title: Does MySQL create an extra index for primary key or uses the data itself as an "index"
body: <p>Cant find a explicit answer of that.
  I know that when you create a primary key, MySQL orders the data according to that primary key, question is, does it actually create another index, or uses the actual data as an index since it should be ordered by the primary key?</p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>if I have a table with has index A and index B and no primary key, I have the data + index A + index B. If I change the table to have columns of index A as the primary key, I will only have data (which is also used as an index) + index B right? The above is in terms of memory usage</p>