elasticsearch/docs/reference/query-dsl/multi-term-rewrite.asciidoc

[[query-dsl-multi-term-rewrite]]
== `rewrite` parameter

WARNING: This parameter is for expert users only. Changing the value of
this parameter can impact search performance and relevance.

{es} uses https://lucene.apache.org/core/[Apache Lucene] internally to power
indexing and searching. In their original form, Lucene cannot execute the
following queries:

* <<query-dsl-fuzzy-query, `fuzzy`>>
* <<query-dsl-prefix-query, `prefix`>>
* <<query-dsl-query-string-query, `query_string`>>
* <<query-dsl-regexp-query, `regexp`>>
* <<query-dsl-wildcard-query, `wildcard`>>

To execute them, Lucene changes these queries to a simpler form, such as a
<<query-dsl-bool-query, `bool` query>> or a
{wikipedia}/Bit_array[bit set].

The `rewrite` parameter determines:

* How Lucene calculates the relevance scores for each matching document
* Whether Lucene changes the original query to a `bool`
query or bit set
* If changed to a `bool` query, which `term` query clauses are included

[discrete]
[[rewrite-param-valid-values]]
=== Valid values

`constant_score` (Default)::
Uses the `constant_score_boolean` method for fewer matching terms. Otherwise,
this method finds all matching terms in sequence and returns matching documents
using a bit set.

`constant_score_boolean`::
Assigns each document a relevance score equal to the `boost`
parameter.
+
This method changes the original query to a <<query-dsl-bool-query, `bool`
query>>. This `bool` query contains a `should` clause and
<<query-dsl-term-query, `term` query>> for each matching term.
+
This method can cause the final `bool` query to exceed the clause limit in the
<<indices-query-bool-max-clause-count, `indices.query.bool.max_clause_count`>>
setting. If the query exceeds this limit, {es} returns an error.

`scoring_boolean`::
Calculates a relevance score for each matching document.
+
This method changes the original query to a <<query-dsl-bool-query, `bool`
query>>. This `bool` query contains a `should` clause and
<<query-dsl-term-query, `term` query>> for each matching term.
+
This method can cause the final `bool` query to exceed the clause limit in the
<<indices-query-bool-max-clause-count, `indices.query.bool.max_clause_count`>>
setting. If the query exceeds this limit, {es} returns an error.

`top_terms_blended_freqs_N`::
Calculates a relevance score for each matching document as if all terms had the
same frequency. This frequency is the maximum frequency of all matching terms.
+
This method changes the original query to a <<query-dsl-bool-query, `bool`
query>>. This `bool` query contains a `should` clause and
<<query-dsl-term-query, `term` query>> for each matching term.
+
The final `bool` query only includes `term` queries for the top `N` scoring
terms.
+
You can use this method to avoid exceeding the clause limit in the
<<indices-query-bool-max-clause-count, `indices.query.bool.max_clause_count`>>
setting.

`top_terms_boost_N`::
Assigns each matching document a relevance score equal to the `boost` parameter.
+
This method changes the original query to a <<query-dsl-bool-query, `bool`
query>>. This `bool` query contains a `should` clause and
<<query-dsl-term-query, `term` query>> for each matching term.
+
The final `bool` query only includes `term` queries for the top `N` terms.
+
You can use this method to avoid exceeding the clause limit in the
<<indices-query-bool-max-clause-count, `indices.query.bool.max_clause_count`>>
setting.

`top_terms_N`::
Calculates a relevance score for each matching document.
+
This method changes the original query to a <<query-dsl-bool-query, `bool`
query>>. This `bool` query contains a `should` clause and
<<query-dsl-term-query, `term` query>> for each matching term.
+
The final `bool` query
only includes `term` queries for the top `N` scoring terms.
+
You can use this method to avoid exceeding the clause limit in the
<<indices-query-bool-max-clause-count, `indices.query.bool.max_clause_count`>>
setting.

[discrete]
[[rewrite-param-perf-considerations]]
=== Performance considerations for the `rewrite` parameter
For most uses, we recommend using the `constant_score`,
`constant_score_boolean`, or `top_terms_boost_N` rewrite methods.

Other methods calculate relevance scores. These score calculations are often
expensive and do not improve query results.