title: Why does Google App Engine restrict GQL queries?
body: <p>I was reading about <a href="http://en.wikipedia.org/wiki/App_Engine" rel="noreferrer">App Engine</a> on wikipedia and came across some GQL restrictions:</p>
  
  <ul>
  <li><p>JOIN is not supported</p></li>
  <li><p>can SELECT from at most one table at a time</p></li>
  <li><p>can put at most 1 column in the WHERE clause</p></li>
  </ul>
  
  <p>What are the advantages of these restrictions?</p>
  
  <p>Are these restrictions common in other places where scalability is a priority?</p>