[GitHub] [druid] gianm commented on issue #9321: Performance degradation in topN queries when SQL-compatible null handling is enabled

[GitBox <gi...@apache.org>]

gianm commented on issue #9321: Performance degradation in topN queries when SQL-compatible null handling is enabled
URL: https://github.com/apache/druid/issues/9321#issuecomment-591671621
 
 
   It could be that something about calling groupBy defeats de-virtualization or inlining as described on https://shipilev.net/blog/2015/black-magic-method-dispatch/. Maybe something that was monomorphic or bimorphic with only topNs in the mix becomes megamorphic once groupBys are in the mix as well (due to different usage patterns of some shared component). I don't think I can speculate about what component that might be, since there are a lot of shared components and some of them are pretty complex. It might not even have anything to do with groupBy — it's possible that your groupBy queries are using some features (specific aggregators, filters, etc) that your topNs aren't using.
   
   But if this is what's going on, then vectorization should fix it, since it makes function call overhead less of an issue. So we should continue with our program of vectorizing everything (topN isn't yet).

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