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In Prioritizing Clauses, we explained how you could use the
parameter at search time to give one query clause more importance than
another. For instance:
A query clause without a
Query-time boosting is the main tool that you can use to tune relevance. Any
type of query accepts a
boost parameter. Setting a
simply double the final
_score; the actual boost value that is applied
goes through normalization and some internal optimization. However, it does
imply that a clause with a boost of
2 is twice as important as a clause with
a boost of
Practically, there is no simple formula for deciding on the “correct” boost
value for a particular query clause. It’s a matter of try-it-and-see.
boost is just one of the factors involved in the relevance
score; it has to compete with the other factors. For instance, in the preceding
title field will probably already have a “natural” boost over
content field thanks to the field-length norm (titles
are usually shorter than the related content), so don’t blindly boost fields
just because you think they should be boosted. Apply a boost and check the
results. Change the boost and check again.
When searching across multiple indices, you can boost an entire index over
the others with the
indices_boost parameter. This could be used, as in the
next example, to give more weight to documents from a more recent index:
These boost values are represented in the Lucene’s Practical Scoring Function by
t.getBoost() element. Boosts are not applied at the level that they
appear in the query DSL. Instead, any boost values are combined and passed
down to the individual terms. The
t.getBoost() method returns any
value applied to the term itself or to any of the queries higher up the chain.
In fact, reading the
explain output is a little more complex
than that. You won’t see the
boost value or
t.getBoost() mentioned in the
explanation at all. Instead, the boost is rolled into the
queryNorm that is applied to a particular term. Although we said that
queryNorm is the same for every term, you will see that the
for a boosted term is higher than the
queryNorm for an unboosted term.