[GitHub] [spark] dongjoon-hyun commented on pull request #31966: [SPARK-34638][SQL] Single field nested column prune on generator output

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dongjoon-hyun commented on pull request #31966:
URL: https://github.com/apache/spark/pull/31966#issuecomment-822188605


   Hi, @viirya . While testing this PR, I found the following regression.
   
   **BEFORE (3.1.1)**
   ```scala
   scala> sql("select * from values array(array(named_struct('a', 1, 'b', 3), named_struct('a', 2, 'b', 4))) T(items)").write.parquet("/tmp/nested_array")
   
   scala> spark.read.parquet("/tmp/nested_array").createOrReplaceTempView("t")
   
   scala> sql("select d.a from (select explode(c) d from (select explode(items) c from t))").show()
   +---+
   |  a|
   +---+
   |  1|
   |  2|
   +---+
   ```
   
   **BEFORE(master)**
   ```scala
   scala> spark.read.parquet("/tmp/nested_array").createOrReplaceTempView("t")
   
   scala> sql("select d.a from (select explode(c) d from (select explode(items) c from t))").show()
   +---+
   |  a|
   +---+
   |  1|
   |  2|
   +---+
   ```
   
   **AFTER (This PR)**
   ```scala
   scala> spark.read.parquet("/tmp/nested_array").createOrReplaceTempView("t")
   
   scala> sql("select d.a from (select explode(c) d from (select explode(items) c from t))").show()
   java.lang.ClassCastException: org.apache.spark.sql.types.ArrayType cannot be cast to org.apache.spark.sql.types.StructType
     at org.apache.spark.sql.catalyst.expressions.SelectedField$.selectField(SelectedField.scala:81)
     at org.apache.spark.sql.catalyst.expressions.SelectedField$.unapply(SelectedField.scala:62)
     at org.apache.spark.sql.catalyst.expressions.SchemaPruning$.getRootFields(SchemaPruning.scala:124)
     at org.apache.spark.sql.catalyst.expressions.SchemaPruning$.$anonfun$identifyRootFields$1(SchemaPruning.scala:81)
   ```
   
   Could you double-check this and add some test coverage?


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