[Perf] GetUsers triangle rewrite + partial album index for GetTracks

api/dbv1/queries/get_users.sql

@@ -83,19 +83,31 @@ SELECT
   is_storage_v2,
   creator_node_endpoint,
 
+  -- "Of the people I follow, how many also follow this user?"
+  --
+  -- Drive the loop from my followees (always small — a few thousand at
+  -- most) and probe whether each follows the target user. The previous
+  -- shape let Postgres pick a Merge Join that walked the full follower
+  -- list of the target — for popular target users that's millions of
+  -- rows. The LIMIT 1 OFFSET 0 inside the EXISTS is the same
+  -- optimization fence used by the feed query: it pins the planner to
+  -- nested-loop semantics so the plan never flips to merge join.
   (
     SELECT count(*)
-    FROM follows f
-    JOIN (
-      SELECT followee_user_id
-      FROM follows mf
-      WHERE mf.follower_user_id = @my_id
-        AND mf.is_delete = false
-    ) mf ON f.follower_user_id = mf.followee_user_id
+    FROM follows mf
     WHERE @my_id > 0
-    AND @my_id != u.user_id -- don't compute when viewing own profile
-    AND f.followee_user_id = u.user_id
-    AND f.is_delete = false
+      AND @my_id != u.user_id -- don't compute when viewing own profile
+      AND mf.follower_user_id = @my_id
+      AND mf.is_delete = false
+      AND EXISTS (
+        SELECT 1 FROM (
+          SELECT 1 FROM follows f
+          WHERE f.follower_user_id = mf.followee_user_id
+            AND f.followee_user_id = u.user_id
+            AND f.is_delete = false
+          LIMIT 1 OFFSET 0
+        ) x
+      )
   ) AS current_user_followee_follow_count,
 
   (

ddl/migrations/0197_playlists_albums_partial_idx.sql

@@ -0,0 +1,21 @@
+-- Partial index covering only public, undeleted, current album playlists.
+--
+-- Speeds up the album_backlink subquery in GetTracks: that subquery does
+-- ~200 random `playlists_pkey` lookups per popular track to filter for
+-- (is_album=true AND is_delete=false AND is_current=true) — and ~99.98%
+-- of those lookups end up rejected by the filter. With this partial index
+-- the planner can resolve "is this an album playlist?" without ever
+-- touching the playlists heap for non-album rows.
+--
+-- Size budget: ~55k matching rows × ~12 bytes ≈ 700 KB.
+--
+-- NOTE: intentionally NOT wrapped in BEGIN/COMMIT so that
+-- CREATE INDEX CONCURRENTLY can run without holding an ACCESS EXCLUSIVE
+-- lock on playlists. IF NOT EXISTS makes the migration idempotent.
+
+create index concurrently if not exists idx_playlists_albums_published
+    on playlists (playlist_id)
+    where is_album = true and is_delete = false and is_current = true;
+
+comment on index idx_playlists_albums_published is
+    'Partial index for GetTracks album_backlink subquery; lets non-album lookups skip the heap entirely.';