Clarifying projection query cost?

[Alex Kerr]

Hi,

Can anyone clarify the cost of projection queries please? (both in operations and financially)

Datastore pricing page at: https://cloud.google.com/datastore/docs/pricing says a regular query (is that "SELECT * " ?) is 1 read + 1 read per entity retrieved. I get that, but then projection queries are just "1 read" - which implies a cost of 1 read total regardless of the number of results returned and the number of properties projected - that can't be right?

Also I note the new pricing section from July but still not clear on projection query cost!

Many thanks

Alex

[Nick (Cloud Platform Support)]

Hi Alex,

I believe this old App Engine Groups thread answers your question adequately. Let me know if you have any questions after reading.

Cheers,

Nick
Cloud Platform Community Support 

[Alex Kerr]

Thanks Nick. So can you confirm costs are:

1.) For a projection query, cost is a single read for the entire query regardless of number of results returned (and same under the new Datastore pricing)

2.) For a projection query with distinct option, cost is 1 read + 1 read per entity retrieved

3.) For normal queries ("SELECT * FROM ... " in GQL) under the new (from July) Datastore pricing, cost is 1 read per entity

Are there drawbacks to the suggestion made by the poster of that link you specified, in achieving a "SELECT * " at projection query cost, by adding in a dummy property to the entity kind and then doing a projection query on the remaining properties? (I suppose the obvious drawback being you'd need an index for every property)

Thanks,

Alex

[Nick (Cloud Platform Support)]

Hey Alex,

As described in the other thread, the cost of maintaining indexes matters here, and there are other restrictions described in the documentation. I hope this helps clarify the trade-offs for you.

Regards,

Nick
Cloud Platform Community Support