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https://issues.apache.org/jira/browse/SPARK-22018?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16167166#comment-16167166
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Apache Spark commented on SPARK-22018:
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User 'tdas' has created a pull request for this issue:
https://github.com/apache/spark/pull/19240
> Catalyst Optimizer does not preserve top-level metadata while collapsing
> projects
> ---------------------------------------------------------------------------------
>
> Key: SPARK-22018
> URL: https://issues.apache.org/jira/browse/SPARK-22018
> Project: Spark
> Issue Type: Bug
> Components: Optimizer, Structured Streaming
> Affects Versions: 2.1.1, 2.2.0
> Reporter: Tathagata Das
> Assignee: Tathagata Das
>
> If there are two projects like as follows.
> {code}
> Project [a_with_metadata#27 AS b#26]
> +- Project [a#0 AS a_with_metadata#27]
> +- LocalRelation <empty>, [a#0, b#1]
> {code}
> Child Project has an output column with a metadata in it, and the parent
> Project has an alias that implicitly forwards the metadata. So this metadata
> is visible for higher operators. Upon applying CollapseProject optimizer
> rule, the metadata is not preserved.
> {code}
> Project [a#0 AS b#26]
> +- LocalRelation <empty>, [a#0, b#1]
> {code}
> This is incorrect, as downstream operators that expect certain metadata (e.g.
> watermark in structured streaming) to identify certain fields will fail to do
> so.
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