Create a limited datastore index

I have a query in my GAE app that looks like this:

datastore.NewQuery("item").Ancestor(fk).Order("-PubDate").Limit(10).Run(c)

In order for this to work I need an index of items ordered by PubDate; the autogenerated one looks like:

- kind: item
  ancestor: yes
  properties:
  - name: PubDate
    direction: desc

This index is pretty big (about 4 GB) but most of it will never be touched because of that Limit() call. Is it possible to have the index only remember 10 results for each ancestor?

It is possible to have two entities of the same kind to have the same property, but one entity having this property indexed and the other unindexed.

The low-level Datastore API in Java runtime allows an app to decide whether to index or not each property for each individual entity. I don't know if an equivalent exists in other runtimes. If not, you can use two different property names to indicate an indexed date and unindexed date.

So technically, yes, you can keep only a small number of entities in the index. Note, however, that you will have to re-save an entity with a property unindexed in order to remove it from that index. Re-saving all entities will incur additional costs, so this solution probably makes sense if you re-save an entity anyway for any other reason.