perf(api): collapse N+1 in /api/artists/{id} + 1 round-trip in /api/albums/{id}

Two new sqlc queries replace three sequential per-album round trips that were dominating detail-screen latency. GetAlbumWithArtist: handleGetAlbum was doing GetAlbumByID then GetArtistByID — separate round trips for one logical lookup. The new query joins albums + artists with sqlc.embed and returns both in one SELECT. Detail-page DB cost: 3 trips → 2. ListAlbumsByArtistWithTrackCount: handleGetArtist was loading the artist's album list, then issuing one CountTracksByAlbum per album to populate track_count. On a 30-album artist that's 32 sequential queries — each ~5ms over a local DB, ~30ms over a remote one. The new query embeds the album row + a correlated count(*) subquery, so every album's track count comes back in one SELECT regardless of album count. Detail-page DB cost: 1 + N → 1 + 1. Together these account for the bulk of cold-cache navigation latency on the Flutter client. Combined with the existing SWR + nav hydration on the client side, detail screens should render their header instantly and the body within one round trip instead of N+constant.
2026-05-11 18:13:27 -04:00
parent 9cac664679
commit c7549bbe48
3 changed files with 132 additions and 20 deletions
@@ -40,19 +40,28 @@ func (h *handlers) handleGetTrack(w http.ResponseWriter, r *http.Request) {
 // handleGetAlbum implements GET /api/albums/{id}. Returns the album plus its
 // tracks (ordered by disc/track number via the underlying query) with
 // duration summed from the track list — keeps one source of truth.
+//
+// Two DB round trips: the album+artist join and the per-user tracks
+// list. Down from three (separate album, artist, tracks) before
+// GetAlbumWithArtist landed.
 func (h *handlers) handleGetAlbum(w http.ResponseWriter, r *http.Request) {
 	q := dbq.New(h.pool)
-	album, apiErr := resolveByID(r, "id", q.GetAlbumByID, "album")
-	if apiErr != nil {
-		writeErr(w, apiErr)
+	id, ok := requireURLUUID(w, r, "id")
+	if !ok {
 		return
 	}
-	artist, err := q.GetArtistByID(r.Context(), album.ArtistID)
+	row, err := q.GetAlbumWithArtist(r.Context(), id)
 	if err != nil {
-		h.logger.Error("api: get album artist failed", "err", err)
+		if errors.Is(err, pgx.ErrNoRows) {
+			writeErr(w, apierror.NotFound("album"))
+			return
+		}
+		h.logger.Error("api: get album+artist failed", "err", err)
 		writeErr(w, apierror.InternalMsg("lookup failed", err))
 		return
 	}
+	album := row.Album
+	artistName := row.ArtistName
 	var userID pgtype.UUID
 	if user, ok := auth.UserFromContext(r.Context()); ok {
 		userID = user.ID
@@ -68,20 +77,24 @@ func (h *handlers) handleGetAlbum(w http.ResponseWriter, r *http.Request) {
 	refs := make([]TrackRef, 0, len(tracks))
 	durSec := 0
 	for _, t := range tracks {
-		ref := trackRefFrom(t, album.Title, artist.Name)
+		ref := trackRefFrom(t, album.Title, artistName)
 		refs = append(refs, ref)
 		durSec += ref.DurationSec
 	}
 	detail := AlbumDetail{
-		AlbumRef: albumRefFrom(album, artist.Name, len(tracks), durSec),
+		AlbumRef: albumRefFrom(album, artistName, len(tracks), durSec),
 		Tracks:   refs,
 	}
 	writeJSON(w, http.StatusOK, detail)
 }

