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Query Caching
Druid supports query result caching through an LRU cache. Results are stored as a whole or either on a per segment basis along with the parameters of a given query. Segment level caching allows Druid to return final results based partially on segment results in the cache and partially on segment results from scanning historical/real-time segments. Result level caching enables Druid to cache the entire result set, so that query results can be completely retrieved from the cache for identical queries.
Segment results can be stored in a local heap cache or in an external distributed key/value store. Segment query caches can be enabled at either the Historical and Broker level (it is not recommended to enable caching on both).
Query caching on Brokers
Enabling caching on the broker can yield faster results than if query caches were enabled on Historicals for small clusters. This is the recommended setup for smaller production clusters (< 20 servers). Take note that when caching is enabled on the Broker, results from Historicals are returned on a per segment basis, and Historicals will not be able to do any local result merging. Result level caching is enabled only on the Broker side.
Query caching on Historicals
Larger production clusters should enable caching only on the Historicals to avoid having to use Brokers to merge all query results. Enabling caching on the Historicals instead of the Brokers enables the Historicals to do their own local result merging and puts less strain on the Brokers.