Google App Engine index costs

Question

From what I've understood, App Engine indexes are costly both in terms of increased overall storage size and by slowing down your writes.

But do indexes only cost when they're actually used in queries and are explicitly defined in index.yaml? Or do properties such as StringProperty cost more than their non-indexed counterpart (e.g. TextProperty) simply by existing, even though they're not used in index.yaml?

Answer 1

You could also set indexed=False for properties which you don't want indexed:

http://code.google.com/appengine/docs/python/datastore/propertyclass.html#Property