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This impairs the ability of the Druid cluster to scale well. Segment-level caching is controlled by the query context parameters .
Larger production clusters should enable segment-level cache population on Historicals only (not on Brokers) to avoid having to use Brokers to merge all query results.
In all cases, the Druid cache is a query result cache.
Each time the Named Query is called, if the arguments passed differ from those used to create the current cache, a new cache is created.
Apache Druid (incubating) supports query result caching at both the segment and whole-query result level.
Cache data can be stored in the local JVM heap or in an external distributed key/value store.
Segment-level caching does require Druid to merge the per-segment results on each query, even when they are served from the cache.
For this reason, whole-query result level caching can be more efficient if invalidation due to real-time ingestion is not an issue.