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Posted to issues@drill.apache.org by "Boaz Ben-Zvi (JIRA)" <ji...@apache.org> on 2018/11/13 02:05:00 UTC

[jira] [Created] (DRILL-6845) Eliminate duplicates for Semi Hash Join

Boaz Ben-Zvi created DRILL-6845:
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             Summary: Eliminate duplicates for Semi Hash Join
                 Key: DRILL-6845
                 URL: https://issues.apache.org/jira/browse/DRILL-6845
             Project: Apache Drill
          Issue Type: Sub-task
          Components: Execution - Relational Operators
    Affects Versions: 1.14.0
            Reporter: Boaz Ben-Zvi
            Assignee: Boaz Ben-Zvi
             Fix For: 1.15.0


Following DRILL-6735: The performance of the new Semi Hash Join may degrade if the build side contains excessive number of join-key duplicate rows; this mainly a result of the need to store all those rows first, before the hash table is built.

  Proposed solution: For Semi, the Hash Agg would create a Hash-Table initially, and use it to eliminate key-duplicate rows as they arrive.

  Proposed extra: That Hash-Table has an added cost (e.g. resizing). So perform "runtime stats" – Check initial number of incoming rows (e.g. 32k), and if the number of duplicates is less than some threshold (e.g. %20) – cancel that "early" hash table.

 



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