[jira] [Created] (DRILL-4188) Change the default value of planner.enable_hash_single_key to false

["Aman Sinha (JIRA)" <ji...@apache.org>]

Aman Sinha created DRILL-4188:
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             Summary: Change the default value of planner.enable_hash_single_key to false
                 Key: DRILL-4188
                 URL: https://issues.apache.org/jira/browse/DRILL-4188
             Project: Apache Drill
          Issue Type: Bug
          Components: Query Planning & Optimization
    Affects Versions: 1.4.0
            Reporter: Aman Sinha
            Assignee: Aman Sinha


The planner.enable_hash_single_key flag is used by the HashJoin and MergeJoin plans to do hash distribution on both sides of the join when it is a multi-column join (e.g T1.a1 = T2.a2 AND T1.b1 = T2.b2).   The default value of this parameter is True, which means that Drill will generate multiple plans each with hash distribute on only 1 column.  The final plan chosen is based on costing.  

However, due to lack of column statistics, this approach is problematic because we could end up picking the first column for hash distribution if all plans cost the same and if this column has low number of distinct values, there could be substantial skew in distribution.  

Doing the hash distribution on all columns should be the default, so I propose to change planner.enable_hash_single_key to False.  The scenario where we might still want single column hash distribution is when the join is done after some other operation (e.g window function, grouped-aggregation) where the child already does a hash-distribution on 1 column that is part of the join.  However, for those case, we may want to selectively enable this flag. 



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