title: Deadlock mechanism in MySql and the use of Index on top of Primary key
body: <p>I have beed reading quite a lot about he InnoDB engine internals and its locking mechanism (different locks and types of deadlocks)</p>
  <p>I've seen this error when doing update operation:</p>
  <pre><code>'Deadlock found when trying to get lock; try restarting transaction' 
  </code></pre>
  <p>when performing update operation and followed what is advised here to
  create <a href="https://stackoverflow.com/questions/2332768/how-to-avoid-mysql-deadlock-found-when-trying-to-get-lock-try-restarting-trans">an index on field called resource_id</a>.</p>
  <p>I wonder if there is a difference/ added value to avoid deadlocks in MySql DB, by adding new Index on a column that is <strong>already</strong> defined as PrimaryKey.</p>
  <p>Why MySql does not block you to add Index on the same column that is already primary key? this leads me to the understanding that there is  a difference.</p>