How many Datastore reads consume each Fetch, Count and Query operations?

Question

I\'m reading on Google App Engine groups many users (Fig1, Fig2, Fig3) that can\'t figure out where the high number of Datastore reads in their billing reports come from.

Answer 1

See http://code.google.com/appengine/docs/billing.html#Billable_Resource_Unit_Cost . A query costs you 1 read plus 1 read for each entity returned. "Returned" includes entities skipped by offset or count. So that is 1001 reads for each of these:

Example.all(keys_only = True).filter('bars=','spam').count() 
Example.all().count(1000)
Example.all().fetch(1000)
Example.all().fetch(1000, offset=500)

For these, the number of reads charged is 1 plus the number of entities that match the filters:

Example.all().filter('bars=','spam').filter('bars=','fu').fetch()
Example.all().filter('foo>=', filtr).filter('foo<', filtr+ u'\ufffd').fetch()

Instead of using count you should consider storing the count in the datastore, sharded if you need to update the count more than once a second. http://code.google.com/appengine/articles/sharding_counters.html

Whenever possible you should use cursors instead of an offset.

Answer 2

Just to make sure:

I'm almost sure:

Example.all().count(10000)

This one uses small datastore operations (no need to fetch the entities, only keys), so this would count as 1 read + 10,000 (max) small operations.