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https://issues.apache.org/jira/browse/SPARK-17995?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15589692#comment-15589692
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Ryan Blue commented on SPARK-17995:
-----------------------------------
I'm not sure how that would work.
Here's an example. Say I have two tables:
{code}
scala> l.show
+---+----+
| id|left|
+---+----+
| 1| a|
| 2| b|
+---+----+
scala> r.show
+---+-----+
| id|right|
+---+-----+
| 1| x|
+---+-----+
{code}
When I join those two on {{id}} and add an extra derived column, I end up with
this *parsed plan*:
{code}
== Parsed Logical Plan ==
Project [id#5 AS id#165, left#6, right#16, isnull(right#16) AS (right IS
NULL)#166]
+- Join LeftOuter, (id#5 = id#15)
:- Project [_1#2 AS id#5, _2#3 AS left#6]
: +- LocalRelation [_1#2, _2#3]
+- Project [_1#12 AS id#15, _2#13 AS right#16]
+- LocalRelation [_1#12, _2#13]
{code}
That has a reference to {{right#16}} in the outer-most project, which is coming
from {{select(r("right"), ...)}}. That reference -- and any reference like it
in the possible tree above the outer join -- needs to be replaced with a new
attribute that correctly shows the {{right}} column as nullable, which wasn't
inferred when I created the dataframe. It's fairly easy to replace the join
itself, but replacing all references to an attribute above the join is where I
don't see an easy solution.
> 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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