posts: add index on rating column.

Should improve performance for rating:e and rating:q searches. Rating:s
isn't isn't indexed because Postgres is unlikely to use the index for
rating:s searches (the selectivity is too low, ~77% of all posts are
rating:s).

db/migrate/20210127012303_add_rating_index_to_posts.rb

@@ -0,0 +1,5 @@
+class AddRatingIndexToPosts < ActiveRecord::Migration[6.1]
+  def change
+    add_index :posts, :rating, where: "rating != 's'"
+  end
+end

db/structure.sql

@@ -7239,6 +7239,13 @@ CREATE INDEX index_posts_on_parent_id ON public.posts USING btree (parent_id) WH
 CREATE INDEX index_posts_on_pixiv_id ON public.posts USING btree (pixiv_id) WHERE (pixiv_id IS NOT NULL);
 
 
+--
+-- Name: index_posts_on_rating; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_posts_on_rating ON public.posts USING btree (rating) WHERE (rating <> 's'::bpchar);
+
+
 --
 -- Name: index_posts_on_source_trgm; Type: INDEX; Schema: public; Owner: -
 --
@@ -7948,6 +7955,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20210110090656'),
 ('20210115015308'),
 ('20210123112752'),
-('20210127000201');
+('20210127000201'),
+('20210127012303');