[GitHub] [arrow-datafusion] alamb opened a new issue #128: Optimize nested joins

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

alamb opened a new issue #128:
URL: https://github.com/apache/arrow-datafusion/issues/128


   *Note*: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-10964
   
   Once [https://github.com/apache/arrow/pull/8961] is merged, we have an optimization for a JOIN that operates on two tables.
   
   The next step is to extend this optimization to work with nested joins, and this is not trivial. See discussion in [https://github.com/apache/arrow/pull/8961] for context.
   
    


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

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

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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