perf(preview): Optimize retriving all previews from oc_filecache

Previoulsy we used a PATH LIKE expression which is fine for small
instances but doesn't scale for big instance (timeout).

Manually moving accross the tree with getFolderContentsById is
significantly faster as we can use the index and this also reuse common
APIs from OCP/Files instead of directly manipulating the filecache with
the query builder.

Assisted-by: ClaudeCode:claude-opus-4-8
Signed-off-by: Carl Schwan <carl@carlschwan.eu>

core/BackgroundJobs/PreviewMigrationJob.php

@@ -12,11 +12,10 @@ namespace OC\Core\BackgroundJobs;
 use OC\Preview\PreviewMigrationService;
 use OCP\AppFramework\Utility\ITimeFactory;
 use OCP\BackgroundJob\TimedJob;
-use OCP\DB\IResult;
+use OCP\Files\FileInfo;
 use OCP\Files\IRootFolder;
 use OCP\IAppConfig;
 use OCP\IConfig;
-use OCP\IDBConnection;
 use Override;
 use Psr\Log\LoggerInterface;
 
@@ -27,7 +26,6 @@ class PreviewMigrationJob extends TimedJob {
 		ITimeFactory $time,
 		private readonly IAppConfig $appConfig,
 		private readonly IConfig $config,
-		private readonly IDBConnection $connection,
 		private readonly IRootFolder $rootFolder,
 		private readonly PreviewMigrationService $migrationService,
 		private readonly LoggerInterface $logger,
@@ -45,79 +43,60 @@ class PreviewMigrationJob extends TimedJob {
 			return;
 		}
 
-		$startTime = time();
-		while (true) {
-			$qb = $this->connection->getQueryBuilder();
-			$qb->select('path')
-				->from('filecache')
-				->where($qb->expr()->orX(
-					// Hierarchical preview folder structure
-					$qb->expr()->like('path', $qb->createNamedParameter($this->previewRootPath . '%/%/%/%/%/%/%/%/%')),
-					// Legacy flat preview folder structure
-					$qb->expr()->like('path', $qb->createNamedParameter($this->previewRootPath . '%/%.%'))
-				))->andWhere(
-					$qb->expr()->eq('storage', $qb->createNamedParameter($this->rootFolder->getMountPoint()->getNumericStorageId()))
-				)
-				->hintShardKey('storage', $this->rootFolder->getMountPoint()->getNumericStorageId())
-				->setMaxResults(100);
-
-			$result = $qb->executeQuery();
-			$foundPreviews = $this->processQueryResult($result);
-
-			if (!$foundPreviews) {
-				break;
-			}
+		$storage = $this->rootFolder->getMountPoint()->getStorage();
+		if ($storage === null) {
+			$this->appConfig->setValueBool('core', 'previewMovedDone', true);
+			return;
+		}
 
-			// Stop if execution time is more than one hour.
-			if (time() - $startTime > 3600) {
-				return;
-			}
+		$cache = $storage->getCache();
+		$previewRootId = $cache->getId(rtrim($this->previewRootPath, '/'));
+		if ($previewRootId === -1) {
+			// No previews have ever been generated on this instance.
+			$this->appConfig->setValueBool('core', 'previewMovedDone', true);
+			return;
 		}
 
-		$this->appConfig->setValueBool('core', 'previewMovedDone', true);
-	}
+		$startTime = time();
 
-	private function processQueryResult(IResult $result): bool {
-		$foundPreview = false;
-		$fileIds = [];
-		$flatFileIds = [];
-		while ($row = $result->fetch()) {
-			$pathSplit = explode('/', $row['path']);
-			assert(count($pathSplit) >= 2);
-			$fileId = (int)$pathSplit[count($pathSplit) - 2];
-			if (count($pathSplit) === 11) {
-				// Hierarchical structure
-				if (!in_array($fileId, $fileIds)) {
-					$fileIds[] = $fileId;
-				}
-			} else {
-				// Flat structure
-				if (!in_array($fileId, $flatFileIds)) {
-					$flatFileIds[] = $fileId;
+		// Walk the preview folder tree via the `parent` column, which is indexed on
+		// every supported database platform.
+		//
+		// Depth from the preview root tells us which structure a leaf folder holds:
+		// - depth 1: legacy flat structure, e.g. preview/<fileid>/<size>.png
+		// - depth 8: hierarchical structure, e.g. preview/a/b/c/d/e/f/g/<fileid>/<size>.png
+		$foldersToVisit = [[$previewRootId, '', 0]];
+
+		while ($foldersToVisit !== []) {
+			[$folderId, $folderName, $depth] = array_pop($foldersToVisit);
+
+			$hasPreviewFiles = false;
+			foreach ($cache->getFolderContentsById($folderId) as $entry) {
+				if ($entry->getMimeType() === FileInfo::MIMETYPE_FOLDER) {
+					$foldersToVisit[] = [$entry->getId(), $entry->getName(), $depth + 1];
+				} else {
+					$hasPreviewFiles = true;
 				}
 			}
-			$foundPreview = true;
-		}
 
-		foreach ($fileIds as $fileId) {
-			try {
-				$this->migrationService->migrateFileId($fileId, flatPath: false);
-			} catch (\Exception $e) {
-				$this->logger->error('Failed to migrate preview with fileId: ' . $fileId . ' (hierarchical file structure)', [
-					'exception' => $e,
-				]);
+			if (!$hasPreviewFiles || !ctype_digit($folderName)) {
+				continue;
 			}
-		}
 
-		foreach ($flatFileIds as $fileId) {
 			try {
-				$this->migrationService->migrateFileId($fileId, flatPath: true);
+				$this->migrationService->migrateFileId((int)$folderName, flatPath: $depth === 1);
 			} catch (\Exception $e) {
-				$this->logger->error('Failed to migrate preview with fileId: ' . $fileId . ' (legacy file structure)', [
+				$this->logger->error('Failed to migrate preview with fileId: ' . $folderName, [
 					'exception' => $e,
 				]);
 			}
+
+			// Stop if execution time is more than one hour.
+			if (time() - $startTime > 3600) {
+				return;
+			}
 		}
-		return $foundPreview;
+
+		$this->appConfig->setValueBool('core', 'previewMovedDone', true);
 	}
 }

tests/lib/Preview/PreviewMigrationJobTest.php

@@ -119,7 +119,6 @@ class PreviewMigrationJobTest extends TestCase {
 			Server::get(ITimeFactory::class),
 			$this->appConfig,
 			$this->config,
-			Server::get(IDBConnection::class),
 			Server::get(IRootFolder::class),
 			new PreviewMigrationService(
 				$this->config,
@@ -157,7 +156,6 @@ class PreviewMigrationJobTest extends TestCase {
 			Server::get(ITimeFactory::class),
 			$this->appConfig,
 			$this->config,
-			Server::get(IDBConnection::class),
 			Server::get(IRootFolder::class),
 			new PreviewMigrationService(
 				$this->config,
@@ -203,7 +201,6 @@ class PreviewMigrationJobTest extends TestCase {
 			Server::get(ITimeFactory::class),
 			$this->appConfig,
 			$this->config,
-			Server::get(IDBConnection::class),
 			Server::get(IRootFolder::class),
 			new PreviewMigrationService(
 				$this->config,