db: sync production schema with declared schema.

Fix various minor inconsistencies between the production database schema
and the declared schema in db/structure.sql.

* tags.category was a smallint instead of an integer in production.
* The unique_schema_migrations index didn't exist outside production.
* The index_posts_on_tag_index index was called index_posts_on_tags_index
  outside production.
* The posts.tag_index column didn't have a statistics target defined
  outside production.
* ID sequences didn't have `AS integer` defined in production.

db/migrate/20210127000201_resync_schema_with_production.rb

@@ -0,0 +1,84 @@
+class ResyncSchemaWithProduction < ActiveRecord::Migration[6.1]
+  def change
+    enable_extension :pgstattuple
+
+    add_index :schema_migrations, :version, name: "unique_schema_migrations", unique: true, if_not_exists: true
+
+    reversible do |dir|
+      dir.up do
+        change_column :tags, :category, :integer
+
+        if index_exists?(:posts, :tag_index, name: :index_posts_on_tags_index)
+          rename_index :posts, :index_posts_on_tags_index, :index_posts_on_tag_index
+        end
+
+        remove_index :posts, name: "index_posts_on_source", if_exists: true
+        execute "ALTER TABLE posts ALTER column tag_index SET STATISTICS 3000"
+
+        fix_id_sequences
+      end
+
+      dir.down do
+        change_column :tags, :category, :smallint
+
+        if index_exists?(:posts, :tag_index, name: :index_posts_on_tag_index)
+          rename_index :posts, :index_posts_on_tag_index, :index_posts_on_tags_index
+        end
+
+        remove_index :posts, name: "index_posts_on_source", if_exists: true
+        add_index :posts, "lower(source)", name: "index_posts_on_source", if_not_exists: true
+        execute "ALTER TABLE posts ALTER column tag_index SET STATISTICS -1"
+      end
+    end
+  end
+
+  def fix_id_sequences
+    tables = %w[
+      api_keys
+      artist_commentaries
+      artist_commentary_versions
+      artist_urls
+      artist_versions
+      artists
+      bans
+      bulk_update_requests
+      comment_votes
+      comments
+      delayed_jobs
+      dmails
+      favorite_groups
+      favorites
+      forum_posts
+      forum_topic_visits
+      forum_topics
+      ip_bans
+      mod_actions
+      news_updates
+      note_versions
+      notes
+      pixiv_ugoira_frame_data
+      pools
+      post_appeals
+      post_approvals
+      post_disapprovals
+      post_flags
+      post_replacements
+      post_votes
+      posts
+      saved_searches
+      tag_aliases
+      tag_implications
+      tags
+      uploads
+      user_feedback
+      user_name_change_requests
+      users
+      wiki_pages
+      wiki_page_versions
+    ]
+
+    tables.each do |table|
+      execute "ALTER SEQUENCE #{table}_id_seq AS integer"
+    end
+  end
+end