title: Index for multiple columns in ActiveRecord
body: <p>In ActiveRecord there are two ways to declare indexes for multiple columns:</p>
  
  <pre>add_index :classifications, [:species, :family, :trivial_names]</pre>
  
  <pre>add_index :classifications, :species
  add_index :classifications, :family
  add_index :classifications, :trivial_names</pre>
  
  <p>Is there any difference between the first approach and the second one? If so, when should I use the first and when the second?</p>