Query DSL

Elasticsearch provides a full Query DSL (Domain Specific Language) based on JSON to define queries. Think of the Query DSL as an AST (Abstract Syntax Tree) of queries, consisting of two types of clauses:

Leaf query clauses: Leaf query clauses look for a particular value in a particular field, such as the match, term or range queries. These queries can be used by themselves.
Compound query clauses: Compound query clauses wrap other leaf or compound queries and are used to combine multiple queries in a logical fashion (such as the bool or dis_max query), or to alter their behaviour (such as the constant_score query).

Query clauses behave differently depending on whether they are used in query context or filter context.

Allow expensive queries

Certain types of queries will generally execute slowly due to the way they are implemented, which can affect the stability of the cluster. Those queries can be categorised as follows:

Queries that need to do linear scans to identify matches:
- script queries
- queries on numeric, date, boolean, ip, geo_point or keyword fields that are not indexed but have doc values enabled
Queries that have a high up-front cost:
- fuzzy queries (except on wildcard fields)
- regexp queries (except on wildcard fields)
- prefix queries (except on wildcard fields or those without index_prefixes)
- wildcard queries (except on wildcard fields)
- range queries on text and keyword fields
Joining queries
Queries that may have a high per-document cost:
- script_score queries
- percolate queries

The execution of such queries can be prevented by setting the value of the search.allow_expensive_queries setting to false (defaults to true).

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