title: Objectify - how efficient is a key-only query filtered by ancestor?
body: <p>I am trying to get all the children of a given parent entity efficiently. The only way to do that now is to use a query in objectify, which is not efficient because it bypasses the cache. Objectify-4 adds hybrid queries, which you can mimic in Objectify 3.x by issuing a key only query and then doing a batch get on those keys.</p>
  
  <p>My question is how efficient is a key-only query that filters using an ancestor? Something like:</p>
  
  <pre><code>ofy.query(Car.class).ancestor(someKey).fetchKeys();
  </code></pre>
  
  <p>I don't want to get all keys for all "Cars" here, I only want "Cars" that belong to a specific parent.  </p>