title: I've indexed a combined column, should I index separate column too?
body: <p>I have a table like this</p>
  
  <pre><code>CREATE TABLE `tbl` (
      `id` int(10) unsigned NOT NULL,
      `pid` int(10) unsigned NOT NULL,
      `cid` int(10) unsigned NOT NULL,
      `name` int(11) DEFAULT NULL,
      `desc` varchar(500),
      `deleted` tinyint(1) unsigned NOT NULL DEFAULT 0,
      `createdAt` datetime DEFAULT NULL,
      PRIMARY KEY `pid_cid` (`pid`,`cid`)
      ) ENGINE=InnoDB 
  </code></pre>
  
  <p><code>WHERE</code> clause of select queries are those:</p>
  
  <pre><code>where pid=pid [and deleted = 1];
  where cid=cid [and deleted = 1];
  where pid=pid and cid=cid [and deleted = 1];
  </code></pre>
  
  <ol>
  <li>Should I index <code>pid</code> and <code>cid</code> separately?</li>
  <li>Should I index <code>deleted</code>?</li>
  </ol>