Google Appengine Datastore Indexes

If you're adding a new index with a release, be careful as the index must be built before it can be used.

You can watch the index build progress in the Admin Console -> Data Store Index screen. When complete you can switch to your new deployment.

Or you'll get:-
com.google.appengine.api.datastore.DatastoreNeedIndexException: The index for this query is not ready to serve. See the Datastore Indexes page in the Admin Console.