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.
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Developer Programs Engineer, Google App Engine
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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
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