title: Google App Engine index costs
body: <p>From what I've understood, App Engine indexes are costly both in terms of increased overall storage size and by slowing down your writes.</p>
  
  <p>But do indexes only cost when they're actually used in queries and are explicitly defined in index.yaml? Or do properties such as StringProperty cost more than their non-indexed counterpart (e.g. TextProperty) simply by existing, even though they're not used in index.yaml?</p>