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Posted to github@arrow.apache.org by GitBox <gi...@apache.org> on 2021/04/26 13:24:40 UTC

[GitHub] [arrow-datafusion] alamb commented on issue #128: Optimize nested joins

alamb commented on issue #128:
URL: https://github.com/apache/arrow-datafusion/issues/128#issuecomment-826832822


   Comment from Daniël Heres(Dandandan) @ 2020-12-22T13:07:20.151+0000:
   <pre>Found some nice material from Spark on this:
   [https://databricks.com/blog/2017/08/31/cost-based-optimizer-in-apache-spark-2-2.html]
   
   basically the idea to use column level statistics such as:
   * min/max
   * nr of distinct values
   * null count
   
   to come up with e.g. selectivity of a filter.
   
   Also there is a formula for (inner) join cardinality:
   
   {{num(A IJ B) = num(A)*num(B)/max(distinct(A.k),distinct(B.k))}}</pre>


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