[[query-dsl-combined-fields-query]]
=== Combined fields
++++
<titleabbrev>Combined fields</titleabbrev>
++++

The `combined_fields` query supports searching multiple text fields as if their
contents had been indexed into one combined field. The query takes a term-centric
view of the input string: first it analyzes the query string into individual terms,
then looks for each term in any of the fields. This query is particularly
useful when a match could span multiple text fields, for example the `title`,
`abstract`, and `body` of an article:

[source,console]
----
GET /_search
{
  "query": {
    "combined_fields" : {
      "query":      "database systems",
      "fields":     [ "title", "abstract", "body"],
      "operator":   "and"
    }
  }
}
----

The `combined_fields` query takes a principled approach to scoring based on the
simple BM25F formula described in
http://www.staff.city.ac.uk/~sb317/papers/foundations_bm25_review.pdf[The Probabilistic Relevance Framework: BM25 and Beyond].
When scoring matches, the query combines term and collection statistics across
fields to score each match as if the specified fields had been indexed into a
single, combined field. This scoring is a best attempt; `combined_fields` makes
some approximations and scores will not obey the BM25F model perfectly.

:query-type: combined fields
// tag::max-clause-limit[]
[WARNING]
.Field number limit
===================================================
By default, there is a limit to the number of clauses a query can contain. This
limit is defined by the
<<indices-query-bool-max-clause-count,`indices.query.bool.max_clause_count`>>
setting, which defaults to `4096`. For {query-type} queries, the number of
clauses is calculated as the number of fields multiplied by the number of terms.
===================================================
// end::max-clause-limit[]
:query-type!:

==== Per-field boosting

Field boosts are interpreted according to the combined field model. For example,
if the `title` field has a boost of 2, the score is calculated as if each term
in the title appeared twice in the synthetic combined field.

[source,console]
----
GET /_search
{
  "query": {
    "combined_fields" : {
      "query" : "distributed consensus",
      "fields" : [ "title^2", "body" ] <1>
    }
  }
}
----
<1> Individual fields can be boosted with the caret (`^`) notation.

NOTE: The `combined_fields` query requires that field boosts are greater than
or equal to 1.0. Field boosts are allowed to be fractional.

[[combined-field-top-level-params]]
==== Top-level parameters for `combined_fields`

`fields`::
(Required, array of strings) List of fields to search. Field wildcard patterns
are allowed. Only <<text,`text`>> fields are supported, and they must all have
the same search <<analyzer,`analyzer`>>.

`query`::
+
--
(Required, string) Text to search for in the provided `<fields>`.

The `combined_fields` query <<analysis,analyzes>> the provided text before
performing a search.
--

`auto_generate_synonyms_phrase_query`::
+
--
(Optional, Boolean) If `true`, <<query-dsl-match-query-phrase,match phrase>>
queries are automatically created for multi-term synonyms. Defaults to `true`.

See <<query-dsl-match-query-synonyms,Use synonyms with match query>> for an
example.
--

`operator`::
+
--
(Optional, string) Boolean logic used to interpret text in the `query` value.
Valid values are:

`or` (Default)::
For example, a `query` value of `database systems` is interpreted as `database
OR systems`.

`and`::
For example, a `query` value of `database systems` is interpreted as `database
AND systems`.
--

`minimum_should_match`::
+
--
(Optional, string) Minimum number of clauses that must match for a document to
be returned. See the <<query-dsl-minimum-should-match, `minimum_should_match`
parameter>> for valid values and more information.
--

`zero_terms_query`::
+
--
(Optional, string) Indicates whether no documents are returned if the `analyzer`
removes all tokens, such as when using a `stop` filter. Valid values are:

`none` (Default)::
No documents are returned if the `analyzer` removes all tokens.

`all`::
Returns all documents, similar to a <<query-dsl-match-all-query,`match_all`>>
query.

See <<query-dsl-match-query-zero>> for an example.
--

===== Comparison to `multi_match` query

The `combined_fields` query provides a principled way of matching and scoring
across multiple <<text, `text`>> fields. To support this, it requires that all
fields have the same search <<analyzer,`analyzer`>>.

If you want a single query that handles fields of different types like
keywords or numbers, then the <<query-dsl-multi-match-query,`multi_match`>>
query may be a better fit. It supports both text and non-text fields, and
accepts text fields that do not share the same analyzer.

The main `multi_match` modes `best_fields` and `most_fields` take a
field-centric view of the query. In contrast, `combined_fields` is
term-centric: `operator` and `minimum_should_match` are applied per-term,
instead of per-field. Concretely, a query like

[source,console]
----
GET /_search
{
  "query": {
    "combined_fields" : {
      "query":      "database systems",
      "fields":     [ "title", "abstract"],
      "operator":   "and"
    }
  }
}
----

is executed as:

[source,txt]
----
+(combined("database", fields:["title" "abstract"]))
+(combined("systems", fields:["title", "abstract"]))
----

In other words, each term must be present in at least one field for a
document to match.

The `cross_fields` `multi_match` mode also takes a term-centric approach and
applies `operator` and `minimum_should_match per-term`. The main advantage of
`combined_fields` over `cross_fields` is its robust and interpretable approach
to scoring based on the BM25F algorithm.

[NOTE]
.Custom similarities
===================================================
The `combined_fields` query currently only supports the BM25 similarity,
which is the default unless a <<index-modules-similarity, custom similarity>>
is configured. <<similarity, Per-field similarities>> are also not allowed.
Using `combined_fields` in either of these cases will result in an error.
===================================================