PERF: add an index on slug to make sure that slug lookups are quick

db/migrate/20170824172615_add_slug_index_on_topic.rb

@@ -0,0 +1,9 @@
+class AddSlugIndexOnTopic < ActiveRecord::Migration
+  def up
+    execute 'CREATE INDEX idxTopicSlug ON topics(slug) WHERE deleted_at IS NULL AND slug IS NOT NULL'
+  end
+
+  def down
+    execute 'DROP INDEX idxTopicSlug'
+  end
+end