Google App Engine, Does empty properties in datastore entities cost money

Google App Engine, Does empty properties in datastore entities cost money??

In 'Datastore Statistics' tab of my app's admin page

i get 'NULL 56 Bytes '

i guessed from the following docs that the empty properties will not use memory

"a given row in a Bigtable table may have different columns than any other row before it. In this sense, a Bigtable functions as a large, sorted, multi-dimensional array with each row having a collection of columns, which may or may not be the same set of columns of other rows in the same table."

http://code.google.com/appengine/articles/storage_bre开发者_StackOverflow社区akdown.html

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Empty properties have to be stored as well, because the key exists, even if the value does not. There is a difference between an empty property and a non-existing property.