PERF: add a filtered index for banners

This ensures we can very quickly figure out which topics are banners if
a banner is set.

Previously you would have to scan an entire table to find banners

app/models/draft.rb

@@ -278,6 +278,7 @@ end
 #  updated_at :datetime         not null
 #  sequence   :integer          default(0), not null
 #  revisions  :integer          default(1), not null
+#  owner      :string
 #
 # Indexes
 #

app/models/topic.rb

@@ -1544,6 +1544,7 @@ end
 #  index_topics_on_bumped_at               (bumped_at)
 #  index_topics_on_created_at_and_visible  (created_at,visible) WHERE ((deleted_at IS NULL) AND ((archetype)::text <> 'private_message'::text))
 #  index_topics_on_id_and_deleted_at       (id,deleted_at)
+#  index_topics_on_id_filtered_banner      (id) UNIQUE WHERE (((archetype)::text = 'banner'::text) AND (deleted_at IS NULL))
 #  index_topics_on_lower_title             (lower((title)::text))
 #  index_topics_on_pinned_at               (pinned_at) WHERE (pinned_at IS NOT NULL)
 #  index_topics_on_pinned_globally         (pinned_globally) WHERE pinned_globally

db/migrate/20191101001705_add_banner_index_to_topics.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+class AddBannerIndexToTopics < ActiveRecord::Migration[6.0]
+  def change
+    # this speeds up the process for finding banners on the site
+    add_index :topics, [:id], name: 'index_topics_on_id_filtered_banner', where: "archetype = 'banner' AND deleted_at IS NULL", unique: true
+  end
+end