_path_keys WHERE item_id IN '
+          '(SELECT id FROM library WHERE file_path IN ($placeholders))',
+          chunk,
+        );
+        totalDeleted += await txn.rawDelete(
+          'DELETE FROM library WHERE file_path IN ($placeholders)',
+          chunk,
         );
       }
-      totalDeleted += await db.rawDelete(
-        'DELETE FROM library WHERE file_path IN ($placeholders)',
-        chunk,
-      );
-    }
+    });
     if (totalDeleted > 0) {
       _log.i('Deleted $totalDeleted items from library');
     }

lib/services/sqlite_helpers.dart

@@ -328,13 +328,18 @@ Future<void> backfillPathKeys(
   await batch.commit(noResult: true);
 }
 
+/// [replaceExisting] may be false when the caller knows [table] holds no keys
+/// for [id] yet, such as the first row for an id in a fresh staging table.
 void putPathKeysInBatch(
   Batch batch,
   String table,
   String id,
-  String? filePath,
-) {
-  batch.delete(table, where: 'item_id = ?', whereArgs: [id]);
+  String? filePath, {
+  bool replaceExisting = true,
+}) {
+  if (replaceExisting) {
+    batch.delete(table, where: 'item_id = ?', whereArgs: [id]);
+  }
   for (final key in buildPathMatchKeys(filePath)) {
     batch.insert(table, {
       'item_id': id,

test/sqlite_path_keys_batch_test.dart

@@ -0,0 +1,56 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/services/sqlite_helpers.dart';
+import 'package:spotiflac_android/utils/path_match_keys.dart';
+import 'package:sqflite/sqflite.dart';
+
+class _RecordingBatch implements Batch {
+  final operations = <String>[];
+
+  @override
+  void delete(String table, {String? where, List<Object?>? whereArgs}) {
+    operations.add('delete $table ${whereArgs?.single}');
+  }
+
+  @override
+  void insert(
+    String table,
+    Map<String, Object?> values, {
+    String? nullColumnHack,
+    ConflictAlgorithm? conflictAlgorithm,
+  }) {
+    expect(conflictAlgorithm, ConflictAlgorithm.ignore);
+    operations.add('insert $table ${values['item_id']} ${values['path_key']}');
+  }
+
+  @override
+  dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
+}
+
+void main() {
+  const path = '/storage/emulated/0/Music/Album/01 Song.flac';
+
+  test('path keys replace existing rows by default', () {
+    final batch = _RecordingBatch();
+    putPathKeysInBatch(batch, 'stage_keys', 'lib_1', path);
+    expect(batch.operations.first, 'delete stage_keys lib_1');
+    expect(batch.operations.skip(1), [
+      for (final key in buildPathMatchKeys(path))
+        'insert stage_keys lib_1 $key',
+    ]);
+  });
+
+  test('first row in a fresh stage writes the same keys without a delete', () {
+    final batch = _RecordingBatch();
+    putPathKeysInBatch(
+      batch,
+      'stage_keys',
+      'lib_1',
+      path,
+      replaceExisting: false,
+    );
+    expect(batch.operations, [
+      for (final key in buildPathMatchKeys(path))
+        'insert stage_keys lib_1 $key',
+    ]);
+  });
+}