feat: add performance indexes for improved query efficiency

src/main/resources/db/migration/V0012__ADD_INDEXES.sql

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+-- ============================================================
+-- V0012 — Add performance indexes
+-- ============================================================
+
+-- mangas
+-- follow: findByFollowTrue() hot path for the scheduled follow-update task
+CREATE INDEX IF NOT EXISTS idx_mangas_follow ON mangas (follow);
+
+-- state: natural filter (PENDING, ACTIVE, etc.)
+CREATE INDEX IF NOT EXISTS idx_mangas_state ON mangas (state);
+
+-- adult: MangaSpecification always filters adult = false by default
+CREATE INDEX IF NOT EXISTS idx_mangas_adult ON mangas (adult);
+
+-- score: MangaSpecification filters score >= ?
+CREATE INDEX IF NOT EXISTS idx_mangas_score ON mangas (score);
+
+-- status: plain index for exact-match; functional index for LOWER(status) IN (...)
+CREATE INDEX IF NOT EXISTS idx_mangas_status ON mangas (status);
+CREATE INDEX IF NOT EXISTS idx_mangas_status_lower ON mangas (LOWER(status));
+
+-- title: findByTitleIgnoreCase + LIKE search via LOWER(title)
+CREATE INDEX IF NOT EXISTS idx_mangas_title_lower ON mangas (LOWER(title));
+
+-- ============================================================
+
+-- images
+-- object_key: needed for the page-by-page IN-query in ImageCleanupTask
+CREATE INDEX IF NOT EXISTS idx_images_object_key ON images (object_key);
+
+-- ============================================================
+
+-- manga_contents
+-- manga_content_provider_id: FK join for every chapter lookup
+CREATE INDEX IF NOT EXISTS idx_manga_contents_provider_id ON manga_contents (manga_content_provider_id);
+
+-- (manga_content_provider_id, LOWER(url)): covers existsByMangaContentProvider_IdAndUrlIgnoreCase
+CREATE INDEX IF NOT EXISTS idx_manga_contents_provider_url ON manga_contents (manga_content_provider_id, LOWER(url));
+
+-- downloaded: ingestion pipelines filter on downloaded = false
+CREATE INDEX IF NOT EXISTS idx_manga_contents_downloaded ON manga_contents (downloaded);
+
+-- ============================================================
+
+-- manga_content_images
+-- manga_content_id: findAllByMangaContent + existsByMangaContent_IdAndPosition
+CREATE INDEX IF NOT EXISTS idx_manga_content_images_content_id ON manga_content_images (manga_content_id);
+
+-- (manga_content_id, position): covers the position-existence check and ordered reads
+CREATE INDEX IF NOT EXISTS idx_manga_content_images_content_position ON manga_content_images (manga_content_id, position);
+
+-- image_id: FK into the 500k-row images table — cascade deletes and joins
+CREATE INDEX IF NOT EXISTS idx_manga_content_images_image_id ON manga_content_images (image_id);
+
+-- ============================================================
+
+-- manga_content_provider
+-- manga_id: findByManga_IdAndContentProvider_Id + @OneToMany collection load
+CREATE INDEX IF NOT EXISTS idx_manga_content_provider_manga_id ON manga_content_provider (manga_id);
+
+-- LOWER(manga_title): existsByMangaTitleIgnoreCaseAndContentProvider_Id
+CREATE INDEX IF NOT EXISTS idx_manga_content_provider_title_lower ON manga_content_provider (LOWER(manga_title));
+
+-- ============================================================
+
+-- manga_ingest_reviews
+-- content_provider_id: FK lookups and cascade deletes
+CREATE INDEX IF NOT EXISTS idx_manga_ingest_reviews_provider_id ON manga_ingest_reviews (content_provider_id);
+
+-- ============================================================
+
+-- user_manga_content_read
+-- user_id alone: countByUser, countDistinctMangaByUser
+-- (The UNIQUE constraint on (user_id, manga_content_id) exists but a single-column index
+--  on user_id allows efficient count/scan queries without reading the full composite key.)
+CREATE INDEX IF NOT EXISTS idx_user_manga_content_read_user_id ON user_manga_content_read (user_id);
+
+-- (user_id, created_at DESC): eliminates the sort step in findTop10ByUserOrderByCreatedAtDesc
+CREATE INDEX IF NOT EXISTS idx_user_manga_content_read_user_created ON user_manga_content_read (user_id, created_at DESC);
+
+-- ============================================================
+
+-- user_manga_follow
+-- user_id: findByUser, existsByUserAndManga
+CREATE INDEX IF NOT EXISTS idx_user_manga_follow_user_id ON user_manga_follow (user_id);
+
+-- manga_id: findByManga, existsByManga
+CREATE INDEX IF NOT EXISTS idx_user_manga_follow_manga_id ON user_manga_follow (manga_id);
+
+-- Composite unique: existsByUserAndManga / findByUserAndManga point lookups
+-- (no constraint existed on this table before)
+CREATE UNIQUE INDEX IF NOT EXISTS idx_user_manga_follow_user_manga ON user_manga_follow (user_id, manga_id);
+
+-- ============================================================
+
+-- manga_import_job
+-- status: MangaImportJobSpecification filters on status
+CREATE INDEX IF NOT EXISTS idx_manga_import_job_status ON manga_import_job (status);