Add index on topic_links post_id. Remove a redundant index.

db/migrate/20140617193351_add_and_remove_indexes_on_topic_links.rb

@@ -0,0 +1,13 @@
+class AddAndRemoveIndexesOnTopicLinks < ActiveRecord::Migration
+  def up
+    # Index (topic_id) is a subset of (topic_id, post_id, url)
+    remove_index :topic_links, :topic_id
+
+    add_index :topic_links, :post_id
+  end
+
+  def down
+    remove_index :topic_links, :post_id
+    add_index :topic_links, :topic_id
+  end
+end