How to use Lucene FastVectorHighlighter on multiple fields?

Question

I've got a basic search working, and I'm highlighting using FastVectorHighlighter. When you ask the highlighter for a "best fragment" you have a few overloads of getBestFragment(s) to choose from, documented here. I'm now using the simplest one, like this:

highlightedText = highlighter.getBestFragment(fieldQuery, searcher.getIndexReader(), 
     scoreDoc.doc, "description", 100)

So I'm highlighting the match from the "description" field. My query however searches another field, "notes". How do I include that in the highlighting? There is an overload that takes a Set<String> matchedFields and one String storedField, but I don't understand the docs. The doc for the method says:

it is advisable that all matchedFields share the same source as storedField or are at least a prefix of it.

What does that mean? How do I index the "notes" and "description" Strings, and what do I pass for matchedFields and storedField?

Answer 1

That call, I believe, is intended to highlight against multiple indexed forms of the same content. That is, if you have one stored full-text content field, but you have indexed it in a number of different ways to expand how you can search it. Perhaps you have one indexed field that uses standard analysis, another with language-specific stemming, another that uses ngrams, and another indexing metaphones.

If you want to highlight two different stored fields, two calls to getBestFragment would be called for. Or you could use a different highlighter that allows multiple stored fields to be highlighted at the same time, PostingsHighlighter, for instance.