Hi,

Could anybody help me out with understand keys-only query costs?

I've done the following tests:

Fetch 2 items using a regular query (fully entity fetched)
- (3 RPCs, cost=350, billed_ops=[DATASTORE_READ:5])
Fetch 2 items using a keys-only query, and then fetch the full item (cache 
miss)
- (9 RPCs, cost=370, billed_ops=[DATASTORE_READ:5, DATASTORE_SMALL:2])
Fetch 2 items using a keys-only query, and fetch the full entity from 
memcache (cache hit)
- (5 RPCs, cost=230, billed_ops=[DATASTORE_READ:3, DATASTORE_SMALL:2]) 

So even with full cache hit, a keys-only query still uses 3 full datastore 
reads?
I'm assuming keys-only queries should only be used when the hit chance is 
high? eg only use keys-only queries on data that is expected to have been 
accessed recently.

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