The cheapest way is to use the value you are checking for as a datastore key, then to do a get by key. A query always results in an index operation followed by a fetch by key, so you are reducing the need for an index lookup.
-- Ikai Lan Developer Programs Engineer, Google App Engine Blogger: http://googleappengine.blogspot.com Reddit: http://www.reddit.com/r/appengine Twitter: http://twitter.com/app_engine On Tue, Oct 26, 2010 at 10:02 AM, Dan Dubois <uvico...@gmail.com> wrote: > What is the absolute cheapest way (CPU cost) theoretically to run an > SQL EXISTS query with the Datastore API? > > I imagine FetchOptions would be > FetchOptions.Builder.withChunkSize(1).limit(1).prefetchSize(1) and > probably option 1 below. > > Query query = new Query("KIND"); > //add filters and sorts here, if querying by key use > query.setKeysOnly() > DatastoreService datastoreService = > DatastoreServiceFactory.getDatastoreService(); > PreparedQuery pq = datastoreService.prepare(query); > > //Option 1 > long count = > > pq.countEntities(FetchOptions.Builder.withChunkSize(1).limit(1).prefetchSize(1)); > > if (count == 0) { > //does not exist > } > else { > //exists > } > > // or maybe > //Option 2 > if (pq.asIterator().hasNext()) { > //exists > } > else { > //does not exist > } > > Is there anything cheaper? > > Best wishes, > Dan > > -- > You received this message because you are subscribed to the Google Groups > "Google App Engine for Java" group. > To post to this group, send email to > google-appengine-j...@googlegroups.com. > To unsubscribe from this group, send email to > google-appengine-java+unsubscr...@googlegroups.com<google-appengine-java%2bunsubscr...@googlegroups.com> > . > For more options, visit this group at > http://groups.google.com/group/google-appengine-java?hl=en. > > -- You received this message because you are subscribed to the Google Groups "Google App Engine for Java" group. To post to this group, send email to google-appengine-j...@googlegroups.com. To unsubscribe from this group, send email to google-appengine-java+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine-java?hl=en.
