[jira] [Commented] (SPARK-42704) SubqueryAlias should propagate metadata columns its child already selects

["Apache Spark (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-42704?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17697565#comment-17697565 ] 

Apache Spark commented on SPARK-42704:
--------------------------------------

User 'ryan-johnson-databricks' has created a pull request for this issue:
https://github.com/apache/spark/pull/40321

> SubqueryAlias should propagate metadata columns its child already selects 
> --------------------------------------------------------------------------
>
>                 Key: SPARK-42704
>                 URL: https://issues.apache.org/jira/browse/SPARK-42704
>             Project: Spark
>          Issue Type: Bug
>          Components: Spark Core
>    Affects Versions: 3.3.2, 3.4.0
>            Reporter: Ryan Johnson
>            Priority: Major
>
> The `AddMetadataColumns` analyzer rule intends to make resolve available metadata columns, even if the plan already contains projections that did not explicitly mention the metadata column.
> The `SubqueryAlias` plan node intentionally does not propagate metadata columns automatically from a non-leaf/non-subquery child node, because the following should _not_ work:
>  
> {code:java}
> spark.read.table("t").select("a", "b").as("s").select("_metadata"){code}
> However, today it is too strict in breaks the metadata chain, in case the child node's output already includes the metadata column:
>  
> {code:java}
> // expected to work (and does)
> spark.read.table("t")
>   .select("a", "b").select("_metadata")
> // by extension, should also work (but does not)
> spark.read.table("t").select("a", "b", "_metadata").as("s")
>   .select("a", "b").select("_metadata"){code}
> The solution is for `SubqueryAlias` to always propagate metadata columns that are already in the child's output, thus preserving the `metadataOutput` chain for that column.



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