1 entity delete = 2 Writes + 2 Writes per indexed property value + 1 Write per composite index value
All from this page: http://code.google.com/appengine/docs/billing.html#Billable_... And more about why it is so: http://code.google.com/appengine/articles/life_of_write.html
Well, the OP is just another coder who can't read docs, but can write a blog.
The correct behavior would be to recalculate the indices just once, instead of reindexing after every single delete operation.
It then becomes
2*entities + 2*indexed property values + composite index values
operations to delete all entities in the datastore, instead of 2*entities + 2*entities*indexed property values + entities*composite index values
operations.If you create your own bulk delete method, you will find that it takes exactly as many write ops as the admin console tool.
You probably have defined more indexes on your entities than you need to - you will likely be able to make your app cheaper by removing unnecessary indexes. Managing indexes carefully is a critical part of making apps affordable on GAE.
Also, note that the deletions were through the "Datasore Admin" app, which was recently added. It is different from the classic Datastore Viewer.
There are two kinds of indexes:
* multi-property indexes which you configure via datastore-indexes.xml (or yaml). You can remove these by removing them from the xml/yaml and vacuuming.
* single-property indexes, which you decide when you define your data model. You can't vacuum these, and they are defined on a per-entity basis. The only way to make them go away is to re-save the relevant entities without the index defined. Note: multiproperty indexes require single-property indexes on all the properties covered.
These single-property indexes are almost certainly causing your high write op counts. You really should examine your data model with this new understanding; by removing unnecessary single-property indexes, you may be able to dramatically reduce your bill.
One could argue it is a bug in GAE that allows developers to make an expensive mistake when they don't fully understand how something (fairly complicated) works.
Someone else could argue that we are all developers and we should know the costs associated with the systems we are building. There is a real cost associated with PaaS systems like GAE.
What do you think?
And with regards to your script, you can't just delete 3k keys in one request. If you want I'll send you the script I've adapted for jobs that make large changes to the datastore.
I can't remember the exact number but it was about 10 times less than deleting via admin interface and finish in 5 minutes rather than 3 hours.
Why don't you try filing a bug report/suggesting a warning and send an email requesting something of a refund. They tend to be a friendly bunch who give refunds to obvious problems.
Moving to AWS will of course save you lots of money in the longer term, depending on what your hosting requirements are.