title: MySQL 5.6 - table locks even when ALGORITHM=inplace is used
body: <p>I'm running the following <code>ALTER</code> command on a MySQL 5.6 database on a large table with 60 million rows:</p>
  
  <pre><code>ALTER TABLE `large_table` ADD COLUMN `note` longtext NULL, 
  ALGORITHM=INPLACE, LOCK=NONE;
  </code></pre>
  
  <p>Despite specifying both <code>ALGORITHM=INPLACE</code> and <code>LOCK=NONE</code>, the table gets locked and essentially takes down the app until the migration is complete.</p>
  
  <p>I verified that the table was indeed locked by checking the value of the <code>In_use</code> column on the output of the <code>SHOW OPEN TABLES</code> command. It was set to <code>1</code>.</p>
  
  <p>From what I gather in the MySQL documentation, this operation should not be locking the table. And, MySQL is supposed to fail the command if it is not able to proceed without a lock. I upgraded the database to MySQL 5.7 to see if it's any better, but I face the same problem on 5.7 too.</p>
  
  <p>Is this an expected behavior? How do I find out what's going wrong here?</p>