ML-169.8: add StatsLive TTFB performance fix task

backlog/tasks/ml-169.8 - Fix-StatsLive.Index-10-sync-queries-blocking-TTFB.md

@@ -0,0 +1,73 @@
+---
+id: ML-169.8
+title: "Fix: StatsLive.Index 10 sync queries blocking TTFB"
+status: To Do
+assignee: []
+created_date: "2026-05-19 11:48"
+labels:
+  - perf
+  - fix
+dependencies: []
+references:
+  - >-
+    backlog/documents/doc-27 -
+    Phase-4-Performance-Audit-Low-Hanging-Fruit-Findings.md
+modified_files:
+  - lib/music_library_web/live/stats_live/index.ex
+parent_task_id: ML-169
+priority: high
+ordinal: 29000
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+
+Fix performance issue where the StatsLive.Index mount callback executes 10 synchronous Ecto queries before returning, blocking the initial render (TTFB) on the home page.
+
+## Problem
+
+The mount callback executes 10 synchronous queries before returning `{:ok, socket}`:
+
+1. `Collection.get_latest_record()` — ORDER BY purchased_at LIMIT 1
+2. `Collection.count_records_by_artist(limit: 20)` — json_each + GROUP BY
+3. `Collection.count_records_by_genre(limit: 20)` — json_each + GROUP BY + subquery exclusion
+4. `Collection.count_records_by_release_year(limit: 20)` — substr + GROUP BY
+5. `Collection.get_records_on_this_day(date)` — strftime + ORDER BY
+6. `Collection.count_records_by_format()` — GROUP BY format
+7. `Collection.count_records_by_type()` — GROUP BY type
+8. `Wishlist.count()` — FTS search count
+9. `ListeningStats.recent_activity(timezone)` — 4 correlated subqueries × 100 rows
+10. `ListeningStats.scrobble_count()` — simple COUNT
+
+## Impact
+
+10 blocking queries on the home page is the worst TTFB bottleneck in the app. The `assign_async` pattern is already in use for TopAlbums/TopArtists on the same page — apply it consistently.
+
+## Files
+
+- `lib/music_library_web/live/stats_live/index.ex:67-109` — mount callback
+
+## Fix
+
+Use `assign_async` to defer non-critical queries. The simple ones (collection_count, wishlist_count, scrobble_count for the counter badges) can stay synchronous since they're fast. Move the heavy ones to `assign_async`:
+
+- `count_records_by_artist` / `count_records_by_genre` / `count_records_by_release_year`
+- `get_records_on_this_day`
+- `recent_activity` (the most expensive — 4 correlated subqueries)
+- `get_latest_record`
+
+Show skeleton/loading placeholders while async data loads.
+
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+
+<!-- AC:BEGIN -->
+
+- [ ] #1 StatsLive.Index mount completes in under 200ms (sync queries only)
+- [ ] #2 Charts and activity feed load with loading skeleton placeholders
+- [ ] #3 Counter badges (collection, wishlist, scrobbles) show immediately
+- [ ] #4 No visual flash of empty data containers
+- [ ] #5 Assigns that are now async handle nil/failure states gracefully
+<!-- AC:END -->