 // handleGetArtist implements GET /api/artists/{id}. Returns artist + albums;
-// each album carries its own track_count (one count query per album, same
-// pattern as subsonic). At M1 sizes (albums-per-artist << 100) this is fine.
+// each album carries its own track_count.
+//
+// Down from 1 + 1 + N queries (artist, albums, per-album CountTracksByAlbum)
+// to 1 + 1 (artist, albums-with-track-count via correlated subquery).
+// On a 30-album artist that's ~32 round trips collapsed to 2 — the
+// difference between "feels slow" and "feels instant" on detail nav.
 func (h *handlers) handleGetArtist(w http.ResponseWriter, r *http.Request) {
 	q := dbq.New(h.pool)
 	artist, apiErr := resolveByID(r, "id", q.GetArtistByID, "artist")
@@ -89,25 +102,19 @@ func (h *handlers) handleGetArtist(w http.ResponseWriter, r *http.Request) {
 		writeErr(w, apiErr)
 		return
 	}
-	albums, err := q.ListAlbumsByArtist(r.Context(), artist.ID)
+	rows, err := q.ListAlbumsByArtistWithTrackCount(r.Context(), artist.ID)
 	if err != nil {
 		h.logger.Error("api: list albums by artist failed", "err", err)
 		writeErr(w, apierror.InternalMsg("lookup failed", err))
 		return
 	}
-	refs := make([]AlbumRef, 0, len(albums))
-	for _, a := range albums {
-		count, cerr := q.CountTracksByAlbum(r.Context(), a.ID)
-		if cerr != nil {
-			h.logger.Error("api: count tracks failed", "err", cerr)
-			writeErr(w, apierror.InternalMsg("lookup failed", cerr))
-			return
-		}
+	refs := make([]AlbumRef, 0, len(rows))
+	for _, row := range rows {
 		// durationSec=0: not aggregated for nested album lists per spec data flow.
-		refs = append(refs, albumRefFrom(a, artist.Name, int(count), 0))
+		refs = append(refs, albumRefFrom(row.Album, artist.Name, int(row.TrackCount), 0))
 	}
 	detail := ArtistDetail{
-		ArtistRef: artistRefFrom(artist, len(albums)),
+		ArtistRef: artistRefFrom(artist, len(rows)),
 		Albums:    refs,
 	}
 	writeJSON(w, http.StatusOK, detail)
@@ -168,6 +168,41 @@ func (q *Queries) GetAlbumCoverageRollup(ctx context.Context) (GetAlbumCoverageR
 	return i, err
 }

+const getAlbumWithArtist = `-- name: GetAlbumWithArtist :one
+SELECT albums.id, albums.title, albums.sort_title, albums.artist_id, albums.release_date, albums.mbid, albums.cover_art_path, albums.created_at, albums.updated_at, albums.cover_art_source, albums.cover_art_sources_version, artists.name AS artist_name
+FROM albums
+JOIN artists ON artists.id = albums.artist_id
+WHERE albums.id = $1
+`
+
+type GetAlbumWithArtistRow struct {
+	Album      Album
+	ArtistName string
+}
+
+// Combined fetch for /api/albums/{id}: returns the album row + the
+// joined artist name in a single round trip. Replaces a sequential
+// GetAlbumByID + GetArtistByID pair on the hot detail-page path.
+func (q *Queries) GetAlbumWithArtist(ctx context.Context, id pgtype.UUID) (GetAlbumWithArtistRow, error) {
+	row := q.db.QueryRow(ctx, getAlbumWithArtist, id)
+	var i GetAlbumWithArtistRow
+	err := row.Scan(
+		&i.Album.ID,
+		&i.Album.Title,
+		&i.Album.SortTitle,
+		&i.Album.ArtistID,
+		&i.Album.ReleaseDate,
+		&i.Album.Mbid,
+		&i.Album.CoverArtPath,
+		&i.Album.CreatedAt,
+		&i.Album.UpdatedAt,
+		&i.Album.CoverArtSource,
+		&i.Album.CoverArtSourcesVersion,
+		&i.ArtistName,
+	)
+	return i, err
+}
+
 const getAlbumsByIDs = `-- name: GetAlbumsByIDs :many
 SELECT id, title, sort_title, artist_id, release_date, mbid, cover_art_path, created_at, updated_at, cover_art_source, cover_art_sources_version FROM albums WHERE id = ANY($1::uuid[])
 `
@@ -389,6 +424,56 @@ func (q *Queries) ListAlbumsByArtist(ctx context.Context, artistID pgtype.UUID)
 	return items, nil
 }

