[GitHub] [incubator-druid] peferron opened a new pull request #7223: Improve caching documentation

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peferron opened a new pull request #7223: Improve caching documentation
URL: https://github.com/apache/incubator-druid/pull/7223
 
 
   I noticed a few issues while reading the caching docs.
   
   1. The [Query Context](http://druid.io/docs/latest/querying/query-context.html) doc indicates that `useResultLevelCache` and `populateResultLevelCache` default to `false`. According to the parameter description, this means that in addition to enabling result caching in the broker config, every query would have to explicitly set these parameters to `true` in order to use the result cache. This is odd, and looks like an error in the doc, since the actual default value for both parameters in `QueryContexts.java` is `true`.
   
   2. The [Query Caching](http://druid.io/docs/latest/querying/caching.html) doc indicates that "when caching is enabled on the Broker, results from Historicals are returned on a per segment basis". However, my understanding of `CachingClusteredClient`, and the discussion in https://github.com/apache/incubator-druid/issues/5796#issuecomment-391937670, indicates that this only happens if the broker populates the cache—if the broker only reads from the cache without populating it, then historicals can do local merging. This is rather important and deserves a mention IMO. Of course, if all segment results are in the cache, then the broker will still have to handle all the merging, but this aspect is well covered in the next section.

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