⚡ Bolt: Optimize SQLite queries via early aggregation

.jules/bolt.md

@@ -0,0 +1,3 @@
+## 2025-01-20 - Avoid repository pollution with benchmark scripts
+**Learning:** Checking in temporary test scripts and binary dummy databases (like `benchmark.py` and `test.db`) during development is considered a blocking issue for merges as it pollutes the repository.
+**Action:** Always place temporary testing artifacts in `/tmp/` and clean them up before requesting code review or submitting changes.

src-tauri/src/library/db.rs

@@ -946,16 +946,21 @@ impl Repository {
     let tx = conn.transaction()?;
 
     let mut stmt = tx.prepare(
+      // Performance optimization: Avoid correlated scalar subqueries in the SELECT clause (EXISTS, COUNT)
+      // during unpaginated full-table scans. Using Early Aggregation via derived tables prevents
+      // an N+1 execution bottleneck.
       "SELECT
          b.id,
          b.title,
          b.authors_json,
          b.isbn10,
          b.isbn13,
          b.updated_at,
-         EXISTS(SELECT 1 FROM manual_overrides mo WHERE mo.book_id = b.id) AS has_manual_overrides,
-         (SELECT COUNT(*) FROM book_files bf WHERE bf.book_id = b.id) AS file_count
-       FROM books b",
+         mo.has_manual IS NOT NULL AS has_manual_overrides,
+         COALESCE(bf.file_count, 0) AS file_count
+       FROM books b
+       LEFT JOIN (SELECT book_id, 1 AS has_manual FROM manual_overrides GROUP BY book_id) mo ON mo.book_id = b.id
+       LEFT JOIN (SELECT book_id, COUNT(*) AS file_count FROM book_files GROUP BY book_id) bf ON bf.book_id = b.id",
     )?;
 
     let mut candidates = Vec::new();
@@ -1339,9 +1344,12 @@ impl Repository {
          (SELECT COUNT(*) FROM book_files bf WHERE bf.book_id = lb.id) AS file_count
        FROM limited_books lb"
     } else {
+      // Performance optimization: Avoid correlated scalar subqueries (COUNT) in the SELECT
+      // clause during large unpaginated table scans. Early Aggregation prevents an N+1 bottleneck.
       "SELECT b.id, b.title, b.authors_json,
-         (SELECT COUNT(*) FROM book_files bf WHERE bf.book_id = b.id) AS file_count
-       FROM books b"
+         COALESCE(bf.file_count, 0) AS file_count
+       FROM books b
+       LEFT JOIN (SELECT book_id, COUNT(*) AS file_count FROM book_files GROUP BY book_id) bf ON bf.book_id = b.id"
     };
     let mut stmt = conn.prepare(query)?;