bersprockets opened a new pull request, #39349:
URL: https://github.com/apache/spark/pull/39349

   ### What changes were proposed in this pull request?
   
   Change `InterpretedUnsafeProjection#getElementSize` to choose the 
appropriate element size for the underlying SQL type of a UDT, rather than 
simply using the the default size of the underlying SQL type.
   
   ### Why are the changes needed?
   
   Consider this query:
   ```
   // create a file of vector data
   import org.apache.spark.ml.linalg.{DenseVector, Vector}
   
   case class TestRow(varr: Array[Vector])
   val values = Array(0.1d, 0.2d, 0.3d)
   val dv = new DenseVector(values).asInstanceOf[Vector]
   
   val ds = Seq(TestRow(Array(dv, dv))).toDS
   ds.coalesce(1).write.mode("overwrite").format("parquet").save("vector_data")
   
   // this works
   spark.read.format("parquet").load("vector_data").collect
   
   sql("set spark.sql.codegen.wholeStage=false")
   sql("set spark.sql.codegen.factoryMode=NO_CODEGEN")
   
   // this will get an error
   spark.read.format("parquet").load("vector_data").collect
   ```
   The failures vary, incuding
   * `VectorUDT` attempting to deserialize to a `SparseVector` (rather than a 
`DenseVector`)
   * negative array size (for one of the nested arrays)
   * JVM crash (SIGBUS error).
   
   This is because `InterpretedUnsafeProjection` initializes the outer-most 
array writer with an element size of 17 (the size of the UDT's underlying 
struct), rather than an element size of 8, which would be appropriate for an 
array of structs.
   
   When the outer-most array is later accessed, `UnsafeArrayData` assumes an 
element size of 8, so it picks up a garbage offset/size tuple for the second 
element.
   
   ### Does this PR introduce _any_ user-facing change?
   
   No.
   
   ### How was this patch tested?
   
   New unit test.
   


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