[GitHub] [incubator-druid] peferron commented on issue #7187: Improve topN algorithm

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peferron commented on issue #7187: Improve topN algorithm
URL: https://github.com/apache/incubator-druid/issues/7187#issuecomment-470307248
 
 
   @leerho Going back to your song example, could FIS support getting the top song titles by number of unique listeners over the past week? The current topN can do that using HLL sketches as metric (weight). That's one example of the usefulness of accepting any Druid aggregation or post-aggregation as a metric. (Interestingly, this specific case could benefit from using a HLL Map sketch.)
   
   Also, I don't think that the current topN algorithm orders all distinct items on each node (or segment) prior to selecting the top 1000, since that would be unnecessarily wasteful when the number of distinct items is >> 1000. Would be great if someone more familiar with the topN implementation could confirm.

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