The issue is this. Imagine you have two fields, title and document, both of which you want to search with simple queries like: albino elephant. There are two general approaches, either a) create a combined field that concatenates the two individual fields, or b) expand the simple query into a BooleanQuery that searches for each term in both fields.
With approach a), you lose the flexibility to set separate boost factors on the individual fields. I wanted title to be much more important than description for ranking results, and wanted to control this explicitly, as length norm was not always doing the right thing; e.g., descriptions are not always long.
With approach b) you run into another problem. Suppose the example query is expanded into (title:albino description:albino title:elephant description:elephant). Then, assuming tf/idf doesn't affect ranking, a document with albino in both title and description will score the same as a document with albino in title and elephant in description. The latter document for most applications is much better since it matches both query terms. If albino is the more important term according to idf, then the less desirable documents (albino in both fields) will rank consistently ahead of the albino elephants (which is what was happening to me, yielding horrible results).
Another way to handle this would be to generate a query like:
title:(albino elephant) description(albino elephant)
In this case the coord factor would boost titles and descriptions which contained both terms. You may or may not want to disable the coord factor for the outer query, which can be done with:
BooleanQuery title = new BooleanQuery();
title.add(new TermQuery(new Term("title", "albino")), false, false);
title.add(new TermQuery(new Term("title", "elephant")), false, false);BooleanQuery desc = new BooleanQuery();
desc.add(new TermQuery(new Term("desc", "albino")), false, false);
desc.add(new TermQuery(new Term("desc", "elephant")), false, false);BooleanQuery outer = new BooleanQuery() {
public getSimilarity() {
new DefaultSimilarity() {
public coord(int overlap, int length) { return 1.0f; }
}
}
};
outer.add(title, false, false);
outer.add(desc, false, false);In general, doesn't coord() handle this situation?
Also, you can separately boost title and desc here, if you like:
title:(albino elephant)^4.0 description(albino elephant)
or
title.boost(4.0f);
Doug
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