[[query-time-boosting]] === Query-Time Boosting
In <<prioritising-clauses,Prioritizing Clauses>>, we explained ((("relevance", "controlling", "query time boosting")))((("boosting", "query-time")))how you could use the boost
parameter at search time to give one query clause more importance than
another. For instance:
GET /_search { "query": { "bool": { "should": [ { "match": { "title": { "query": "quick brown fox", "boost": 2 <1> } } }, { "match": { <2> "content": "quick brown fox" } } ] } } }
<1> The title query clause is twice as important as the content query
clause, because it has been boosted by a factor of 2.
<2> A query clause without a boost value has a neutral boost of 1.
Query-time boosting is the main tool that you can use to tune relevance. Any
type of query accepts a boost parameter.((("boost parameter", "setting value"))) Setting a boost of 2 doesn't
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 1.
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. Remember that `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 example, the `title` field will probably already have a natural'' boost over
the content field thanks ((("field-length norm")))to the <<field-norm,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.
==== Boosting an Index
When searching across multiple indices, you((("boosting", "query-time", "boosting an index")))((("indices", "boosting an index"))) can boost an entire index over
the others with the indices_boost parameter.((("indices_boost parameter"))) This could be used, as in the
next example, to give more weight to documents from a more recent index:
GET /docs_2014_*/_search <1> { "indices_boost": { <2> "docs_2014_10": 3, "docs_2014_09": 2 }, "query": { "match": { "text": "quick brown fox" } } }
<1> This multi-index search covers all indices beginning with
docs_2014_.
<2> Documents in the docs_2014_10 index will be boosted by 3, those
in docs_2014_09 by 2, and any other matching indices will have
a neutral boost of 1.
==== t.getBoost()
These boost values are represented in the <> by
the t.getBoost() element.((("practical scoring function", "t.getBoost() method")))((("boosting", "query-time", "t.getBoost()")))((("t.getBoost() method"))) Boosts are not applied at the level that they
appear in the query DSL. Instead, any boost values are combined and passsed
down to the individual terms. The t.getBoost() method returns any boost
value applied to the term itself or to any of the queries higher up the chain.
In fact, reading the <<explain,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
<<query-norm,queryNorm>> that is applied to a particular term. Although we said that
the queryNorm is the same for every term, you will see that the queryNorm
for a boosted term is higher than the queryNorm for an unboosted term.
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