Index threads.forum_id and comments.thread_id

schema.txt

@@ -39,3 +39,8 @@ create table forums (
 	description         text,
 	allowed_roles_mask  integer      not null
 );
+
+-- Both of these speed up searches significantly if there are many threads or comments.
+-- Other indices have no measureable impact (yet, at least).
+create index forum_id on threads(forum_id);
+create index thread_id on comments(thread_id);