[jira] [Commented] (SPARK-17995) Use new attributes for columns from outer joins

["Ryan Blue (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-17995?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15586972#comment-15586972 ] 

Ryan Blue commented on SPARK-17995:
-----------------------------------

[~cloud_fan] and [~yhuai], I'd like to help fix this, but I'm not sure the best way.

I started to write an analyzer rule that uses transformUp on the initial logical plan, before unresovled aliases are resolved. That rule would find outer joins and generate a map of attributes to replace to the new attribute above the join, with the schema updated to be nullable and with a new exprId. The attributes to replace come from the output of the outer join.

Where I ran into trouble was in replacing the attributes in the logical plan above the a join. I don't think it is a good idea to have cases in the rule for every possible plan, so I think we need a method to substitute attributes that is implemented by nodes in the plan. That sounds like a larger patch than I originally thought, so I wanted to make sure I'm going down the right path before I put up a PR for it. What do you think?

> Use new attributes for columns from outer joins
> -----------------------------------------------
>
>                 Key: SPARK-17995
>                 URL: https://issues.apache.org/jira/browse/SPARK-17995
>             Project: Spark
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions: 1.6.2, 2.0.0, 2.1.0
>            Reporter: Ryan Blue
>
> Plans involving outer joins use the same attribute reference (by exprId) to reference columns above the join and below the join. This is a false equivalence that leads to bugs like SPARK-16181, in which an attributes were incorrectly replaced by the optimizer. The column has a different schema above the outer join because its values may be null. The fix for that issue, [PR #13884](https://github.com/apache/spark/pull/13884) has a TODO comment from [~cloud_fan] to fix this by using different attributes instead of needing to special-case outer joins in rules and this issue is to track that improvement.



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