+const listAlbumsByArtistWithTrackCount = `-- name: ListAlbumsByArtistWithTrackCount :many
+SELECT albums.id, albums.title, albums.sort_title, albums.artist_id, albums.release_date, albums.mbid, albums.cover_art_path, albums.created_at, albums.updated_at, albums.cover_art_source, albums.cover_art_sources_version,
+       (SELECT count(*) FROM tracks t WHERE t.album_id = albums.id)::bigint
+           AS track_count
+FROM albums
+WHERE albums.artist_id = $1
+ORDER BY release_date NULLS LAST, sort_title
+`
+
+type ListAlbumsByArtistWithTrackCountRow struct {
+	Album      Album
+	TrackCount int64
+}
+
+// Replaces the N+1 pattern in handleGetArtist (1 + N CountTracksByAlbum
+// per album). Returns each album joined with its track count via a
+// correlated subquery — single round trip regardless of album count.
+func (q *Queries) ListAlbumsByArtistWithTrackCount(ctx context.Context, artistID pgtype.UUID) ([]ListAlbumsByArtistWithTrackCountRow, error) {
+	rows, err := q.db.Query(ctx, listAlbumsByArtistWithTrackCount, artistID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ListAlbumsByArtistWithTrackCountRow
+	for rows.Next() {
+		var i ListAlbumsByArtistWithTrackCountRow
+		if err := rows.Scan(
+			&i.Album.ID,
+			&i.Album.Title,
+			&i.Album.SortTitle,
+			&i.Album.ArtistID,
+			&i.Album.ReleaseDate,
+			&i.Album.Mbid,
+			&i.Album.CoverArtPath,
+			&i.Album.CreatedAt,
+			&i.Album.UpdatedAt,
+			&i.Album.CoverArtSource,
+			&i.Album.CoverArtSourcesVersion,
+			&i.TrackCount,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const listAlbumsByGenre = `-- name: ListAlbumsByGenre :many
 SELECT DISTINCT ON (albums.id) albums.id, albums.title, albums.sort_title, albums.artist_id, albums.release_date, albums.mbid, albums.cover_art_path, albums.created_at, albums.updated_at, albums.cover_art_source, albums.cover_art_sources_version
 FROM albums
@@ -14,6 +14,26 @@ RETURNING *;
 -- name: GetAlbumByID :one
 SELECT * FROM albums WHERE id = $1;

+-- name: GetAlbumWithArtist :one
+-- Combined fetch for /api/albums/{id}: returns the album row + the
+-- joined artist name in a single round trip. Replaces a sequential
+-- GetAlbumByID + GetArtistByID pair on the hot detail-page path.
+SELECT sqlc.embed(albums), artists.name AS artist_name
+FROM albums
+JOIN artists ON artists.id = albums.artist_id
+WHERE albums.id = $1;
+
+-- name: ListAlbumsByArtistWithTrackCount :many
+-- Replaces the N+1 pattern in handleGetArtist (1 + N CountTracksByAlbum
+-- per album). Returns each album joined with its track count via a
+-- correlated subquery — single round trip regardless of album count.
+SELECT sqlc.embed(albums),
+       (SELECT count(*) FROM tracks t WHERE t.album_id = albums.id)::bigint
+           AS track_count
+FROM albums
+WHERE albums.artist_id = $1
+ORDER BY release_date NULLS LAST, sort_title;
+
 -- name: GetAlbumByArtistAndTitle :one
 -- Scanner uses this for the no-mbid dedupe path: resolve-or-create.
 SELECT * FROM albums WHERE artist_id = $1 AND title = $2 LIMIT 1;