title: Is there any performance gain in indexing a boolean field?
body: <p>I'm just about to write a query that includes a <code>WHERE isok=1</code>. As the name implies, <code>isok</code> is a boolean field (actually a <code>TINYINT(1) UNSIGNED</code> that is set to 0 or 1 as needed).</p>
  
  <p>Is there any performance gain in indexing this field? Would the engine (InnoDB in this case) perform better or worse looking up the index?</p>