title: create an index without locking the DB
body: <p>I have a table with 10+ million rows.  I need to create an index on a single column, however, the index takes so long to create that I get locks against the table.</p>
  
  <p>It may be important to note that the index is being created as part of a 'rake db:migrate' step... I'm not adverse to creating the index manually if that will work.</p>
  
  <p>UPDATE: I suppose I should have mentioned that this a write often table.</p>