title: Why does MySQL use a temporary table to drop a primary key?
body: <p>When using the command:</p>
  
  <pre><code>  ALTER TABLE my_table DROP PRIMARY KEY;
  </code></pre>
  
  <p>The state (when SHOW PROCESSLIST) appears as:</p>
  
  <pre><code>  copy to tmp table
  </code></pre>
  
  <p>Why would it need to use a tmp table to "drop" a primary key constraint?</p>