how do I use jdo deletepersistenceall

Check the paragraph below on this page:
http://code.google.com/appengine/docs/java/datastore/jdo/queries.html#Delete_By_Query

They are saying that INSTEAD OF execute they do deletePersistentAll - not after it


Deleting Entities By Query

If you are issuing a query with the goal of deleting all the entities that match the query filter, you can save yourself a bit of coding by using JDO's "delete by query" feature. The following deletes all People over a given height:

    Query query = pm.newQuery(Person.class);
    query.setFilter("height > maxHeightParam");
    query.declareParameters("int maxHeightParam");
    query.deletePersistentAll(maxHeight);

You'll notice that the only difference here is that instead of calling query.execute(...) we are instead calling query.deletePersistentAll(...). All of the rules and restrictions relating to filters, sort orders, and indexes explained above apply to queries whether you are selecting or deleting the result set. Note, however, that just as if you had deleted these Person entities with pm.deletePersistent(...), any dependent children of the entities deleted by the query will also be deleted. For more information on dependent children please see Dependent Children and Cascading Deletes.