[Perf] Split feed repost branch by entity type

[raymondjacobson]

Summary

Rewrite the feed query so the repost branch handles track-type and playlist-type reposts separately. Each side INNER-joins against just the entity type it needs (tracks_pkey or playlists_pkey), eliminating the upfront 94k-row hash that the planner was building over every public playlist on every call.

Why

The old query did:

LEFT JOIN tracks    ON repost_type = 'track'  AND repost_item_id = track_id    AND ... 
LEFT JOIN playlists ON repost_type != 'track' AND repost_item_id = playlist_id AND ...
WHERE (tracks.track_id IS NOT NULL OR playlists.playlist_id IS NOT NULL)

To satisfy the playlist LEFT JOIN, Postgres scanned the idx_playlist_status partial index for every public playlist (94,306 rows on prod, 4.3 MB hash, 2,420 buffers) on every feed call — even though the average user's repost set contains only a handful of playlist-type reposts.

Per pg_stat_statements this query had two variants with mean exec times of 860ms and 4,478ms; in Axiom, /v1/users/:userId/feed shows p50 4.4s / p95 13s — the worst signed-in endpoint by total time.

Impact

EXPLAIN ANALYZE on prod read replica, user 20 (1,752 follows):

Metric	Before	After
Public-playlist hash build	94,306 rows / 2,420 buffers	gone
Per-row entity lookup	n/a	tracks_pkey + playlists_pkey

Warm-cache timings are similar in my tests (PG plan caching obscures the win at the read-replica). The savings show up in cold cache and at the tail — production tail latency should drop noticeably with the upfront hash gone.

Risk

• Logic-preserving rewrite. Branch 1a returns 'track'-typed entities, Branch 1b returns whatever repost_type was ('playlist' or 'album') — same as before. The outer GROUP BY (entity_type, entity_id) and max(created_at) semantics are unchanged.
• New TestUsersFeed covers both repost branches and the owned-track/owned-playlist branches.

Test plan

• go test -count=1 ./api/... (full suite, all green)
• TestUsersFeed exercises track-repost, playlist-repost, owned-track, and owned-playlist branches plus the no-followees empty case
• Local server hits /v1/users/Wem1e/feed?limit=20 (Phuture, 1752 follows): 500-750ms warm

🤖 Generated with Claude Code

[dylanjeffers]

love it