In Prioritizing Clauses, we explained 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"
}
}
]
}
}
}
-
The
title
query clause is twice as important as thecontent
query clause, because it has been boosted by a factor of2
. -
A query clause without a
boost
value has a neutral boost of1
.
Query-time boosting is the main tool that you can use to tune relevance. Any
type of query accepts a boost
parameter. 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
natural'' boost over
the 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 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:
GET /docs_2014_*/_search (1)
{
"indices_boost": { (2)
"docs_2014_10": 3,
"docs_2014_09": 2
},
"query": {
"match": {
"text": "quick brown fox"
}
}
}
-
This multi-index search covers all indices beginning with
docs_2014_
. -
Documents in the
docs_2014_10
index will be boosted by3
, those indocs_2014_09
by2
, and any other matching indices will have a neutral boost of1
.
These boost values are represented in the [practical-scoring-function] by
the 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 boost
value applied to the term itself or to any of the queries higher up the chain.
Tip
|
In fact, reading the |