What Happens When Multiple Fields with the Same Name have Different Boosts?

Question

Boost is by-Field in Lucene, and that is then set in the Index. You can add multiple Fields with the Name property set to the same value, and they'll then be searched for that Document as though you just added the union of those Terms to that Document under that Field name.

var field1 = new Field("Text", "aquaculture");
var field2 = new Field("Text", "fish");
field1.setBoost(1.0f);
field2.setBoost(2.0f);
var doc = new Document();
doc.AddField(field1);
doc.AddField(field2);

But you can set the Boost to multiple values as you do so - if you do, what happens? Do the Terms in each Field get set to the separate Boost levels, or does one shared Boost level get used? If one, is it Last-In, First-In, random?

(The above code is pseudocode just to illustrate the Fields getting added to a single Doc to help coders visualize it, I realize the actual code to do the above is a bit different in actual Lucene API)

femtoRgon femtoRgon · Accepted Answer · 2018-12-18T10:03:30

For the current version of Lucene, this question is no longer relevant. Setting boosts on fields was deprecated in 6.5, and no longer supported in 7.0. See LUCENE-6819 for discussion. The replacement, per the migration guide:

...index-time scoring factors should be indexed in a doc value field and combined with the score at query time using FunctionScoreQuery for instance.

In previous versions, if multiple boosts added to the same field, they are multiplied. This specified by the FieldInvertState, which is the information that gets passed to the Similarity, which performs the translation of that data into what gets stored in the index.

This is the cumulative product of document boost and field boost for all field instances sharing the same field name.

What Happens When Multiple Fields with the Same Name have Different Boosts?

1 Answers