perf: optimize cross-library move detection for single-library setups (#4888)

* feat: skip cross-library detection for single library setup

When only one library is configured, skip the cross-library move detection stage entirely as there are no other libraries to search in. This eliminates unnecessary database queries - the primary performance issue reported by users (5-6 hour scans with 13.5k missing files).

Implementation:
- Added library count check in processCrossLibraryMoves
- Returns input unchanged when len(state.libraries) == 1
- Logs debug message for troubleshooting

* refactor: use lightweight queries for cross-library move detection

Replace selectMediaFile() with newSelect() in FindRecentFilesByMBZTrackID and FindRecentFilesByProperties. These queries only need basic media file columns for hash and path comparisons, not annotations/bookmarks.

Benefits:
- Removes unnecessary LEFT JOINs with annotation and bookmark tables
- Reduces query overhead for cross-library file matching
- Follows existing pattern used by GetMissingAndMatching

The annotation/bookmark joins are user-specific (using loggedUser context) and unused in cross-library matching logic where only Equals() and IsEquivalent() checks are performed.

* test: add coverage for single-library and multi-library cross-library detection

Add test cases to verify:
1. Single-library setup correctly skips cross-library move detection
2. Multi-library setup continues to process cross-library moves

Implementation:
- New test verifies processCrossLibraryMoves returns input unchanged for single library
- Wrapped existing multi-library tests in Context with multiple libraries setup
- Ensures no regressions in multi-library matching behavior

Tests verify:
- Single-library: no database queries, input passed through unchanged
- Multi-library: cross-library matching still works correctly
- Reduces the likelihood of introducing single-library skip bugs in future

* fix: enhance cross-library detection by introducing totalLibraryCount

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

persistence/mediafile_repository.go

@@ -332,8 +332,11 @@ func (r *mediaFileRepository) GetMissingAndMatching(libId int) (model.MediaFileC
 }
 
 // FindRecentFilesByMBZTrackID finds recently added files by MusicBrainz Track ID in other libraries
+// It uses a lightweight query without annotation/bookmark joins since those are not needed for matching
 func (r *mediaFileRepository) FindRecentFilesByMBZTrackID(missing model.MediaFile, since time.Time) (model.MediaFiles, error) {
-	sel := r.selectMediaFile().Where(And{
+	sel := r.newSelect().Columns("media_file.*", "library.path as library_path", "library.name as library_name").
+		LeftJoin("library on media_file.library_id = library.id").
+		Where(And{
 			NotEq{"media_file.library_id": missing.LibraryID},
 			Eq{"media_file.mbz_release_track_id": missing.MbzReleaseTrackID},
 			NotEq{"media_file.mbz_release_track_id": ""}, // Exclude empty MBZ Track IDs
@@ -351,8 +354,11 @@ func (r *mediaFileRepository) FindRecentFilesByMBZTrackID(missing model.MediaFil
 }
 
 // FindRecentFilesByProperties finds recently added files by intrinsic properties in other libraries
+// It uses a lightweight query without annotation/bookmark joins since those are not needed for matching
 func (r *mediaFileRepository) FindRecentFilesByProperties(missing model.MediaFile, since time.Time) (model.MediaFiles, error) {
-	sel := r.selectMediaFile().Where(And{
+	sel := r.newSelect().Columns("media_file.*", "library.path as library_path", "library.name as library_name").
+		LeftJoin("library on media_file.library_id = library.id").
+		Where(And{
 			NotEq{"media_file.library_id": missing.LibraryID},
 			Eq{"media_file.title": missing.Title},
 			Eq{"media_file.size": missing.Size},

scanner/phase_2_missing_tracks.go

@@ -187,6 +187,13 @@ func (p *phaseMissingTracks) processCrossLibraryMoves(in *missingTracks) (*missi
 		return nil, nil
 	}
 
