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Posted to issues@spark.apache.org by "Tanel Kiis (Jira)" <ji...@apache.org> on 2020/09/25 16:48:00 UTC

[jira] [Created] (SPARK-32995) CostBasedJoinReorder optimizer rule should be idempotent

Tanel Kiis created SPARK-32995:
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             Summary: CostBasedJoinReorder optimizer rule should be idempotent
                 Key: SPARK-32995
                 URL: https://issues.apache.org/jira/browse/SPARK-32995
             Project: Spark
          Issue Type: Improvement
          Components: SQL
    Affects Versions: 3.1.0
            Reporter: Tanel Kiis


The CostBasedJoinReorder has currently the following comment:
{code}
    // Since join costs in AQP can change between multiple runs, there is no reason that we have an
    // idempotence enforcement on this batch. We thus make it FixedPoint(1) instead of Once.
{code} 

This is incorrect reasoning.
In the optimizer, we mean that a batch is idempotent, when applying it for the second time in a row (without changing anything) it does not change the plan at all. 



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