Add comment_threads reverse lookup index

ddl/migrations/0226_comment_threads_comment_id_idx.sql

@@ -0,0 +1,10 @@
+-- Supports deferred comment triggers that classify a freshly inserted comment
+-- by checking whether it has a comment_threads row:
+--
+--   WHERE comment_id = ?
+--
+-- The primary key is ordered as (parent_comment_id, comment_id), which is ideal
+-- for parent-thread lookups but not for this reverse lookup used during comment
+-- create handling.
+CREATE INDEX CONCURRENTLY IF NOT EXISTS comment_threads_comment_id_idx
+    ON public.comment_threads USING btree (comment_id);