title: How to retrieve only the PKs of a table
body: <p>I'm working with Java Apache Cayenne, under a MySQL DB.
  I have quite a large table with a single bigint PK and some fields.
  I'd like to retrieve just only the PK values and not all the object that maps this entity, as it would be too resource-consuming.</p>
  
  <p>Is there a snippet that I can use, instead of this one that retrieves all the objects?</p>
  
  <pre><code>    ObjectContext context = ... 
      SelectQuery select = new SelectQuery(MyClass.class);
      List&lt;MyClass&gt; result = context.performQuery(select);
  </code></pre>