Fastest way to check if an object exists

I'm using GAE/Java with Objectify, and I'm trying to find the fastest way to check if a given object exists in the datastore, given the key. W开发者_开发百科hat I'm doing right now is .get(key) with @Cached on, but either way that still retrieves the entire object, which is unnecessary.

Any ideas on how to do this with an index only hit? I was also thinking of a keys only query, but I'm seeing (on the system status dashboard ) that the latency is much more than a get.

Just perform a get() with the cache turned on. Unless you have a lot of expensive logic in a @PostLoad method, it should be significantly cheaper to fetch the data out of memcache than it is to go all the way to the datastore for even a keys-only query. The cache is your friend.

As a side note, this sounds like premature optimization. Build your app using the most convenient code, then run appstats and find out where the actual costs are in your application. You'll probably find that the expensive parts are not what you think.

Any ideas on how to do this with an index only hit? I was also thinking of a keys only query

A keys-only query is the only way to get an index-only hit. Whether it's faster than a get depends on the size of your entity and the size of your index. In a trivial example, I'm getting around 8ms for a get and 13ms for a query. You can use AppStats to figure out which is cheaper for you with real data.

A keys only query with a filter on __key__ will be substantially faster than the queries benchmarked on the status dashboard. Whether or not it's faster than simply fetching the entity I'm not sure - try it and let us know!

You can try a keys only query from Objectify like this:

public static boolean objectExists(String id, Class<?> objectClass) {
  return OfyService.ofy().load().filterKey(Key.create(objectClass, id)).keys().first() != null;
}