[GitHub] [druid] jihoonson commented on pull request #11989: Add segment merged result cache for broker

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jihoonson commented on pull request #11989:
URL: https://github.com/apache/druid/pull/11989#issuecomment-983963886


   @yuanlihan thanks for the explanation. Your last comment is what I was looking for :slightly_smiling_face: I haven't read code yet, so perhaps my questions are very basic. So based on your comment, I suppose the problem you want to solve is that the current result-level cache could be not much useful if your query involves realtime segments? And the new post-merge cache can address it by caching only the merge results of historical results? This sounds reasonable to me. It would be nice if you can add a summarized version of the answers to these questions in the PR description. It will help others understand what problem you are solving.
   
   > Can this feature instead cache pre-merge results? That way, we cache each minute slice once. The merge is still needed to gather up the slices required by the query. But, that merge is in-memory and should be pretty fast. The result would be, in this example, caching 1/10 the amount of data compared to caching post-merge data. And, less load on the historicals since we don't hit them each time the query time range shifts forward one minute.
   
   This sounds like the segment-level cache on broker to me too. We already support it (https://druid.apache.org/docs/0.22.0/querying/using-caching.html#enabling-query-caching-on-brokers).


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