Lucene: Score calculation with a PrefixQuery

Question

I have a problem with the score calculation with a PrefixQuery. To change score of each document, when add document into index, I have used setBoost to change the boost of the document. Then I create PrefixQuery to search, but the result have not been changed according to the boost. It seems setBoost totally doesn't work for a PrefixQuery. Please check my code below:

 @Test
 public void testNormsDocBoost() throws Exception {
    Directory dir = new RAMDirectory();
    IndexWriter writer = new IndexWriter(dir, new StandardAnalyzer(Version.LUCENE_CURRENT), true,
            IndexWriter.MaxFieldLength.LIMITED);
    Document doc1 = new Document();
    Field f1 = new Field("contents", "common1", Field.Store.YES, Field.Index.ANALYZED);
    doc1.add(f1);
    doc1.setBoost(100);
    writer.addDocument(doc1);
    Document doc2 = new Document();
    Field f2 = new Field("contents", "common2", Field.Store.YES, Field.Index.ANALYZED);
    doc2.add(f2);
    doc2.setBoost(200);
    writer.addDocument(doc2);
    Document doc3 = new Document();
    Field f3 = new Field("contents", "common3", Field.Store.YES, Field.Index.ANALYZED);
    doc3.add(f3);
    doc3.setBoost(300);
    writer.addDocument(doc3);
    writer.close();

    IndexReader reader = IndexReader.open(dir);
    IndexSearcher searcher = new IndexSearcher(reader);

    TopDocs docs = searcher.search(new PrefixQuery(new Term("contents", "common")), 10);
    for (ScoreDoc doc : docs.scoreDocs) {
        System.out.println("docid : " + doc.doc + " score : " + doc.score + " "
                + searcher.doc(doc.doc).get("contents"));
    }
} 

The output is :

 docid : 0 score : 1.0 common1
 docid : 1 score : 1.0 common2
 docid : 2 score : 1.0 common3

Answer 1

By default, PrefixQuery rewrites the query to use ConstantScoreQuery, which gives every single matching document a score of 1.0. I think this is to make PrefixQuery faster. So your boosts are getting ignored.

If you want the boosts to take effect in your PrefixQuery, you need to call setRewriteMethod(), using the SCORING_BOOLEAN_QUERY_REWRITE constant on your prefix query instance. See http://lucene.apache.org/java/2_9_1/api/all/index.html .

For debugging, you can use searcher.explain().