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

["Ryan Johnson (Jira)" <ji...@apache.org>]

Ryan Johnson created SPARK-42704:
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             Summary: 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


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
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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