it depends on how big your data is, but every query operation, every fetch, every write operation it charged. http://code.google.com/appengine/docs/billing.html has details on how many read/write datastore API calls are made per operation. if u have to write in a new column for thousands of entries, then it will cost a bit, $0.10/100k operations, one re-write has 7 operations + incoming/outgoing bandwidth cost + instance usage cost will apply, it is hard to estimate actual cost.
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it depends on how big your data is, but every query operation, every fetch, every write operation it charged. http://code.google.com/appengine/docs/billing.html has details on how many read/write datastore API calls are made per operation. if u have to write in a new column for thousands of entries, then it will cost a bit, $0.10/100k operations, one re-write has 7 operations + incoming/outgoing bandwidth cost + instance usage cost will apply, it is hard to estimate actual cost.