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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