perf(contests): partial index for shadowban CTE on aggregate_user.score

ddl/migrations/0198_aggregate_user_score_negative_idx.sql

@@ -0,0 +1,29 @@
+-- Partial index covering only shadowbanned users (aggregate_user.score < 0).
+--
+-- Several handlers — v1_event_comments, v1_events_remix_contests,
+-- v1_fan_club_feed, v1_track_comments, v1_track_comment_count — materialize
+-- a `low_abuse_score` CTE via:
+--
+--     SELECT user_id FROM aggregate_user WHERE score < 0
+--
+-- Without a covering index this is a sequential scan over the entire
+-- aggregate_user table (one row per user, in the millions). On cold cache,
+-- /v1/events/remix-contests?status=all measured ~22s end-to-end, dominated
+-- by that seq scan; on warm cache the same call returns in ~100ms.
+--
+-- aggregate_user.score < 0 is a very small fraction of users (shadowbanned
+-- accounts only), so a partial index is dramatically cheaper than a full
+-- btree on `score`.
+--
+-- Size budget: thousands of matching rows × ~12 bytes ≈ tens of KB.
+--
+-- NOTE: intentionally NOT wrapped in BEGIN/COMMIT so CREATE INDEX
+-- CONCURRENTLY can run without holding ACCESS EXCLUSIVE on aggregate_user.
+-- IF NOT EXISTS keeps the migration idempotent.
+
+create index concurrently if not exists idx_aggregate_user_score_negative
+    on aggregate_user (user_id)
+    where score < 0;
+
+comment on index idx_aggregate_user_score_negative is
+    'Partial index for the shadowban-author CTEs (score < 0); avoids a full table scan of aggregate_user.';