Google app engine: Cost and efficiency of retrieving entities using keys vs using queries

Question

I have read in various places that:

projection queries run faster and cost less than one that returns complete entities

because the query results are satisfied from the indexes rather than disk reads. See:Google docs here and Comment from Guido Van Rossum here.

The comments from Guido seem to imply that it is faster to retrieve an entity's details through a projection query than through a key.

However, Pg 135 of 'Programming GAE with Python' by Dan Sanderson mentions:

if you have a key...you can retrieve the entity quickly, much more quickly than with a datastore query.

Given a situation where i can retrieve my data using either a projection query or the entities' key (with get_multi):

From Guido answer from the part "in terms of fewer I/O operations) " I can understand that they are the same. With an exception of bandwidth as he said. About your second question you can check the bullets from the pricing doc. — alcohealism

Jim Morrison Jim Morrison · Accepted Answer · 2020-02-25T19:25:44

The advise is consistent. I'll try to clarify.

If you have a key, fetch an entity with that
If you don't have a key AND you can solve you problem with a projection, use that
If you don't have a key, need full entity(s), then use a query that returns full entities