title: What happens to the hidden MySQL [InnoDB] PK when you add a PK?
body: <p>When using the InnoDB storage engine in MySQL if you do not specify a PRIMARY KEY when creating a table than a hidden unique index is used as the clustered index.  Due to a mutex lock on the data dictionary, I've read that these hidden indexes can cause contention.</p>
  
  <p>My question is - if I add a PRIMARY KEY to a table, what happens to the hidden PK?</p>