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As a general rule, filters should be used instead of queries:
- for binary yes/no searches
- for queries on exact values
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, 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).