Second thing after @FetchGroup I've found is read-only hint for EM. You can specify this hint to EM when executing query meaning that returned data will never be edited or persisted. My understanding of this is that each entity returned from query is in detached state as default state.

query.setHint("", "true");

This could be useful in situation when you are sure, that you won't modify those entities - like listing entities, paging, logging, reporting.