Add reposts user created-at index

ddl/migrations/0223_reposts_user_created_at_idx.sql

@@ -0,0 +1,14 @@
+-- Supports /v1/users/:id/reposts pagination:
+--
+--   WHERE user_id = ?
+--     AND is_delete = false
+--   ORDER BY created_at DESC
+--
+-- The existing reposts_user_idx is ordered by repost_type and repost_item_id
+-- before created_at. For users whose reposts are not recent globally, Postgres
+-- can choose reposts_new_created_at_idx and scan millions of unrelated rows to
+-- find the user's newest reposts.
+CREATE INDEX CONCURRENTLY IF NOT EXISTS reposts_user_created_at_active_idx
+    ON public.reposts USING btree (user_id, created_at DESC)
+    INCLUDE (repost_type, repost_item_id)
+    WHERE is_delete = false;