AppEngine: weird WriteOperation count when deleting

Question

I'm trying to delete 300 000 entities in GAE (java), using this:

 PersistenceManager pm = PMF.get().getPersistenceManager();

 Date date70DaysAgo = Calendar.getInstance().getTime();
 date70DaysAgo.setDate(date70DaysAgo.getDate()-70);

 Query query = pm.newQuery(PositionApplication.class);
 query.setFilter("date <= yyy");
 query.declareParameters("java.util.Date yyy");
 query.setRange(0,750);
 Collection<PositionApplication> elements = (Collection<PositionApplication>)query.execute(date70DaysAgo);

 pm.deletePersistentAll(elements);

I'm limited to 50 000 write ops a day (free quota). I naively thought that in 6-7 days it should be done, but it appears that 1 call uses 1/5 of the daily quota.

--> Datastore Write Operations 21%      0.01 of 0.05 Million Ops

10 000 write ops, why not 750?

Is it related to indexes?

Answer 1

When you delete an entity, the datastore also has to delete the per-property indexes and any entries in the composite indexes.

The calculation for the deletion of each entity is:

2 Writes + 2 Writes per indexed property value + 1 Write per composite index value

See the Billing and Budgeting Resources document for the calculations of all datastore operations.