elasticsearch provides a full Query DSL based on JSON to define queries. In general, there are basic queries such as term or prefix. There are also compound queries like the bool query. Queries can also have filters associated with them such as the filtered or constant_score queries, with specific filter queries.
Think of the Query DSL as an AST of queries. Certain queries can contain other queries (like the bool query), other can contain filters (like the constant_score), and some can contain both a query and a filter (like the filtered). Each of those can contain any query of the list of queries or any filter from the list of filters, resulting in the ability to build quite complex (and interesting) queries.
Both queries and filters can be used in different APIs. For example, within a search query, or as a facet filter. This section explains the components (queries and filters) that can form the AST one can use.
Filters are very handy since they perform an order of magnitude better than plain queries since no scoring is performed and they are automatically cached.
Filters and Caching
Filters can be a great candidate for caching. Caching the result of a filter does not require a lot of memory, and will cause other queries executing against the same filter (same parameters) to be blazingly fast.
Some filters already produce a result that is easily cacheable, and the difference between caching and not caching them is the act of placing the result in the cache or not. These filters, which include the term, terms, prefix, and range filters, are by default cached and are recommended to use (compared to the equivalent query version) when the same filter (same parameters) will be used across multiple different queries (for example, a range filter with age higher than 10).
Other filters, usually already working with the field data loaded into memory, are not cached by default. Those filters are already very fast, and the process of caching them requires extra processing in order to allow the filter result to be used with different queries than the one executed. These filters, including the geo, numeric_range, and script filters are not cached by default.
All filters allow to set
_cache element on them to explicitly control caching. They also allow to set
_cache_key which will be used as the caching key for that filter. This can be handy when using very large filters (like a terms filter with many elements in it).