Field ids are not present in Avro data except in the schema. A record's fields are serialized in the order that the fields occur in the records schema, with no per-field annotations whatsoever. For example, a record that contains a string and an int is serialized simply as a string followed by an int, nothing before, nothing between and nothing after. So, yes, it is a different data format.
So you can't serialize nulls? It also seems like this would make forward/backward compatibility a little more complex. Thrift solves this problem by using tags to indicate what kind of field you're working with.
-Bryan
