PERF: add 'lower(title)' index on topics

db/migrate/20151103233815_add_lower_title_index_on_topics.rb

@@ -0,0 +1,9 @@
+class AddLowerTitleIndexOnTopics < ActiveRecord::Migration
+  def up
+    execute "CREATE INDEX index_topics_on_lower_title ON topics (LOWER(title))"
+  end
+
+  def down
+    execute "DROP INDEX index_topics_on_lower_title"
+  end
+end