A good read on table index.
http://www.railway.at/articles/2008/04/24/database-agnostic-database-ignorant
Sometimes, it is easy to forget when someone or something else handles it for you. For instance, ActiveRecords. Rails does not create foreign keys for you. It only creates the primary key. It does not impose constraint on the database level. The association is handled on the application level. So always remember to create an index for your model associations. It could save your database server from working too hard.
