Since Lucene 4.0, there is a different terms dictionary per field, so your postings lists for abstract and body will be stored separately.
Apart from that I wonder if there is some support for a feature like searching in all/any field. If implemented like i assume it to be, this should be troublesome or require redundancy by keeping lists like above as well. Implementing fields by means of subranges of full wordlists could certainly perform better.
No sure to understand what you mean by "means of subranges or full wordlists", but if you run a BooleanQuery on several fields, Lucene will merge the postings lists on the fly.
Would be great to know what Lucene actually does.
The source code is freely accessible. :-)