title: AWS Aurora Adding Foreign Key Fails with Syntax Error
body: <p>The AWS Aurora docs indicate that it's MySQL 5.6 compliant.  </p>
  
  <p>I execute this: </p>
  
  <pre><code>    ALTER TABLE slave 
  ADD FOREIGN KEY (id(20)) 
       REFERENCES master(id);
  </code></pre>
  
  <p>It fails: </p>
  
  <blockquote>
    <p>ERROR 1215 (HY000): Cannot add foreign key constraint</p>
  </blockquote>
  
  <pre><code> SHOW ENGINE INNODB STATUS; 
  
      ------------------------
      LATEST FOREIGN KEY ERROR
      ------------------------
      2017-08-31 22:29:13 Error in foreign key constraint of table xxx:
      FOREIGN KEY (id(20)) REFERENCES master(id):
      Syntax error close to:
      (20)) REFERENCES master(id)
  </code></pre>
  
  <p>I verified that:</p>
  
  <ul>
  <li>The collation of <code>slave</code> is  <code>latin1_swedish_ci</code>.</li>
  <li>The collation of <code>master</code> is <code>latin1_swedish_ci</code>.</li>
  <li>The collation of <code>slave.id</code> is  <code>latin1_swedish_ci</code>.</li>
  <li>The collation of <code>master.id</code> is <code>latin1_swedish_ci</code>.</li>
  <li>The type of <code>slave.id</code> is <code>tinytext</code>.</li>
  <li>The type of <code>master.id</code> is <code>tinytext</code>.</li>
  <li><code>master.id</code> is the PRIMARY KEY on <code>master</code>.</li>
  <li>The engine is InnoDB.</li>
  </ul>
  
  <p>What is the syntax error?  My command seems to conform to the MySQL 5.6 documentation.</p>