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Date: Wed, 2 Aug 2017 18:42:00 +0000 (UTC)
From: "Thomas Tauber-Marshall (JIRA)" <jira@apache.org>
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Subject: [jira] [Created] (IMPALA-5753) Consider Expr nullability in rewrite
 rules
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archived-at: Wed, 02 Aug 2017 18:42:04 -0000

Thomas Tauber-Marshall created IMPALA-5753:
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             Summary: Consider Expr nullability in rewrite rules
                 Key: IMPALA-5753
                 URL: https://issues.apache.org/jira/browse/IMPALA-5753
             Project: IMPALA
          Issue Type: Improvement
          Components: Frontend
    Affects Versions: Impala 2.10.0
            Reporter: Thomas Tauber-Marshall


A recent change, IMPALA-5016, added an expr rewrite rule that simplifies coalesce(). This rule eliminated the coalesce when its first parameter (that isn't constant null) is non-nullable.

It considered parameters to be non-nullable if they were SlotRefs that pointed to non-nullable SlotDescriptors. However, this approach has two problems:
- Its incorrect if the slot has been outer joined on (IMPALA-5725)
- It misses some opportunities for optimizations, for example '<non-nullable SlotRef> + <non-null constant>' would be non-nullable but wouldn't have been eliminated since its not just a simple SlotRef.

What we really need is a concept of the nullability of Exprs, so for example a SlotRefs nullability would be passed on the nullability of its SlotDescriptor and whether its the output of an outer join, the nullability of a BinaryPredicate would be based on the nullability of its children, etc.

This would be useful for other expr rewrite rules besides coalesce() (such as simplifying 'ifnull')


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