Why do my queries need to be covered by indexes, and how do I include them?
If you run a query that filters on multiple entity properties, or orders your entities in a descending order, you will need an index for that query. You must have an index for every query of that kind that you run with your application. The datastore index for a query maintains and updates a list of keys sorted in the manner that the query specifies to allow speedy access to the data in your datastore. A full explanation of datastore indexes can be found in our documentation.
When you develop your application with Google App Engine SDK, every query you run automatically gets indexed when necessary. If you thoroughly test your application before uploading it to your website, all of the indexes your application will need will be included in your application's index.yaml file. You may manually add indexes to index.yaml if you find a query that was not covered by your development testing. Our documentation includes information on how to write indexes for your application.
How to backup your datastore:
http://groups.google.com/group/google-appengine/browse_thread/thread/1df762e80cbd9890?hl=en
http://aralbalkan.com/1784
