title: optimal activerecord query format for number of X
body: <p>If for example a <code>Tweet</code> <code>has_many :favorites</code>, is it faster to call <code>Tweet.favorites.size</code> or to have a separate attribute/column in the database for a <code>Tweet</code> such as <code>num_of_favs</code>. While my initial thought was that <code>Tweet.favorites.size</code> was faster, I know that certain Rails optimization techniques detail creating your own counter_cache methods such as the <code>Tweet</code> model having a <code>favorites_size</code> attribute which isn't much different from <code>num_of_favs</code>. I am using <code>Postgres</code> btw.</p>