+	// Skip cross-library move detection when only one library is configured
+	// since there are no other libraries to search in.
+	if p.state.totalLibraryCount == 1 {
+		log.Debug(p.ctx, "Scanner: Skipping cross-library move detection (single library)")
+		return in, nil
+	}
+
 	log.Debug(p.ctx, "Scanner: Processing cross-library moves", "pid", in.pid, "missing", len(in.missing), "lib", in.lib.Name)
 
 	for _, missing := range in.missing {

scanner/phase_2_missing_tracks_test.go

@@ -30,6 +30,7 @@ var _ = Describe("phaseMissingTracks", func() {
 		ds = &tests.MockDataStore{MockedMediaFile: mr, MockedLibrary: lr}
 		state = &scanState{
 			libraries:         model.Libraries{{ID: 1, LastScanStartedAt: time.Date(2021, 1, 1, 0, 0, 0, 0, time.UTC)}},
+			totalLibraryCount: 1,
 		}
 		phase = createPhaseMissingTracks(ctx, state, ds)
 	})
@@ -330,6 +331,47 @@ var _ = Describe("phaseMissingTracks", func() {
 			Expect(result).To(BeNil())
 		})
 
+		It("should skip cross-library move detection when only one library is configured", func() {
+			// Default BeforeEach sets up single library, so we just need to verify skip behavior
+			Expect(state.totalLibraryCount).To(Equal(1))
+
+			missingTrack := model.MediaFile{
+				ID:                "missing1",
+				LibraryID:         1,
+				MbzReleaseTrackID: "mbz-track-123",
+				Title:             "Test Track",
+				Size:              1000,
+				Suffix:            "mp3",
+				Path:              "/lib1/track.mp3",
+				Missing:           true,
+				CreatedAt:         time.Now().Add(-30 * time.Minute),
+			}
+
+			in := &missingTracks{
+				lib:     model.Library{ID: 1, Name: "Library 1"},
+				missing: []model.MediaFile{missingTrack},
+			}
+
+			result, err := phase.processCrossLibraryMoves(in)
+			Expect(err).ToNot(HaveOccurred())
+			// Should return input unchanged (no processing done)
+			Expect(result).To(Equal(in))
+			// No matches should be found since cross-library search was skipped
+			Expect(phase.totalMatched.Load()).To(Equal(uint32(0)))
+			// No changes should be detected
+			Expect(state.changesDetected.Load()).To(BeFalse())
+		})
+
+		Context("with multiple libraries", func() {
+			BeforeEach(func() {
+				// Set up multiple libraries for cross-library move tests
+				state.libraries = model.Libraries{
+					{ID: 1, LastScanStartedAt: time.Date(2021, 1, 1, 0, 0, 0, 0, time.UTC)},
+					{ID: 2, LastScanStartedAt: time.Date(2021, 1, 1, 0, 0, 0, 0, time.UTC)},
+				}
+				state.totalLibraryCount = 2
+			})
+
 			It("should process cross-library moves using MusicBrainz Track ID", func() {
 				scanStartTime := time.Now().Add(-1 * time.Hour)
 				missingTrack := model.MediaFile{
@@ -679,6 +721,7 @@ var _ = Describe("phaseMissingTracks", func() {
 				Expect(phase.totalMatched.Load()).To(Equal(uint32(0)))
 				Expect(state.changesDetected.Load()).To(BeFalse())
 			})
+		}) // End of Context "with multiple libraries"
 	})
 
 	Describe("Album Annotation Reassignment", func() {

scanner/scanner.go

@@ -33,6 +33,7 @@ type scanState struct {
 	changesDetected   atomic.Bool
 	libraries         model.Libraries  // Store libraries list for consistency across phases
 	targets           map[int][]string // Optional: map[libraryID][]folderPaths for selective scans
+	totalLibraryCount int              // Total number of libraries (unfiltered), for cross-library move detection
 }
 
 func (s *scanState) sendProgress(info *ProgressInfo) {
@@ -73,6 +74,7 @@ func (s *scannerImpl) scanFolders(ctx context.Context, fullScan bool, targets []
 		state.sendWarning(fmt.Sprintf("getting libraries: %s", err))
 		return
 	}
+	state.totalLibraryCount = len(allLibs)
 
 	if len(targets) > 0 {
 		// Selective scan: filter libraries and build targets map