title: Why does MySQL Workbench create two indexes on a n:m relation?
body: <p>I've been wondering this for a while now. When you use MySQL Workbench to create a n:m relation between two tables (let's say A and B) the resulting table (let's say A_B) has three indexes (PRIMARY, one on A's primary key, one on B's primary key).</p>
  
  <p>According to <a href="https://stackoverflow.com/questions/1071180/is-the-primary-key-automatically-indexed-in-mysql"><strong>this question</strong></a> PRIMARY KEY is always indexed by itself, so why would there be the need to index each singular field again with a singular index ?
  Also if this is MySQL's policy, is it different with other DBMSs ?</p>