Google Datastore Emulator

For a test, we start a local instance of the cloud-datastore-emulator with the LocalDatastoreHelper class, provided by Google.

The interesting obersvation, we made was, that we can insert data with our code and then find it again, by performing a GQL query

SELECT [..] WHERE myfield = true

if we do it against a "live" store, hosted at Google Cloud.

But:

When we run the same code against the emulator running locally, the insert works, but the query does not. A findAll() works fine, so it looks like inserting and reading works generally, but somehow the indexes are missing?

After reading the documentation for hours now, I did not find any hint, why this might happen.

Can anybody help?